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density functional theory, lattice energy, many-body problems, phonons, specific heat, van der Waals forces, vibrational modes
Abstract:
The development and application of computational methods for studying molecular
crystals, particularly density-functional theory (DFT), is a large and ever-growing
field, driven by their numerous applications. Here we expand on our recent study
of the importance of many-body van der Waals interactions in molecular crystals
[Reilly and Tkatchenko, J. Phys. Chem. Lett., 4, 1028–1033, (2013)], with a larger
database of 23 molecular crystals. Particular attention has been paid to the role of
the vibrational contributions that are required to compare experiment sublimation
enthalpies with calculated lattice energies, employing both phonon calculations and
experimental heat-capacity data to provide harmonic and anharmonic estimates of
the vibrational contributions. Exact exchange, which is rarely considered in DFT
studies of molecular crystals, is shown to have a significant contribution to lattice
energies, systematically improving agreement between theory and experiment. When
the vibrational and exact-exchange contributions are coupled with a many-body
approach to dispersion, DFT yields a mean absolute error (3.92 kJ/mol) within the
coveted “chemical accuracy” target (4.2 kJ/mol). The role of many-body dispersion
for structures has also been investigated for a subset of the database, showing good
performance compared to X-ray and neutron diffraction crystal structures. The
results show that the approach employed here can reach the demanding accuracy
of crystal-structure prediction and organic material design with minimal empiricism.
