Repository: tenpy/tenpy (language: python)

Issue:
Re-Licensing under Apache v2
Up until now, TeNPy is licensed under the [GPL v3](https://www.gnu.org/licenses/gpl-3.0.en.html) license.
It is one of the major open source licenses, e.g. the Linux kernel is released with it, and it is the reason why all linux distros are open source. However, the license is quite strict in enforcing what's called "copyleft": anybody who want's to modify and extend the code has to publish his code as open source under the very same GPL license. This is a red flag for companies like IBM developing quiskit or quantum computing start-ups and hinders IBM to use TeNPy as a backend for qiskit.

@bartandrews hence suggested re-license TeNPy under the [Apache v2](http://www.apache.org/licenses/LICENSE-2.0) license instead. Although I was the one originally choosing the GPLv3 license, I fully support the change to Apache v2. I put many hours into TeNpy in the hope that it will be useful for the scientific community, and blocking the further development of it due to licensing issues seems counter-productive.
Apache v2 is also a popular license with many other scientific python projects, including [quiskit](https://github.com/Qiskit/qiskit/blob/main/LICENSE.txt),  [tensorflow](https://github.com/tensorflow/tensorflow/blob/master/LICENSE) and the [cytnx](https://github.com/Cytnx-dev/Cytnx/blob/master/LICENSE) library that we're planning to collaborate with.

To compare the licenses, you might want to read the full texts linked above or check out the bullet point summaries on
choosealicense.com, [Apache v2](https://choosealicense.com/licenses/apache-2.0/) vs [GPL v3](https://choosealicense.com/licenses/gpl-3.0/). By changing to Apache v2, we're essentially dropping the copyleft, i.e. become more permissive towards future developers to choose their own license for their contributions. In theory, someone could even do that closed source (which the copyleft of GPL would avoid), but I think the risk for that is just a very negligible epsilon > 0.

With this issue, I'm hence asking the contributors of TeNPy for their consent or possible objections to this re-licensing. 
For the legal documentation, it would be great if everyone of the current contributors (in particular the authors of our [TeNPy v1 paper](https://arxiv.org/abs/2408.02010)) could give a short comment like "changing the license is fine with me" - or raise objections and concerns, should there be any. Thank you!



Discussion / hints:
changing the license is fine with me
changing the license is fine with me
Changing the license is fine by me. Thanks! 
Wilhem Kadow answered by Mail: 

> Thanks so much for taking care of this. Changing the license is fine with me.

Changing the license is fine with me.
Changing the license is fine with me.
Changing the license is fine with me. Thank you for organising this.
Changing the license is fine with me.
changing the license is fine with me. Thanks! 
Changing the license is fine with me. Thanks!
Changing the license is fine with me. Thanks!
I support changing the license!
Changing the license is fine with me, thank you!
Changing the license is fine with me. Thank you for taking care of this!
I also got  confirmations from Dan Parker, Maximilian Schulz and Samuel Scalet,  Philipp Zechmann and Markus Drescher.

With that, the vast majority of team (making up nearly all explicit code contributions) has explicitly explicitly confirmed, while no one has raised any concerns with the license change.
Given that throughout the repository the copyright is dedicated to the whole "TeNPy developer team" rather than individuals, I think we can proceed and say that we as a whole team agreed to change the license. Hence, I'll proceed to update the license.

Fix patch:
diff --git a/LICENSE b/LICENSE
index f288702d2..d64569567 100644
--- a/LICENSE
+++ b/LICENSE
@@ -1,674 +1,202 @@
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diff --git a/README.rst b/README.rst
index 77a3ddeaf..9babee5ba 100644
--- a/README.rst
+++ b/README.rst
@@ -97,5 +97,5 @@ This work was funded by the U.S. Department of Energy, Office of Science, Office
 
 License
 -------
-The code is licensed under GPL-v3.0 given in the file ``LICENSE`` of the repository, 
+Sinc TeNPy version 1.0.4, the code is licensed under Apache v2 given in the file ``LICENSE`` of the repository,
 in the online documentation readable at https://tenpy.readthedocs.io/en/latest/install/license.html.
diff --git a/doc/conf.py b/doc/conf.py
index f4d075a42..53ad6bfb2 100644
--- a/doc/conf.py
+++ b/doc/conf.py
@@ -1,6 +1,6 @@
 #!/usr/bin/env python3
 # -*- coding: utf-8 -*-
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 #
 import sys
 import os
diff --git a/doc/install/license.rst b/doc/install/license.rst
index 8d5d3073d..709cd2a9e 100644
--- a/doc/install/license.rst
+++ b/doc/install/license.rst
@@ -7,7 +7,8 @@ TeNPy developer team
 License
 =======
 
-The source code documented here is published under a GPL v3 license, which we include below.
+The source code documented here is published under the Apache v2 license, which we include below.
+Up to version 1.0.3, TeNPy was distributed under GPL v3; see :issue:`462` for the discussion around the license change.
 
 .. include:: /../LICENSE
     :literal:
diff --git a/doc/notebooks b/doc/notebooks
index 39300beb9..b6bd1ebfc 160000
--- a/doc/notebooks
+++ b/doc/notebooks
@@ -1,1 +1,1 @@
-Subproject commit 39300beb98484bcb38088bfee39ea09d560b9838
+Subproject commit b6bd1ebfc6dda73ed0013ffd96a6c4eb3425d228
diff --git a/doc/toycodes b/doc/toycodes
index cb9511cbd..cc3937ff1 160000
--- a/doc/toycodes
+++ b/doc/toycodes
@@ -1,1 +1,1 @@
-Subproject commit cb9511cbd08211d010398453fe96713ce1494040
+Subproject commit cc3937ff1a1b05dd19971ebad38a90d69853aa96
diff --git a/examples/a_np_conserved.py b/examples/a_np_conserved.py
index 59bad6c87..4676aba5a 100644
--- a/examples/a_np_conserved.py
+++ b/examples/a_np_conserved.py
@@ -13,7 +13,7 @@
 Compare it to the example `b_mps.py`,
 which does the same steps using a few predefined classes like MPS and MPO.
 """
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import tenpy.linalg.np_conserved as npc
 import numpy as np
diff --git a/examples/advanced/central_charge_ising.py b/examples/advanced/central_charge_ising.py
index bb7677e44..f7bda9db5 100644
--- a/examples/advanced/central_charge_ising.py
+++ b/examples/advanced/central_charge_ising.py
@@ -6,7 +6,7 @@
 
 For the theoretical background why :math:`S = c/6 log(xi)`, see :cite:`pollmann2009`.
 """
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import numpy as np
 import tenpy
diff --git a/examples/advanced/mpo_exponential_decay.py b/examples/advanced/mpo_exponential_decay.py
index 474250771..9271c1c00 100644
--- a/examples/advanced/mpo_exponential_decay.py
+++ b/examples/advanced/mpo_exponential_decay.py
@@ -10,7 +10,7 @@
 We run the iDMRG algorithm to find the ground state and energy density of the
 system in the thermodynamic limit.
 """
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import numpy as np
 
diff --git a/examples/advanced/tfi_phase_transition.py b/examples/advanced/tfi_phase_transition.py
index 44f263a1c..482f1903b 100644
--- a/examples/advanced/tfi_phase_transition.py
+++ b/examples/advanced/tfi_phase_transition.py
@@ -4,7 +4,7 @@
 through the phase transition by changing the field `g`.
 It plots a few observables in the end.
 """
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import numpy as np
 
diff --git a/examples/advanced/tfi_segment.py b/examples/advanced/tfi_segment.py
index 7519a4b53..d270c192b 100644
--- a/examples/advanced/tfi_segment.py
+++ b/examples/advanced/tfi_segment.py
@@ -8,7 +8,7 @@
 :class:`~tenpy.simulations.GroundStateSearch.OrthogonalExcitations` class,
 but this example might be helpful to see the general idea.
 """
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import numpy as np
 
diff --git a/examples/advanced/vumps_and_plane_wave.py b/examples/advanced/vumps_and_plane_wave.py
index d8363cdc9..368b22d1c 100644
--- a/examples/advanced/vumps_and_plane_wave.py
+++ b/examples/advanced/vumps_and_plane_wave.py
@@ -2,7 +2,7 @@
 
