Crystal structures of four new coumarin polymorphs were solved by crystal structure prediction method and their lattice and free energies were calculated by advanced techniques.
Predicting
structures of organic molecular cocrystals is a challenging
task when considering the immense number of possible intermolecular
orientations. Use of the Shannon information entropy, constructed
from an intermolecular orientational spatial distribution function,
to drive a search for crystal structures via enhanced molecular dynamics
can be an efficient way to map out a landscape of putative polymorphs.
Here, the Shannon entropy is used to generate a set of collective
variables for differentiating polymorphs of a 1:1 cocrystal of resorcinol
and urea. We show that driven adiabatic free energy dynamics, a particular
enhanced-sampling approach, combined with these entropy variables,
can transform the stable phase into alternate polymorphs. Density
functional theory calculations confirm that a structure obtained from
the enhanced molecular dynamics is stable at pressures above 1 GPa.
We thus show that enhanced sampling should be considered an integral
component of crystal structure searching protocols for systems with
multiple independent molecules.
Predictions of crystal structures from first-principles electronic structure calculations and molecular simulations have been performed for an energetic molecule, 4-amino-2,3,6trinitrophenol. 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 densityfunctional 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.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.