  When calculating the optical absorption spectra of molecular crystals from
first principles, the influence of the crystalline environment on the
excitations is of significant importance. For such systems, however, methods to
describe the excitations accurately can be computationally prohibitive due to
the relatively large system sizes involved. In this work, we demonstrate a
method that allows optical absorption spectra to be computed both efficiently
and at high accuracy. Our approach is based on the spectral warping method
successfully applied to molecules in solvent. It involves calculating the
absorption spectrum of a supercell of the full molecular crystal using
semi-local TDDFT, before warping the spectrum using a transformation derived
from smaller-scale semi-local and hybrid TDDFT calculations on isolated dimers.
We demonstrate the power of this method on three polymorphs of the well known
colour polymorphic compound ROY, and find that it outperforms both small-scale
hybrid TDDFT dimer calculations and large-scale semi-local TDDFT supercell
calculations, when compared to experiment.