 This example uses VUMPS to find the ground state of the transverse field Ising model and uses the plane wave excitation ansatz to compute the first excited states.
 """
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 from tenpy.models.tf_ising import TFIChain
 from tenpy.algorithms import vumps, plane_wave_excitation
diff --git a/examples/advanced/xxz_corr_length.py b/examples/advanced/xxz_corr_length.py
index 6eb250628..1ba8fafa4 100644
--- a/examples/advanced/xxz_corr_length.py
+++ b/examples/advanced/xxz_corr_length.py
@@ -5,7 +5,7 @@
 :meth:`~tenpy.networks.mps.MPS.correlation_length` to extract the correlation length of the ground
 state, and plots it vs. hz in the end.
 """
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import numpy as np
 
diff --git a/examples/b_mps.py b/examples/b_mps.py
index dad771bf7..26afa54c6 100644
--- a/examples/b_mps.py
+++ b/examples/b_mps.py
@@ -12,7 +12,7 @@
 Note that this example performs the same steps as `a_np_conserved.py`,
 but makes use of other predefined classes except npc.
 """
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import tenpy.linalg.np_conserved as npc
 import numpy as np
diff --git a/examples/c_tebd.py b/examples/c_tebd.py
index 10f04974d..2d547a926 100644
--- a/examples/c_tebd.py
+++ b/examples/c_tebd.py
@@ -3,7 +3,7 @@
 The example functions in this class do the same as the ones in `toycodes/c_tebd.py`, but make use
 of the classes defined in tenpy.
 """
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import numpy as np
 
diff --git a/examples/chern_insulators/chiral_pi_flux.py b/examples/chern_insulators/chiral_pi_flux.py
index c8f75bffe..c6abb4edd 100644
--- a/examples/chern_insulators/chiral_pi_flux.py
+++ b/examples/chern_insulators/chiral_pi_flux.py
@@ -3,7 +3,7 @@
 Based on the model in [Neupert2011]_
 """
 
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import numpy as np
 
diff --git a/examples/chern_insulators/haldane.py b/examples/chern_insulators/haldane.py
index d917e5804..660ba1551 100644
--- a/examples/chern_insulators/haldane.py
+++ b/examples/chern_insulators/haldane.py
@@ -3,7 +3,7 @@
 Reproduces Fig. 2.a,b) in [Grushin2015]_
 """
 
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import numpy as np
 
diff --git a/examples/chern_insulators/haldane_C3.py b/examples/chern_insulators/haldane_C3.py
index 3fe1495d7..3b9083a77 100644
--- a/examples/chern_insulators/haldane_C3.py
+++ b/examples/chern_insulators/haldane_C3.py
@@ -3,7 +3,7 @@
 Based on the model in [Yang2012]_
 """
 
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import numpy as np
 
diff --git a/examples/chern_insulators/haldane_FCI.py b/examples/chern_insulators/haldane_FCI.py
index 2aa9c3e33..1a540c94a 100644
--- a/examples/chern_insulators/haldane_FCI.py
+++ b/examples/chern_insulators/haldane_FCI.py
@@ -6,7 +6,7 @@
 - 1/2 filling of the lowest band (i.e. 1/4 total filling)
 """
 
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import numpy as np
 import warnings
diff --git a/examples/d_dmrg.py b/examples/d_dmrg.py
index a8c48e4a3..11f40f93a 100644
--- a/examples/d_dmrg.py
+++ b/examples/d_dmrg.py
@@ -3,7 +3,7 @@
 The example functions in this class do the same as the ones in `toycodes/d_dmrg.py`,
 but make use of the classes defined in tenpy.
 """
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import numpy as np
 
diff --git a/examples/e_tdvp.py b/examples/e_tdvp.py
index 979b3eae5..2dbe33144 100644
--- a/examples/e_tdvp.py
+++ b/examples/e_tdvp.py
@@ -4,7 +4,7 @@
 difference is that we can run one-site TDVP or two-site TDVP. In the former, the bond dimension can
 not grow; the latter allows to grow the bond dimension and hence requires a truncation.
 """
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 import numpy as np
 import tenpy
 from tenpy.algorithms import tdvp
diff --git a/examples/tfi_exact.py b/examples/tfi_exact.py
index 643cc3caf..dd2fdd20b 100644
--- a/examples/tfi_exact.py
+++ b/examples/tfi_exact.py
@@ -4,7 +4,7 @@
 .. math ::
     H = - J \\sum_{i} \\sigma^x_i \\sigma^x_{i+1} - g \\sum_{i} \\sigma^z_i
 """
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import numpy as np
 import scipy.sparse as sparse
diff --git a/examples/v1_publication/heisenberg_tebd.py b/examples/v1_publication/heisenberg_tebd.py
index 9563379d5..41f64eda4 100644
--- a/examples/v1_publication/heisenberg_tebd.py
+++ b/examples/v1_publication/heisenberg_tebd.py
@@ -1,5 +1,5 @@
 """An example simulating the dynamics of the Neel state under Heisenberg evolution, using TEBD."""
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 import argparse
 import matplotlib.pyplot as plt
 import matplotlib as mpl
diff --git a/examples/v1_publication/tfi_cylinder.py b/examples/v1_publication/tfi_cylinder.py
index 93f0d011b..8156b002d 100644
--- a/examples/v1_publication/tfi_cylinder.py
+++ b/examples/v1_publication/tfi_cylinder.py
@@ -1,5 +1,5 @@
 """An example determining the phase diagram of the 2D transverse field Ising model."""
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 import pickle
 import numpy as np
 import os
diff --git a/examples/z_exact_diag.py b/examples/z_exact_diag.py
index 68eeb487c..163838114 100644
--- a/examples/z_exact_diag.py
+++ b/examples/z_exact_diag.py
@@ -2,7 +2,7 @@
 
 Sorry that this is not well documented! ED is meant to be used for debugging only ;)
 """
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import tenpy.linalg.np_conserved as npc
 from tenpy.models.xxz_chain import XXZChain
diff --git a/pyproject.toml b/pyproject.toml
index 6a4bf757c..41e5c13ae 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -8,7 +8,6 @@ dynamic = ["version"]
 description = "Simulation of quantum many-body systems with tensor networks in Python"
 readme = {file = "README.rst", content-type = "text/x-rst"}
 requires-python = ">=3.8"
-license = {text = "GPLv3"}
 authors = [
     {name = "TeNPy Developer Team"},
 ]
@@ -21,7 +20,7 @@ classifiers = [
     "Development Status :: 5 - Production/Stable",
     "Intended Audience :: Science/Research",
     "Intended Audience :: Developers",
-    "License :: OSI Approved :: GNU General Public License v3 (GPLv3)",
+    "License :: OSI Approved :: Apache Software License",
     "Natural Language :: English",
     "Programming Language :: C",
     "Programming Language :: Python",
diff --git a/setup.py b/setup.py
index 638d31f3b..92ba55e63 100644
--- a/setup.py
+++ b/setup.py
@@ -1,4 +1,4 @@
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 from setuptools import setup, Extension
 try:
     from Cython.Build import cythonize
diff --git a/tenpy/__init__.py b/tenpy/__init__.py
index 674ff7f89..99f4c4e07 100644
--- a/tenpy/__init__.py
+++ b/tenpy/__init__.py
@@ -6,7 +6,7 @@
 The code is intended to be accessible for newcomers
 and yet powerful enough for day-to-day research.
 """
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 # This file marks this directory as a python package.
 
 # Note: all external packages that are imported should be `del`-ed at the end of the file!
diff --git a/tenpy/__main__.py b/tenpy/__main__.py
index cf51defe0..e82754a23 100644
--- a/tenpy/__main__.py
+++ b/tenpy/__main__.py
@@ -1,5 +1,5 @@
 """The tenpy entry point, called by ``python -m tenpy``."""
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import tenpy
 
diff --git a/tenpy/algorithms/__init__.py b/tenpy/algorithms/__init__.py
index a153ff892..f9df99683 100644
--- a/tenpy/algorithms/__init__.py
+++ b/tenpy/algorithms/__init__.py
@@ -19,7 +19,7 @@
     exact_diag
     disentangler
 """
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 from . import algorithm, dmrg, dmrg_parallel, disentangler, mps_common, tebd, tdvp, \
     exact_diag, purification, network_contractor, mpo_evolution, vumps, plane_wave_excitation
diff --git a/tenpy/algorithms/algorithm.py b/tenpy/algorithms/algorithm.py
index 16f9e77d8..01ce61bf3 100644
--- a/tenpy/algorithms/algorithm.py
+++ b/tenpy/algorithms/algorithm.py
@@ -1,5 +1,5 @@
 """This module contains some base classes for algorithms."""
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import time
 import numpy as np
diff --git a/tenpy/algorithms/disentangler.py b/tenpy/algorithms/disentangler.py
index 955022df3..a34dccfc6 100644
--- a/tenpy/algorithms/disentangler.py
+++ b/tenpy/algorithms/disentangler.py
@@ -7,7 +7,7 @@
 
