Derivation of force fields for molecular mechanics and dynamics from <i>ab initio</i> energy surfaces

TLDR

The study introduces a method to derive and compare optimal functional forms and rapidly determine force constants for potential energy functions used in simulating organic, polymeric, and biopolymeric systems and modeling vibrational spectra. The method employs ab initio molecular energy surfaces, including energies and their first and second derivatives, to fit various analytical potential forms to data, exemplified on the formate anion. The results show that anharmonicity and cross terms are essential for accurately capturing structure, energy, and dynamics, yielding a more precise model of out‑of‑plane deformation at a trigonal center.

Abstract

We present a technique for addressing the problem of deriving potential energy functions for the simulation of organic, polymeric, and biopolymeric systems, as well as for modeling vibrational spectroscopic properties. This method is designed to address three major objectives: deriving and comparing optimal functional forms for describing the energies of molecular deformations and interactions, developing a technique to rapidly and objectively determine reasonable force constants for intermolecular and intramolecular interactions, and determining the transferability of these potential forms and constants. The first two of these objectives are addressed in this paper, while the latter problem will be treated elsewhere. The technique uses ab initio molecular energy surfaces, which are described by the energy and its first and second derivatives with respect to coordinates. As an example, application to a small model compound (i.e., the formate anion) is given. A variety of analytical forms for the potential are tested against the data, to find which forms are best. The importance of anharmonicity and cross terms in accounting for structure and energy, as well as for dynamics, is demonstrated and a more accurate representation of the out-of-plane deformation for a trigonal center is derived from the energy surfaces.