Molecular crystals of energetic materials
(EMs) are denser than
typical molecular crystals and are characterized by distinct intermolecular
interactions between nitrogen-containing moieties. To assess the performance
of dispersion-inclusive density functional theory (DFT) methods, we
have compiled a data set of experimental sublimation enthalpies of
31 energetic materials. We evaluate the performance of three methods:
the semilocal Perdew–Burke–Ernzerhof (PBE) functional
coupled with the pairwise Tkatchenko-Scheffler (TS) dispersion correction,
PBE with the many-body dispersion (MBD) method, and the PBE-based
hybrid functional (PBE0) with MBD. Zero-point energy contributions
and thermal effects are described using the quasi-harmonic approximation
(QHA), including explicit treatment of thermal expansion, which we
find to be non-negligible for EMs. The lattice energies obtained with
PBE0+MBD are the closest to experimental sublimation enthalpies with
a mean absolute error of 9.89 kJ/mol. However, the state-of-the-art
treatment of vibrational and thermal contributions makes the agreement
with experiment worse. Pressure–volume curves are also examined
for six representative materials. For pressure–volume curves,
all three methods provide reasonable agreement with experimental data
with mean absolute relative errors of 3% or less. Most of the intermolecular
interactions typical of EMs, namely nitro-amine, nitro–nitro,
and nitro-hydrogen interactions, are more sensitive to the choice
of the dispersion method than to the choice of the exchange-correlation
functional. The exception is π–π stacking interactions,
which are also very sensitive to the choice of the functional. Overall,
we find that PBE+TS, PBE+MBD, and PBE0+MBD do not perform as well
for energetic materials as previously reported for other classes of
molecular crystals. This highlights the importance of testing dispersion-inclusive
DFT methods for diverse classes of materials and the need for further
method development.