 .. autodata:: disentanglers_atom_parse_dict
 """
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import numpy as np
 import logging
diff --git a/tenpy/algorithms/dmrg.py b/tenpy/algorithms/dmrg.py
index 692ce36b8..630d1edfc 100644
--- a/tenpy/algorithms/dmrg.py
+++ b/tenpy/algorithms/dmrg.py
@@ -30,7 +30,7 @@
 A generic protocol for approaching a physics question using DMRG is given in
 :doc:`/intro/dmrg-protocol`.
 """
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import numpy as np
 import time
diff --git a/tenpy/algorithms/dmrg_parallel.py b/tenpy/algorithms/dmrg_parallel.py
index 05db7a678..bfbd28ece 100644
--- a/tenpy/algorithms/dmrg_parallel.py
+++ b/tenpy/algorithms/dmrg_parallel.py
@@ -3,7 +3,7 @@
 .. warning ::
     This module is still under active development. Use with care!
 """
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 from ..tools.thread import Worker
 
diff --git a/tenpy/algorithms/exact_diag.py b/tenpy/algorithms/exact_diag.py
index 43145ebd0..9c76cd5b6 100644
--- a/tenpy/algorithms/exact_diag.py
+++ b/tenpy/algorithms/exact_diag.py
@@ -13,7 +13,7 @@
     but just the ability to diagonalize the defined models for small system sizes
     without additional extra work.
 """
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import numpy as np
 import warnings
diff --git a/tenpy/algorithms/mpo_evolution.py b/tenpy/algorithms/mpo_evolution.py
index 36fcb63d7..1f0a3d89e 100644
--- a/tenpy/algorithms/mpo_evolution.py
+++ b/tenpy/algorithms/mpo_evolution.py
@@ -1,6 +1,6 @@
 """Time evolution using the WI or WII approximation of the time evolution operator."""
 
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import logging
 
diff --git a/tenpy/algorithms/mps_common.py b/tenpy/algorithms/mps_common.py
index 7b697426a..d17aedeeb 100644
--- a/tenpy/algorithms/mps_common.py
+++ b/tenpy/algorithms/mps_common.py
@@ -18,7 +18,7 @@
 The :class:`VariationalCompression` and :class:`VariationalApplyMPO`
 implemented here also directly use the :class:`Sweep` class.
 """
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 from ..linalg import np_conserved as npc
 from .algorithm import Algorithm
diff --git a/tenpy/algorithms/network_contractor.py b/tenpy/algorithms/network_contractor.py
index 3c0f4cb10..45c69be23 100644
--- a/tenpy/algorithms/network_contractor.py
+++ b/tenpy/algorithms/network_contractor.py
@@ -9,7 +9,7 @@
     - implement or wrap netcon.m, a function to find optimal contraction sequences
         (:arxiv:`1304.6112`)
 """
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import numpy as np
 from ..linalg import np_conserved as npc
diff --git a/tenpy/algorithms/plane_wave_excitation.py b/tenpy/algorithms/plane_wave_excitation.py
index b7e0dab4d..e21018110 100644
--- a/tenpy/algorithms/plane_wave_excitation.py
+++ b/tenpy/algorithms/plane_wave_excitation.py
@@ -22,7 +22,7 @@
 in :class:`MultiSitePlaneWaveExcitationEngine`. Note that with the current implementation, the
 numerical costs scale exponentially with the number of exciting sites.
 """
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import numpy as np
 import logging
diff --git a/tenpy/algorithms/purification.py b/tenpy/algorithms/purification.py
index fa5beb005..99e822c1f 100644
--- a/tenpy/algorithms/purification.py
+++ b/tenpy/algorithms/purification.py
@@ -1,6 +1,6 @@
 """Algorithms for using Purification."""
 
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import numpy as np
 import logging
diff --git a/tenpy/algorithms/tdvp.py b/tenpy/algorithms/tdvp.py
index d92c2f506..47bb3e24f 100644
--- a/tenpy/algorithms/tdvp.py
+++ b/tenpy/algorithms/tdvp.py
@@ -30,7 +30,7 @@
 .. todo ::
     allow for increasing bond dimension in SingleSiteTDVPEngine, similar to DMRG Mixer
 """
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 from ..linalg.krylov_based import LanczosEvolution
 from ..linalg.truncation import svd_theta, TruncationError
diff --git a/tenpy/algorithms/tebd.py b/tenpy/algorithms/tebd.py
index 46cb0aa00..ade8e3bde 100644
--- a/tenpy/algorithms/tebd.py
+++ b/tenpy/algorithms/tebd.py
@@ -37,7 +37,7 @@
     Yet, imaginary TEBD might be useful for cross-checks and testing.
 
 """
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import numpy as np
 import time
diff --git a/tenpy/algorithms/truncation.py b/tenpy/algorithms/truncation.py
index 063beffbf..10a57aab3 100644
--- a/tenpy/algorithms/truncation.py
+++ b/tenpy/algorithms/truncation.py
@@ -4,7 +4,7 @@
 Here we just import all the names again for backwards compatibility,
 to support loading pickle and HDF5 data files with `TruncationError` instances in them.
 """
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 # just provide namespace from ..linalg.truncation
 
diff --git a/tenpy/algorithms/vumps.py b/tenpy/algorithms/vumps.py
index df560108e..15837e9e4 100644
--- a/tenpy/algorithms/vumps.py
+++ b/tenpy/algorithms/vumps.py
@@ -31,7 +31,7 @@
 the single-site algorithm, which is the more principled algorithm.
 
 """
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import numpy as np
 import time
diff --git a/tenpy/linalg/__init__.py b/tenpy/linalg/__init__.py
index 91a2ec4f6..2d64de758 100644
--- a/tenpy/linalg/__init__.py
+++ b/tenpy/linalg/__init__.py
@@ -22,7 +22,7 @@
     truncation
 
 """
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 from . import charges, np_conserved, krylov_based, random_matrix, sparse, svd_robust, truncation
 from .charges import *
diff --git a/tenpy/linalg/_npc_helper.pyx b/tenpy/linalg/_npc_helper.pyx
index 46423ecf2..54ed1a3d8 100644
--- a/tenpy/linalg/_npc_helper.pyx
+++ b/tenpy/linalg/_npc_helper.pyx
@@ -8,7 +8,7 @@ functions/classes defined here to overwrite those written in pure Python wheneve
 decorator ``@use_cython`` is used in other python files of tenpy.
 If this module was not compiled and could not be imported, a warning is issued.
 """
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 DEF DEBUG_PRINT = 0  # set this to 1 for debug output (e.g. benchmark timings within the functions)
 DEF USE_MKL_GEMM_BATCH = 1 # whether to use ?gemm_batch function of MKL
diff --git a/tenpy/linalg/charges.py b/tenpy/linalg/charges.py
index d4a2d8254..f409f3617 100644
--- a/tenpy/linalg/charges.py
+++ b/tenpy/linalg/charges.py
@@ -19,7 +19,7 @@
 
 .. autodata:: QTYPE
 """
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import numpy as np
 import bisect
diff --git a/tenpy/linalg/krylov_based.py b/tenpy/linalg/krylov_based.py
index 28eec7ea7..dc3abb1b6 100644
--- a/tenpy/linalg/krylov_based.py
+++ b/tenpy/linalg/krylov_based.py
@@ -1,5 +1,5 @@
 """Lanczos algorithm for np_conserved arrays."""
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import numpy as np
 from .sparse import FlatHermitianOperator, OrthogonalNpcLinearOperator, ShiftNpcLinearOperator
diff --git a/tenpy/linalg/np_conserved.py b/tenpy/linalg/np_conserved.py
index 0b09b487e..ec6ab5797 100644
--- a/tenpy/linalg/np_conserved.py
+++ b/tenpy/linalg/np_conserved.py
@@ -81,7 +81,7 @@
     speigs
 
 """
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import numpy as np
 import scipy.linalg
diff --git a/tenpy/linalg/random_matrix.py b/tenpy/linalg/random_matrix.py
index 6f5dcc7da..a0a95d343 100644
--- a/tenpy/linalg/random_matrix.py
+++ b/tenpy/linalg/random_matrix.py
@@ -29,7 +29,7 @@
     npc.Array.from_func_square(GOE, [leg, leg.conj()])
 
