  The C2DB is a highly curated open database organizing a wealth of computed
properties for more than 4000 atomically thin two-dimensional (2D) materials.
Here we report on new materials and properties that were added to the database
since its first release in 2018. The set of new materials comprise several
hundred monolayers exfoliated from experimentally known layered bulk materials,
(homo)bilayers in various stacking configurations, native point defects in
semiconducting monolayers, and chalcogen/halogen Janus monolayers. The new
properties include exfoliation energies, Bader charges, spontaneous
polarisations, Born charges, infrared polarisabilities, piezoelectric tensors,
band topology invariants, exchange couplings, Raman- and second harmonic
generation spectra. We also describe refinements of the employed material
classification schemes, upgrades of the computational methodologies used for
property evaluations, as well as significant enhancements of the data
documentation and provenance. Finally, we explore the performance of Gaussian
process-based regression for efficient prediction of mechanical and electronic
materials properties. The combination of open access, detailed documentation,
and extremely rich materials property data sets make the C2DB a unique resource
that will advance the science of atomically thin materials.
