Exchange-Hole Dipole Dispersion Model for Accurate Energy Ranking in Molecular Crystal Structure Prediction

Accurate energy ranking is a key
facet to the problem of first-principles
crystal-structure prediction (CSP) of molecular crystals. This work
presents a systematic assessment of B86bPBE-XDM, a semilocal density
functional combined with the exchange-hole dipole moment (XDM) dispersion
model, for energy ranking using 14 compounds from the first five CSP
blind tests. Specifically, the set of crystals studied comprises 11
rigid, planar compounds and 3 co-crystals. The experimental structure
was correctly identified as the lowest in lattice energy for 12 of
the 14 total crystals. One of the exceptions is 4-hydroxythiophene-2-carbonitrile,
for which the experimental structure was correctly identified once
a quasi-harmonic estimate of the vibrational free-energy contribution
was included, evidencing the occasional importance of thermal corrections
for accurate energy ranking. The other exception is an organic salt,
where charge-transfer error (also called delocalization error) is
expected to cause the base density functional to be unreliable. Provided
the choice of base density functional is appropriate and an estimate
of temperature effects is used, XDM-corrected density-functional theory
is highly reliable for the energetic ranking of competing crystal
structures.