 """
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import numpy as np
 
diff --git a/tenpy/linalg/sparse.py b/tenpy/linalg/sparse.py
index ec8f742b1..07dd97c87 100644
--- a/tenpy/linalg/sparse.py
+++ b/tenpy/linalg/sparse.py
@@ -7,7 +7,7 @@
 :class:`FlatLinearOperator` allows to use all the scipy sparse methods by providing functionality
 to convert flat numpy arrays to and from np_conserved arrays.
 """
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import numpy as np
 from . import np_conserved as npc
diff --git a/tenpy/linalg/svd_robust.py b/tenpy/linalg/svd_robust.py
index 177819e7c..75a079783 100644
--- a/tenpy/linalg/svd_robust.py
+++ b/tenpy/linalg/svd_robust.py
@@ -21,7 +21,7 @@
 >>> from tenpy.linalg.svd_robust import svd
 >>> U, S, VT = svd([[1., 1.], [0., 1.]])
 """
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import numpy as np
 import scipy
diff --git a/tenpy/linalg/truncation.py b/tenpy/linalg/truncation.py
index 9124428be..a0589a1c9 100644
--- a/tenpy/linalg/truncation.py
+++ b/tenpy/linalg/truncation.py
@@ -41,7 +41,7 @@
     There might be other sources of error as well, for example TEBD has also an discretization
     error depending on the chosen time step.
 """
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import numpy as np
 from ..linalg import np_conserved as npc
diff --git a/tenpy/models/__init__.py b/tenpy/models/__init__.py
index 5fefe6497..56aef1c74 100644
--- a/tenpy/models/__init__.py
+++ b/tenpy/models/__init__.py
@@ -33,7 +33,7 @@
     mixed_xk
     clock
 """
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 from . import lattice, model
 from . import tf_ising, xxz_chain, spins, spins_nnn
diff --git a/tenpy/models/aklt.py b/tenpy/models/aklt.py
index 1d5613d4e..fcf0badc9 100644
--- a/tenpy/models/aklt.py
+++ b/tenpy/models/aklt.py
@@ -4,7 +4,7 @@
 Writing down the Hamiltonian is easiest done in terms of bond couplings.
 This class thus serves as an example how this can be done.
 """
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import numpy as np
 
diff --git a/tenpy/models/clock.py b/tenpy/models/clock.py
index a3600c9e1..c5073f5cb 100644
--- a/tenpy/models/clock.py
+++ b/tenpy/models/clock.py
@@ -2,7 +2,7 @@
 
 Generalization of transverse field Ising model to higher dimensional on-site Hilbert space.
 """
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import numpy as np
 from .model import CouplingMPOModel, NearestNeighborModel
diff --git a/tenpy/models/fermions_spinless.py b/tenpy/models/fermions_spinless.py
index 89dea3e14..3ba3708a5 100644
--- a/tenpy/models/fermions_spinless.py
+++ b/tenpy/models/fermions_spinless.py
@@ -2,7 +2,7 @@
 
 .. todo ::     add further terms (e.g. c^dagger c^dagger + h.c.) to the Hamiltonian.
 """
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 
 from .model import CouplingMPOModel, NearestNeighborModel
diff --git a/tenpy/models/haldane.py b/tenpy/models/haldane.py
index 19e8b4404..44d8d558e 100644
--- a/tenpy/models/haldane.py
+++ b/tenpy/models/haldane.py
@@ -1,5 +1,5 @@
 """Bosonic and fermionic Haldane models."""
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import numpy as np
 
diff --git a/tenpy/models/hofstadter.py b/tenpy/models/hofstadter.py
index 0eb65e028..e93551356 100644
--- a/tenpy/models/hofstadter.py
+++ b/tenpy/models/hofstadter.py
@@ -4,7 +4,7 @@
     Long term: implement different lattices.
     Long term: implement variable hopping strengths Jx, Jy.
 """
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import numpy as np
 
diff --git a/tenpy/models/hubbard.py b/tenpy/models/hubbard.py
index d64c08dcb..80f669678 100644
--- a/tenpy/models/hubbard.py
+++ b/tenpy/models/hubbard.py
@@ -1,5 +1,5 @@
 """Bosonic and fermionic Hubbard models."""
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import numpy as np
 
diff --git a/tenpy/models/lattice.py b/tenpy/models/lattice.py
index e55f99580..69f4d2028 100644
--- a/tenpy/models/lattice.py
+++ b/tenpy/models/lattice.py
@@ -16,7 +16,7 @@
 :doc:`/notebooks/90_overview_predefined_lattices`
 
 """
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import numpy as np
 from scipy.spatial import ConvexHull, Voronoi
diff --git a/tenpy/models/mixed_xk.py b/tenpy/models/mixed_xk.py
index 934abd241..2fa84a8eb 100644
--- a/tenpy/models/mixed_xk.py
+++ b/tenpy/models/mixed_xk.py
@@ -62,7 +62,7 @@
     exclusion principle implies a possibly large occupation on single k modes, i.e., hard-core
     bosons in x-y-space don't map to hard-core bosons in x-k-space!
 """
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import numpy as np
 import itertools as it
diff --git a/tenpy/models/model.py b/tenpy/models/model.py
index 971ff8821..3207bed78 100644
--- a/tenpy/models/model.py
+++ b/tenpy/models/model.py
@@ -27,7 +27,7 @@
 
 See also the introduction in :doc:`/intro/model`.
 """
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import numpy as np
 import warnings
diff --git a/tenpy/models/spins.py b/tenpy/models/spins.py
index 6869772ff..552142151 100644
--- a/tenpy/models/spins.py
+++ b/tenpy/models/spins.py
@@ -2,7 +2,7 @@
 
 Uniform lattice of spin-S sites, coupled by nearest-neighbor interactions.
 """
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 from ..networks.site import SpinSite
 from .model import CouplingMPOModel, NearestNeighborModel
diff --git a/tenpy/models/spins_nnn.py b/tenpy/models/spins_nnn.py
index ef53f1e72..cc870b9d1 100644
--- a/tenpy/models/spins_nnn.py
+++ b/tenpy/models/spins_nnn.py
@@ -14,7 +14,7 @@
 :meth:`~tenpy.models.model.NearestNeighborModel.from_MPOModel`.
 An example for such a case is given in the file ``examples/c_tebd.py``.
 """
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 from .lattice import Chain
 from ..networks.site import SpinSite, GroupedSite
diff --git a/tenpy/models/tf_ising.py b/tenpy/models/tf_ising.py
index b7680f8ad..f9ff3a8a3 100644
--- a/tenpy/models/tf_ising.py
+++ b/tenpy/models/tf_ising.py
@@ -6,7 +6,7 @@
 
 We choose the field along z to allow to conserve the parity, if desired.
 """
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import numpy as np
 
diff --git a/tenpy/models/tj_model.py b/tenpy/models/tj_model.py
index aa0e5e2cf..19b75cc9d 100644
--- a/tenpy/models/tj_model.py
+++ b/tenpy/models/tj_model.py
@@ -1,5 +1,5 @@
 """tJ model"""
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 from .model import CouplingMPOModel, NearestNeighborModel
 from .lattice import Chain
diff --git a/tenpy/models/toric_code.py b/tenpy/models/toric_code.py
index 83d17560b..ca9943b85 100644
--- a/tenpy/models/toric_code.py
+++ b/tenpy/models/toric_code.py
@@ -3,7 +3,7 @@
 As we put the model on a cylinder, the name "toric code" is a bit misleading, but it is the
 established name for this model...
 """
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import numpy as np
 
diff --git a/tenpy/models/xxz_chain.py b/tenpy/models/xxz_chain.py
index d68deedb8..755a6142f 100644
--- a/tenpy/models/xxz_chain.py
+++ b/tenpy/models/xxz_chain.py
@@ -3,7 +3,7 @@
 The XXZ chain is contained in the more general :class:`~tenpy.models.spins.SpinChain`; the idea of
 this module is more to serve as a pedagogical example for a model.
 """
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 from .lattice import Site, Chain
 from .model import CouplingModel, NearestNeighborModel, MPOModel, CouplingMPOModel
diff --git a/tenpy/networks/__init__.py b/tenpy/networks/__init__.py
index e5bddf1c6..2a1213793 100644
--- a/tenpy/networks/__init__.py
+++ b/tenpy/networks/__init__.py
@@ -16,7 +16,7 @@
     uniform_mps
     momentum_mps
 """
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 from . import site, mps, mpo, purification_mps, momentum_mps, uniform_mps
 
diff --git a/tenpy/networks/momentum_mps.py b/tenpy/networks/momentum_mps.py
index 11dc0d460..4490845ca 100644
--- a/tenpy/networks/momentum_mps.py
+++ b/tenpy/networks/momentum_mps.py
@@ -20,7 +20,7 @@
 always orthogonal to the initial uniform MPS. `X` parametrizes the excited states.
 """
 
