Toward a Reliable Description of the Lattice Vibrations in Organic Molecular Crystals: The Impact of van der Waals Interactions

This
work assesses
the reliability of different van der Waals (vdW)
methods to describe lattice vibrations of molecular crystals in the
framework of density functional theory (DFT). To accomplish this task,
calculated and experimental lattice phonon Raman spectra of a pool
of organic molecular crystals are compared. We show that the many-body
dispersion (MBD@rsSCS) van der Waals method of Ambrosetti et al. and
the pairwise method of Grimme et al. (D3-BJ) outperform the other
tested approaches (i.e., the D2 method of Grimme, the TS method of
Tkatchenko and Scheffler, and the nonlocal functional vdW-DF-optPBE
of Klimeš et al.). For the worse-performing approaches the
results could not even be fixed by the introduction of scaling parameters,
as commonly used for high-energy intramolecular vibrations. Interestingly,
when using the experimentally determined unit cell parameters, DFT
calculations using the PBE functional without corrections for long-range
vdW interactions provide spectra of similar accuracy as the MBD@rsSCS
and D3-BJ simulations.