From dimers to the solid-state: Distributed intermolecular force-fields for pyridine.

Alexander A Aina,Sarah L Price,Alston J Misquitta

doi:10.1063/1.4999789

Abstract

An anisotropic atom-atom force-field for pyridine, using distributed atomic multipoles, polarizabilities, and dispersion coefficients and an anisotropic atom-atom repulsion model derived from symmetry-adapted perturbation theory (density functional theory) dimer calculations, is used to model pyridine crystal structures. Here we show that this distributed intermolecular force-field (DIFF) models the experimental crystal structures as accurately as modelling all but the electrostatic term with an isotropic repulsion-dispersion potential that has been fitted to experimental crystal structures. In both cases, the differences are comparable to the changes in the crystal structure with temperature, pressure, or neglect of zero-point vibrational effects. A crystal structure prediction study has been carried out, and the observed polymorphs contrasted with hypothetical thermodynamically competitive crystal structures. The DIFF model was able to identify the structure of an unreported high pressure phase of pyridine, unlike the empirically fitted potential. The DIFF model approach therefore provides a model of the underlying pair potential energy surface that we have transferred to the crystalline phase with a considerable degree of success, though the treatment of the many-body terms needs improvement and the pair potential is slightly over-binding. Furthermore, this study of a system that exhibits isotopic polymorphism highlights that the use of an empirical potential has partially absorbed temperature and zero-point motion effects as well as the intermolecular forces not explicitly represented in the functional form. This study therefore highlights the complexity in modelling crystallization phenomena from a realistic pair potential energy surface.

Highlights

The true intermolecular potential energy surface of a molecule should be transferable between all phases, allowing simulation of all the physical properties that depend on the forces between the molecules
The distributed intermolecular force-field (DIFF) model approach provides a model of the underlying pair potential energy surface that we have transferred to the crystalline phase with a considerable degree of success, though the treatment of the many-body terms needs improvement and the pair potential is slightly over-binding
Water and other small polyatomic molecules have very accurate potentials available, though these are often not used in Molecular Dynamics (MD) simulations because of the need to compromise between accuracy, speed of evaluation, and functional forms assumed in the suitable codes

Summary

Introduction

The true intermolecular potential energy surface of a molecule should be transferable between all phases, allowing simulation of all the physical properties that depend on the forces between the molecules. The holy grail of defining a sufficiently accurate analytical pair potential to account for all the physical properties of the inert gases was achieved in the 1980’s.3. These potentials only included the three-body dispersion term for the condensed phases. There is a need for force-fields that correctly model the thermodynamics and properties of the observed polymorphs and other hypothetical crystal structures, the liquid, and the gaseous phase if we are to be able to study the phase change behavior of organic molecules

Methods

Results

Discussion

Conclusion