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import numpy as np
 import logging
diff --git a/tenpy/networks/mpo.py b/tenpy/networks/mpo.py
index 20e518b7c..00391a757 100644
--- a/tenpy/networks/mpo.py
+++ b/tenpy/networks/mpo.py
@@ -34,7 +34,7 @@
 Similar as for the MPS, a bond index ``i`` is *left* of site `i`,
 i.e. between sites ``i-1`` and ``i``.
 """
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import numpy as np
 from scipy.linalg import expm
diff --git a/tenpy/networks/mps.py b/tenpy/networks/mps.py
index 6552416a6..13cc9de56 100644
--- a/tenpy/networks/mps.py
+++ b/tenpy/networks/mps.py
@@ -143,7 +143,7 @@
 iMPS, so this is the form *assumed* when calling MPO :meth:`~tenpy.networks.mpo.MPO.apply` on an
 MPS.
 """
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 from abc import ABCMeta, abstractmethod
 import numpy as np
diff --git a/tenpy/networks/purification_mps.py b/tenpy/networks/purification_mps.py
index 32a216338..577d88cbe 100644
--- a/tenpy/networks/purification_mps.py
+++ b/tenpy/networks/purification_mps.py
@@ -116,7 +116,7 @@
     Moreover, we don't split the physical and auxiliary space into separate sites, which makes
     TEBD as costly as :math:`O(d^6 \chi^3)`.
 """
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import copy
 import numpy as np
diff --git a/tenpy/networks/site.py b/tenpy/networks/site.py
index 0419ca2db..1a9b247c2 100644
--- a/tenpy/networks/site.py
+++ b/tenpy/networks/site.py
@@ -3,7 +3,7 @@
 The :class:`Site` is the prototype, read it's docstring.
 
 """
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import numpy as np
 import itertools
diff --git a/tenpy/networks/terms.py b/tenpy/networks/terms.py
index d4dcd361f..8dbe03fb0 100644
--- a/tenpy/networks/terms.py
+++ b/tenpy/networks/terms.py
@@ -5,7 +5,7 @@
 sites it acts on. Moreover, we associate a `strength` to each term, which corresponds to the
 prefactor when specifying e.g. a Hamiltonian.
 """
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import numpy as np
 import warnings
diff --git a/tenpy/networks/uniform_mps.py b/tenpy/networks/uniform_mps.py
index 492183539..92259101c 100644
--- a/tenpy/networks/uniform_mps.py
+++ b/tenpy/networks/uniform_mps.py
@@ -20,7 +20,7 @@
 account for the additional type of tensor structure.
 
 """
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import numpy as np
 import logging
diff --git a/tenpy/simulations/__init__.py b/tenpy/simulations/__init__.py
index d606f7a0f..422f57f69 100644
--- a/tenpy/simulations/__init__.py
+++ b/tenpy/simulations/__init__.py
@@ -13,7 +13,7 @@
     time_evolution
     post_processing
 """
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 from . import measurement, simulation, ground_state_search, time_evolution
 from .measurement import *
diff --git a/tenpy/simulations/ground_state_search.py b/tenpy/simulations/ground_state_search.py
index 12629b4fe..35f285432 100644
--- a/tenpy/simulations/ground_state_search.py
+++ b/tenpy/simulations/ground_state_search.py
@@ -1,5 +1,5 @@
 """Simulations for ground state searches."""
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import numpy as np
 from pathlib import Path
diff --git a/tenpy/simulations/measurement.py b/tenpy/simulations/measurement.py
index d31fe8574..fc38a8223 100644
--- a/tenpy/simulations/measurement.py
+++ b/tenpy/simulations/measurement.py
@@ -10,7 +10,7 @@
 
 Full description and details in :doc:`/intro/measurements`.
 """
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import numpy as np
 import warnings
diff --git a/tenpy/simulations/post_processing.py b/tenpy/simulations/post_processing.py
index a11c32f0e..5aafd5a3c 100644
--- a/tenpy/simulations/post_processing.py
+++ b/tenpy/simulations/post_processing.py
@@ -9,7 +9,7 @@
 the simulation class in a post-processing step. They follow the syntax
 ``def pp_function(DL, *, kwarg1, kwarg_2=default_2):``.
 """
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 
 import os
diff --git a/tenpy/simulations/simulation.py b/tenpy/simulations/simulation.py
index 9d4e9aa6d..12b1a5513 100644
--- a/tenpy/simulations/simulation.py
+++ b/tenpy/simulations/simulation.py
@@ -7,7 +7,7 @@
 See :doc:`/intro/simulations` for an overview and
 :doc:`/examples` for a list of example parameter yaml files.
 """
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import os
 import sys
diff --git a/tenpy/simulations/time_evolution.py b/tenpy/simulations/time_evolution.py
index 12b1ab29f..941dcd775 100644
--- a/tenpy/simulations/time_evolution.py
+++ b/tenpy/simulations/time_evolution.py
@@ -1,6 +1,6 @@
 """Simulations for (real) time evolution, time dependent correlation
 functions and spectral functions."""
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import numpy as np
 import warnings
diff --git a/tenpy/tools/__init__.py b/tenpy/tools/__init__.py
index c3514c9ec..d3279e84f 100644
--- a/tenpy/tools/__init__.py
+++ b/tenpy/tools/__init__.py
@@ -23,7 +23,7 @@
     spectral_function_tools
     prediction
 """
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 from . import (events, fit, hdf5_io, math, misc, params, process, string, optimization, cache,
                thread)
diff --git a/tenpy/tools/cache.py b/tenpy/tools/cache.py
index e65c6bfb4..6de99ec6c 100644
--- a/tenpy/tools/cache.py
+++ b/tenpy/tools/cache.py
@@ -7,7 +7,7 @@
 this is easiest done through a ``with`` statement, see the example in :class:`DictCache`.
 """
 
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import pickle
 import numpy as np
diff --git a/tenpy/tools/docs.py b/tenpy/tools/docs.py
index f40257d6a..f4e8469a6 100644
--- a/tenpy/tools/docs.py
+++ b/tenpy/tools/docs.py
@@ -1,5 +1,5 @@
 """Tools for handling docstrings."""
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 __all__ = ['amend_parent_docstring']
 
diff --git a/tenpy/tools/events.py b/tenpy/tools/events.py
index 585e68336..a26b4d677 100644
--- a/tenpy/tools/events.py
+++ b/tenpy/tools/events.py
@@ -4,7 +4,7 @@
 which can get called once a certain "event" happens.
 Examples are given in the class doc-string.
 """
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 from collections import namedtuple
 import warnings
diff --git a/tenpy/tools/fit.py b/tenpy/tools/fit.py
index 3a49a9bc1..b9ea90225 100644
--- a/tenpy/tools/fit.py
+++ b/tenpy/tools/fit.py
@@ -1,5 +1,5 @@
 """tools to fit to an algebraic decay."""
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import numpy as np
 import scipy.optimize as optimize
diff --git a/tenpy/tools/hdf5_io.py b/tenpy/tools/hdf5_io.py
index cadd4dab8..297bdd9cd 100644
--- a/tenpy/tools/hdf5_io.py
+++ b/tenpy/tools/hdf5_io.py
@@ -65,7 +65,7 @@
 
 .. autodata:: TYPES_FOR_HDF5_DATASETS
 """
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import pickle
 import gzip
diff --git a/tenpy/tools/math.py b/tenpy/tools/math.py
index 44a93bca6..b61f17c33 100644
--- a/tenpy/tools/math.py
+++ b/tenpy/tools/math.py
@@ -2,7 +2,7 @@
 
 .. autodata:: LeviCivita3
 """
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import numpy as np
 import warnings
diff --git a/tenpy/tools/misc.py b/tenpy/tools/misc.py
index 0ce9c9c33..b06ed28d5 100644
--- a/tenpy/tools/misc.py
+++ b/tenpy/tools/misc.py
@@ -1,5 +1,5 @@
 """Miscellaneous tools, somewhat random mix yet often helpful."""
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import operator
 import numpy as np
diff --git a/tenpy/tools/optimization.py b/tenpy/tools/optimization.py
index 8706fa520..39239058e 100644
--- a/tenpy/tools/optimization.py
+++ b/tenpy/tools/optimization.py
@@ -76,7 +76,7 @@
 .. autodata:: have_cython_functions
 .. autodata:: compiled_with_MKL
 """
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 from enum import IntEnum
 import warnings
diff --git a/tenpy/tools/params.py b/tenpy/tools/params.py
index a21517810..57a2a080a 100644
--- a/tenpy/tools/params.py
+++ b/tenpy/tools/params.py
@@ -2,7 +2,7 @@
 
