Exchange-Hole Dipole Dispersion Model for Accurate Energy Ranking in Molecular Crystal Structure Prediction II: Nonplanar Molecules

The crystal structure prediction
(CSP) of a given compound from
its molecular diagram is a fundamental challenge in computational
chemistry with implications in relevant technological fields. A key
component of CSP is the method to calculate the lattice energy of
a crystal, which allows the ranking of candidate structures. This
work is the second part of our investigation to assess the potential
of the exchange-hole dipole moment (XDM) dispersion model for crystal
structure prediction. In this article, we study the relatively large,
nonplanar, mostly flexible molecules in the first five blind tests
held by the Cambridge Crystallographic Data Centre. Four of the seven
experimental structures are predicted as the energy minimum, and thermal
effects are demonstrated to have a large impact on the ranking of
at least another compound. As in the first part of this series, delocalization
error affects the results for a single crystal (compound X), in this
case by detrimentally overstabilizing the π-conjugated conformation
of the monomer. Overall, B86bPBE-XDM correctly predicts 16 of the
21 compounds in the five blind tests, a result similar to the one
obtained using the best CSP method available to date (dispersion-corrected
PW91 by Neumann et al.). Perhaps more importantly, the systems for
which B86bPBE-XDM fails to predict the experimental structure as the
energy minimum are mostly the same as with Neumann’s method,
which suggests that similar difficulties (absence of vibrational free
energy corrections, delocalization error,...) are not limited to B86bPBE-XDM
but affect GGA-based DFT-methods in general. Our work confirms B86bPBE-XDM
as an excellent option for crystal energy ranking in CSP and offers
a guide to identify crystals (organic salts, conjugated flexible systems)
where difficulties may appear.