Crystal Structure Predictions for 4‑Amino-2,3,6-trinitrophenol Using a Tailor-Made First-Principles-Based Force Field

Predictions
of crystal structures from first-principles electronic
structure calculations and molecular simulations have been performed
for an energetic molecule, 4-amino-2,3,6-trinitrophenol. This physics-based
approach consists of a series of steps. First, a tailor-made two-body
potential energy surface (PES) was constructed with recently developed
software, autoPES, using symmetry-adapted perturbation theory based
on a density-functional theory description of monomers [SAPT­(DFT)].
The fitting procedure ensures asymptotic correctness of the PES by
employing a rigorous asymptotic multipole expansion, which seamlessly
integrates with SAPT­(DFT) interaction energies. Next, crystal structure
prediction (CSP) was performed by generating possible crystal structures
with rigid molecules, minimizing these structures using the SAPT­(DFT)
force field, and running isothermal–isobaric molecular dynamics
(MD) simulations with flexible molecules based on the tailor-made
SAPT­(DFT) intermolecular force field and a generic/SAPT­(DFT) intramolecular
one. This workflow led to the experimentally observed structure being
identified as one of the forms with the lowest lattice energy, demonstrating
the success of a first-principles, bottom-up approach to CSP. Importantly,
we argue that the accuracy of the intermolecular potential, here the
SAPT­(DFT)-based potential, is determinative of the crystal structure,
while generic/SAPT­(DFT) force fields can be used to represent the
intramolecular potential. This force field approach simplifies the
CSP workflow, without significantly compromising the accuracy of the
prediction.