 See the doc-string of :class:`Config` for details.
 """
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import warnings
 import numpy
diff --git a/tenpy/tools/prediction.py b/tenpy/tools/prediction.py
index 727c1ad33..1298dc1df 100644
--- a/tenpy/tools/prediction.py
+++ b/tenpy/tools/prediction.py
@@ -1,5 +1,5 @@
 """This module contains functions for linear prediction."""
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import numpy as np
 from scipy.linalg import solve_toeplitz
diff --git a/tenpy/tools/process.py b/tenpy/tools/process.py
index b42a5204d..8acb3076f 100644
--- a/tenpy/tools/process.py
+++ b/tenpy/tools/process.py
@@ -13,7 +13,7 @@
 while still being failsafe if the shared OpenMP library is not found.  In the latter case,
 you might also try the equivalent :func:`mkl_get_nthreads` and :func:`mkl_set_nthreads`.
 """
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import warnings
 import ctypes
diff --git a/tenpy/tools/spectral_function_tools.py b/tenpy/tools/spectral_function_tools.py
index 45f5e40d5..2c18a80cd 100644
--- a/tenpy/tools/spectral_function_tools.py
+++ b/tenpy/tools/spectral_function_tools.py
@@ -8,7 +8,7 @@
 However, they can also be used in a standalone way on available results (i.e. in an interactive ipython or
 jupyter notebook session).
 """
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import numpy as np
 
diff --git a/tenpy/tools/string.py b/tenpy/tools/string.py
index 82793462a..c62c9db98 100644
--- a/tenpy/tools/string.py
+++ b/tenpy/tools/string.py
@@ -1,5 +1,5 @@
 """Tools for handling strings."""
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 from typing import Sequence
 import numpy as np
diff --git a/tenpy/tools/thread.py b/tenpy/tools/thread.py
index c6cb30dac..85ab0c860 100644
--- a/tenpy/tools/thread.py
+++ b/tenpy/tools/thread.py
@@ -1,6 +1,6 @@
 """Tools for thread-based parallelization."""
 
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import threading
 import queue
diff --git a/tenpy/version.py b/tenpy/version.py
index 85e638d1d..47ecaf689 100644
--- a/tenpy/version.py
+++ b/tenpy/version.py
@@ -10,7 +10,7 @@
 .. autodata :: full_version
 .. autodata :: version_summary
 """
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import sys
 import subprocess


Test patch:
diff --git a/doc/changelog/latest/license_change.txt b/doc/changelog/latest/license_change.txt
new file mode 100644
index 000000000..ae71ce9b0
--- /dev/null
+++ b/doc/changelog/latest/license_change.txt
@@ -0,0 +1,1 @@
+- We have changed the license from GPL v3 to the less restrictive Apache v2 license, see :issue:`462` for details.
diff --git a/tests/benchmark/benchmark.py b/tests/benchmark/benchmark.py
index 6d5457092..a832b4ed2 100755
--- a/tests/benchmark/benchmark.py
+++ b/tests/benchmark/benchmark.py
@@ -11,7 +11,7 @@
 Afterwards, you can plot the produced files::
     python benchmark.py -p tensordot_*_benchmark_*.txt
 """
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 from __future__ import print_function  # (backwards compatibility to python 2)
 
diff --git a/tests/benchmark/combine_npc.py b/tests/benchmark/combine_npc.py
index a4ef2b7db..25b72f8b9 100644
--- a/tests/benchmark/combine_npc.py
+++ b/tests/benchmark/combine_npc.py
@@ -1,5 +1,5 @@
 """To be used in the `-m` argument of benchmark.py."""
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import tensordot_npc
 
diff --git a/tests/benchmark/combine_numpy.py b/tests/benchmark/combine_numpy.py
index 48d64eaf7..d1b76b380 100644
--- a/tests/benchmark/combine_numpy.py
+++ b/tests/benchmark/combine_numpy.py
@@ -1,5 +1,5 @@
 """To be used in the `-m` argument of benchmark.py."""
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import numpy as np
 import tensordot_npc
diff --git a/tests/benchmark/dmrg_infinite.py b/tests/benchmark/dmrg_infinite.py
index 5075748d2..267201566 100644
--- a/tests/benchmark/dmrg_infinite.py
+++ b/tests/benchmark/dmrg_infinite.py
@@ -1,5 +1,5 @@
 """To be used in the `-m` argument of benchmark.py."""
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 from tenpy.networks.mps import MPS
 from tenpy.models.spins import SpinChain
diff --git a/tests/benchmark/profiling.py b/tests/benchmark/profiling.py
index 10c476427..665684b7d 100755
--- a/tests/benchmark/profiling.py
+++ b/tests/benchmark/profiling.py
@@ -8,7 +8,7 @@
 
     python profiling.py -p tensordot_*_profile_*.prof --sort time --limit 10
 """
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import numpy as np
 import cProfile
diff --git a/tests/benchmark/run_benchmark.sh b/tests/benchmark/run_benchmark.sh
index 77ffacbca..5e7ee185f 100755
--- a/tests/benchmark/run_benchmark.sh
+++ b/tests/benchmark/run_benchmark.sh
@@ -1,5 +1,5 @@
 #!/bin/bash
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 set -e
 DIR="$(dirname ${BASH_SOURCE[0]})"
 
diff --git a/tests/benchmark/singlerun.py b/tests/benchmark/singlerun.py
index 5f9852298..2ac140053 100755
--- a/tests/benchmark/singlerun.py
+++ b/tests/benchmark/singlerun.py
@@ -4,7 +4,7 @@
 -s 20 -S 50 -q 1 1
 """
 
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import numpy as np
 import time
diff --git a/tests/benchmark/split_npc.py b/tests/benchmark/split_npc.py
index 84fa9a1ae..c57a9b0bf 100644
--- a/tests/benchmark/split_npc.py
+++ b/tests/benchmark/split_npc.py
@@ -1,5 +1,5 @@
 """To be used in the `-m` argument of benchmark.py."""
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import tensordot_npc
 
diff --git a/tests/benchmark/split_numpy.py b/tests/benchmark/split_numpy.py
index 1bd781c83..1e66f386b 100644
--- a/tests/benchmark/split_numpy.py
+++ b/tests/benchmark/split_numpy.py
@@ -1,5 +1,5 @@
 """To be used in the `-m` argument of benchmark.py."""
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import numpy as np
 import tensordot_npc
diff --git a/tests/benchmark/tebd_infinite.py b/tests/benchmark/tebd_infinite.py
index 1dbb2987d..dfa440972 100644
--- a/tests/benchmark/tebd_infinite.py
+++ b/tests/benchmark/tebd_infinite.py
@@ -1,5 +1,5 @@
 """To be used in the `-m` argument of benchmark.py."""
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import numpy as np
 
diff --git a/tests/benchmark/tensordot_npc.py b/tests/benchmark/tensordot_npc.py
index 748305ab0..b41a07f80 100644
--- a/tests/benchmark/tensordot_npc.py
+++ b/tests/benchmark/tensordot_npc.py
@@ -1,5 +1,5 @@
 """To be used in the `-m` argument of benchmark.py."""
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import numpy as np
 import tenpy.linalg.np_conserved as npc
diff --git a/tests/benchmark/tensordot_numpy.py b/tests/benchmark/tensordot_numpy.py
index c100cbb23..06ef687a4 100644
--- a/tests/benchmark/tensordot_numpy.py
+++ b/tests/benchmark/tensordot_numpy.py
@@ -1,5 +1,5 @@
 """To be used in the `-m` argument of benchmark.py."""
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import numpy as np
 import tensordot_npc
diff --git a/tests/export_import_test/io_test.py b/tests/export_import_test/io_test.py
index adadce256..290bdef4b 100644
--- a/tests/export_import_test/io_test.py
+++ b/tests/export_import_test/io_test.py
@@ -7,7 +7,7 @@
 test_pickle.py``. This will generate the files with pre-defined data (see :func:`gen_example_data`)
 and the tenpy version in the filename.
 """
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import numpy as np
 import os
diff --git a/tests/export_import_test/test_cache.py b/tests/export_import_test/test_cache.py
index 3f2e95641..b39e31486 100644
--- a/tests/export_import_test/test_cache.py
+++ b/tests/export_import_test/test_cache.py
@@ -1,4 +1,4 @@
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import numpy as np
 import numpy.testing as npt
diff --git a/tests/linting.py b/tests/linting.py
index 001aabb4e..7139fe4ab 100644
--- a/tests/linting.py
+++ b/tests/linting.py
@@ -4,7 +4,7 @@
 The code may still run fine even if these checks fail.
 We therefore consider them part of a linting routine and do *not* call them from pytest.
 """
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import tenpy
 import types
@@ -80,7 +80,7 @@ def check_copyright_notice():
     for fn in tenpy_files:
         with open(fn, 'r') as f:
             for line in f:
-                if line.startswith('# Copyright (C) TeNPy Developers, GNU GPLv3'):
+                if line.startswith('# Copyright (C) TeNPy Developers, Apache license'):
                     break
             else:  # no break
                 raise AssertionError(f'No/wrong copyright notice in {fn}.')
diff --git a/tests/random_test.py b/tests/random_test.py
index 5fb77c86c..c435d2702 100644
--- a/tests/random_test.py
+++ b/tests/random_test.py
@@ -1,5 +1,5 @@
 """Provide helper functions for test of random Arrays."""
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import numpy as np
 import itertools as it
diff --git a/tests/tdvp_numpy.py b/tests/tdvp_numpy.py
index aa7596579..d07414948 100644
--- a/tests/tdvp_numpy.py
+++ b/tests/tdvp_numpy.py
@@ -2,7 +2,7 @@
 
 .. todo ::     Make this a nice toycode! For the test, use some pre-defined values.
 """
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 from scipy.linalg import expm
 import numpy as np
diff --git a/tests/test_charges.py b/tests/test_charges.py
index ca69925ca..c06c1fbaf 100644
--- a/tests/test_charges.py
+++ b/tests/test_charges.py
@@ -1,5 +1,5 @@
 """A collection of tests for tenpy.linalg.charges."""
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import tenpy.linalg.charges as charges
 import numpy as np
diff --git a/tests/test_cs_projection.py b/tests/test_cs_projection.py
index cccaa37ce..7617f2c1d 100644
--- a/tests/test_cs_projection.py
+++ b/tests/test_cs_projection.py
@@ -1,5 +1,5 @@
 """A test for :meth:`~tenpy.networks.mps.MPS.project_onto_charge_sector`"""
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import numpy as np
 from tenpy.networks.site import SpinSite, FermionSite, SpinHalfFermionSite
diff --git a/tests/test_dmrg.py b/tests/test_dmrg.py
index c4b6b7fcb..6f30ffd22 100644
--- a/tests/test_dmrg.py
+++ b/tests/test_dmrg.py
@@ -1,5 +1,5 @@
 """A collection of tests to check the functionality of `tenpy.dmrg`"""
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import tenpy.linalg.np_conserved as npc
 from tenpy.models.tf_ising import TFIChain
diff --git a/tests/test_exact_diag.py b/tests/test_exact_diag.py
index 272c52b56..0def65e22 100644
--- a/tests/test_exact_diag.py
+++ b/tests/test_exact_diag.py
@@ -1,5 +1,5 @@
 """A collection of tests to check the functionality of algorithms.exact_diagonalization."""
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import tenpy.linalg.np_conserved as npc
 import numpy as np
diff --git a/tests/test_examples.py b/tests/test_examples.py
index e64137340..c33ca872b 100644
--- a/tests/test_examples.py
+++ b/tests/test_examples.py
@@ -4,7 +4,7 @@
 __name__ == "__main__": ...`` clauses, if you want to demonstrate an interactive code, which
 should not be executed as part of the tests.
 """
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import sys
 import os
diff --git a/tests/test_krylov_based.py b/tests/test_krylov_based.py
index 54b080d94..aa140f9b3 100644
--- a/tests/test_krylov_based.py
+++ b/tests/test_krylov_based.py
@@ -1,5 +1,5 @@
 """A collection of tests for tenpy.linalg.krylov_based."""
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import tenpy.linalg.np_conserved as npc
 import numpy as np
diff --git a/tests/test_lattice.py b/tests/test_lattice.py
index 09d1bc20f..3c86320db 100644
--- a/tests/test_lattice.py
+++ b/tests/test_lattice.py
@@ -1,5 +1,5 @@
 """A collection of tests for tenpy.models.lattice."""
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 from tenpy.models import lattice
 import tenpy.linalg.np_conserved as npc
diff --git a/tests/test_model.py b/tests/test_model.py
index 0d449fc0a..a9ce2ab07 100644
--- a/tests/test_model.py
+++ b/tests/test_model.py
@@ -1,5 +1,5 @@
 """A collection of tests for (classes in) :mod:`tenpy.models.model`."""
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import itertools
 
diff --git a/tests/test_model_aklt.py b/tests/test_model_aklt.py
index 479009c74..cf28a1ed0 100644
--- a/tests/test_model_aklt.py
+++ b/tests/test_model_aklt.py
@@ -1,4 +1,4 @@
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 from tenpy.models import aklt
 from test_model import check_general_model
 from tenpy.algorithms.dmrg import TwoSiteDMRGEngine
diff --git a/tests/test_model_clock.py b/tests/test_model_clock.py
index 9e52f9a57..b489e4242 100644
--- a/tests/test_model_clock.py
+++ b/tests/test_model_clock.py
@@ -1,4 +1,4 @@
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 from tenpy.models.clock import ClockModel, ClockChain
 from test_model import check_general_model
diff --git a/tests/test_model_fermions_spinless.py b/tests/test_model_fermions_spinless.py
index a20ddc451..471b4a66d 100644
--- a/tests/test_model_fermions_spinless.py
+++ b/tests/test_model_fermions_spinless.py
@@ -1,4 +1,4 @@
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 import numpy as np
 from tenpy.models.fermions_spinless import FermionChain, FermionModel
 from test_model import check_general_model
diff --git a/tests/test_model_haldane.py b/tests/test_model_haldane.py
index e3a296d0a..b1f577c2a 100644
--- a/tests/test_model_haldane.py
+++ b/tests/test_model_haldane.py
@@ -1,4 +1,4 @@
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 from tenpy.models.haldane import BosonicHaldaneModel, FermionicHaldaneModel
 from test_model import check_general_model
 import pytest
diff --git a/tests/test_model_hofstadter.py b/tests/test_model_hofstadter.py
index f72f54e1e..388d6d01a 100644
--- a/tests/test_model_hofstadter.py
+++ b/tests/test_model_hofstadter.py
@@ -1,4 +1,4 @@
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 from tenpy.models.hofstadter import HofstadterBosons, HofstadterFermions
 from test_model import check_general_model
 import pytest
diff --git a/tests/test_model_hubbard.py b/tests/test_model_hubbard.py
index af9ed064c..d5de15301 100644
--- a/tests/test_model_hubbard.py
+++ b/tests/test_model_hubbard.py
@@ -1,4 +1,4 @@
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 import pytest
 from tenpy.models import hubbard
 from test_model import check_general_model
diff --git a/tests/test_model_mixed_xk.py b/tests/test_model_mixed_xk.py
index 682b8d405..9a57da441 100644
--- a/tests/test_model_mixed_xk.py
+++ b/tests/test_model_mixed_xk.py
@@ -1,4 +1,4 @@
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 from tenpy.models.mixed_xk import SpinlessMixedXKSquare, HubbardMixedXKSquare
 from test_model import check_general_model
 from tenpy.models.fermions_spinless import FermionModel
diff --git a/tests/test_model_spins.py b/tests/test_model_spins.py
index ab295e322..0c44b29c8 100644
--- a/tests/test_model_spins.py
+++ b/tests/test_model_spins.py
@@ -1,4 +1,4 @@
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 from tenpy.models import spins
 from test_model import check_general_model
 
diff --git a/tests/test_model_spins_nnn.py b/tests/test_model_spins_nnn.py
index e0808855c..37ac822b8 100644
--- a/tests/test_model_spins_nnn.py
+++ b/tests/test_model_spins_nnn.py
@@ -1,4 +1,4 @@
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 from tenpy.models import spins_nnn
 from test_model import check_general_model
 from tenpy.models.model import NearestNeighborModel
diff --git a/tests/test_model_tf_ising.py b/tests/test_model_tf_ising.py
index 0b103680b..914062343 100644
--- a/tests/test_model_tf_ising.py
+++ b/tests/test_model_tf_ising.py
@@ -1,4 +1,4 @@
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 from tenpy.models.tf_ising import TFIModel, TFIChain
 from test_model import check_general_model
diff --git a/tests/test_model_tj_model.py b/tests/test_model_tj_model.py
index 0aa5706b2..1a2d76bfc 100644
--- a/tests/test_model_tj_model.py
+++ b/tests/test_model_tj_model.py
@@ -1,4 +1,4 @@
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 from tenpy.models import tj_model
 from test_model import check_general_model
 
diff --git a/tests/test_model_toric_code.py b/tests/test_model_toric_code.py
index fd4a57810..c53d92365 100644
--- a/tests/test_model_toric_code.py
+++ b/tests/test_model_toric_code.py
@@ -1,4 +1,4 @@
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 from tenpy.models.toric_code import ToricCode
 from test_model import check_general_model
diff --git a/tests/test_model_xxz_chain.py b/tests/test_model_xxz_chain.py
index e3489b263..7c0d4ba62 100644
--- a/tests/test_model_xxz_chain.py
+++ b/tests/test_model_xxz_chain.py
@@ -1,5 +1,5 @@
 """test of :class:`tenpy.models.XXZChain`."""
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import numpy as np
 import numpy.testing as npt
diff --git a/tests/test_mpo.py b/tests/test_mpo.py
index f2be79fa0..16394e055 100644
--- a/tests/test_mpo.py
+++ b/tests/test_mpo.py
@@ -1,5 +1,5 @@
 """A collection of tests for (classes in) :module:`tenpy.networks.mpo`."""
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import numpy as np
 import pytest
diff --git a/tests/test_mps.py b/tests/test_mps.py
index bf7cdb322..6e14b01a7 100644
--- a/tests/test_mps.py
+++ b/tests/test_mps.py
@@ -1,5 +1,5 @@
 """A collection of tests for :module:`tenpy.networks.mps`."""
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import numpy as np
 import numpy.testing as npt
diff --git a/tests/test_network_contractor.py b/tests/test_network_contractor.py
index c2aefb1c0..a0adb540e 100644
--- a/tests/test_network_contractor.py
+++ b/tests/test_network_contractor.py
@@ -1,5 +1,5 @@
 """a collection of tests to check the functionality of network_contractor.py."""
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 from tenpy.algorithms.network_contractor import contract, ncon
 import numpy as np
diff --git a/tests/test_np_conserved.py b/tests/test_np_conserved.py
index 986b735b4..9d3424c71 100644
--- a/tests/test_np_conserved.py
+++ b/tests/test_np_conserved.py
@@ -1,5 +1,5 @@
 """A collection of tests for tenpy.linalg.np_conserved."""
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import tenpy.linalg.np_conserved as npc
 import numpy as np
diff --git a/tests/test_package_structure.py b/tests/test_package_structure.py
index b60968d6e..623689bd5 100644
--- a/tests/test_package_structure.py
+++ b/tests/test_package_structure.py
@@ -1,5 +1,5 @@
 """Check if subpackages define proper __all__."""
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 import types
 
 import tenpy
diff --git a/tests/test_params.py b/tests/test_params.py
index 1cc6ccdfa..f2ceb4406 100644
--- a/tests/test_params.py
+++ b/tests/test_params.py
@@ -1,5 +1,5 @@
 """A test for tenpy.tools.params."""
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 from tenpy.tools.params import Config, asConfig
 import copy
diff --git a/tests/test_post_processing.py b/tests/test_post_processing.py
index bb0d3888c..6570d1a02 100644
--- a/tests/test_post_processing.py
+++ b/tests/test_post_processing.py
@@ -1,5 +1,5 @@
 """Short tests for the :class:`DataLoader` and for post_processing functions in a Simulation"""
-# Copyright 2023 TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 
 import tenpy
diff --git a/tests/test_predict_ram.py b/tests/test_predict_ram.py
index 404bba9b9..3aef9b740 100644
--- a/tests/test_predict_ram.py
+++ b/tests/test_predict_ram.py
@@ -1,5 +1,5 @@
 """Short test for vmem prediction."""
-# Copyright 2023 TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 
 import tenpy.algorithms as algo
diff --git a/tests/test_purification.py b/tests/test_purification.py
index f108c3802..b2e0846c7 100644
--- a/tests/test_purification.py
+++ b/tests/test_purification.py
@@ -1,5 +1,5 @@
 """A collection of tests for :module:`tenpy.networks.purification_mps`."""
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import numpy as np
 import numpy.testing as npt
diff --git a/tests/test_random_matrix.py b/tests/test_random_matrix.py
index b85c9654f..e4f2e2b49 100644
--- a/tests/test_random_matrix.py
+++ b/tests/test_random_matrix.py
@@ -1,5 +1,5 @@
 """A collection of tests for :module:`tenpy.linalg.random_matrix`."""
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import numpy as np
 import numpy.testing as npt
diff --git a/tests/test_simulation.py b/tests/test_simulation.py
index f4ec98aad..056720970 100644
--- a/tests/test_simulation.py
+++ b/tests/test_simulation.py
@@ -1,5 +1,5 @@
 """A collection of tests to check the functionality of modules in `tenpy.simulations`"""
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import copy
 import numpy as np
diff --git a/tests/test_site.py b/tests/test_site.py
index 3b0cbfcf9..244fa4857 100644
--- a/tests/test_site.py
+++ b/tests/test_site.py
@@ -1,5 +1,5 @@
 """A collection of tests for :mod:`tenpy.models.site`."""
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import numpy as np
 import numpy.testing as npt
diff --git a/tests/test_sparse.py b/tests/test_sparse.py
index fe273d33a..c21feba86 100644
--- a/tests/test_sparse.py
+++ b/tests/test_sparse.py
@@ -1,4 +1,4 @@
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 import tenpy.linalg.np_conserved as npc
 import numpy as np
 
diff --git a/tests/test_svd_robust.py b/tests/test_svd_robust.py
index 52278c314..903d06a11 100644
--- a/tests/test_svd_robust.py
+++ b/tests/test_svd_robust.py
@@ -1,5 +1,5 @@
 """A collection of tests for tenpy.linalg.svd_robust."""
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import numpy as np
 import numpy.testing as npt
diff --git a/tests/test_tdvp.py b/tests/test_tdvp.py
index 950e55388..ecd2bd6f3 100644
--- a/tests/test_tdvp.py
+++ b/tests/test_tdvp.py
@@ -1,5 +1,5 @@
 #!/usr/bin/python2
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 import numpy as np
 import pytest
 from tenpy.models.spins import SpinChain
diff --git a/tests/test_tebd.py b/tests/test_tebd.py
index 6b55ddcac..377afb7b2 100644
--- a/tests/test_tebd.py
+++ b/tests/test_tebd.py
@@ -1,5 +1,5 @@
 """A collection of tests to check the functionality of `tenpy.tebd`"""
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import numpy.testing as npt
 import tenpy.linalg.np_conserved as npc
diff --git a/tests/test_terms.py b/tests/test_terms.py
index ad7670db5..5655be353 100644
--- a/tests/test_terms.py
+++ b/tests/test_terms.py
@@ -1,5 +1,5 @@
 """A collection of tests for :module:`tenpy.networks.terms`."""
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import numpy as np
 import copy
diff --git a/tests/test_time_evolution.py b/tests/test_time_evolution.py
index 1620d0827..c2d885b99 100644
--- a/tests/test_time_evolution.py
+++ b/tests/test_time_evolution.py
@@ -2,7 +2,7 @@
 
 Setup is a time evolution with TFI Model starting from a spin polarized state.
 """
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import numpy as np
 import scipy.linalg as LA
diff --git a/tests/test_tools.py b/tests/test_tools.py
index af892ccbf..7da96f022 100644
--- a/tests/test_tools.py
+++ b/tests/test_tools.py
@@ -1,5 +1,5 @@
 """A collection of tests for tenpy.tools submodules."""
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import numpy as np
 import numpy.testing as npt
diff --git a/tests/test_truncation.py b/tests/test_truncation.py
index 0481736ff..1050b5dde 100644
--- a/tests/test_truncation.py
+++ b/tests/test_truncation.py
@@ -1,5 +1,5 @@
 """A test for tenpy.algorithms.truncation."""
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import numpy as np
 import pytest
diff --git a/tests/test_umps.py b/tests/test_umps.py
index c5c7573ac..84e6a0edf 100644
--- a/tests/test_umps.py
+++ b/tests/test_umps.py
@@ -1,5 +1,5 @@
 """A collection of tests for :module:`tenpy.networks.uniform_mps`."""
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 import numpy as np
 from tenpy.networks import mps, site, uniform_mps
diff --git a/tests/test_vumps.py b/tests/test_vumps.py
index 481bec74f..557936b5d 100644
--- a/tests/test_vumps.py
+++ b/tests/test_vumps.py
@@ -1,5 +1,5 @@
 """A collection of tests to check the functionality of `tenpy.vumps`"""
-# Copyright (C) TeNPy Developers, GNU GPLv3
+# Copyright (C) TeNPy Developers, Apache license
 
 from tenpy.models.tf_ising import TFIChain
 from tenpy.algorithms import vumps