How close are the classical two-body potentials to ab initio calculations? Insights from linear machine learning based force matching

TL;DR

This paper introduces a linear machine learning force matching method to derive pairwise atomic potentials from ab initio data, revealing that classical silica potentials are overly stiff and may misrepresent thermal properties.

Contribution

The study presents a novel linear ML approach for force matching that directly extracts pair potentials from ab initio calculations, providing a new way to evaluate and improve classical force fields.

Findings

Classical silica potentials are significantly stiffer than ab initio-derived potentials.

The new force field predicts a lower glass transition temperature (~1800 K).

Simulated silica shows positive thermal expansion, contrasting previous negative expansion predictions.

Abstract

In this work, we propose a linear machine learning force matching approach that can directly extract pair atomic interactions from ab initio calculations in amorphous structures. The local feature representation is specifically chosen to make the linear weights a force field as a force/potential function of the atom pair distance. Consequently, this set of functions is the closest representation of the ab initio forces given the two-body approximation and finite scanning in the configurational space. We validate this approach in amorphous silica. Potentials in the new force field (consisting of tabulated Si-Si, Si-O, and O-O potentials) are significantly softer than existing potentials that are commonly used for silica, even though all of them produce the tetrahedral network structure and roughly similar glass properties. This suggests that those commonly used classical force fields do not offer fundamentally accurate representations of the atomic interaction in silica. The new force field furthermore produces a lower glass transition temperature ($T_g\sim$1800 K) and a positive liquid thermal expansion coefficient, suggesting the extraordinarily high $T_g$ and negative liquid thermal expansion of simulated silica could be artifacts of previously developed classical potentials. Overall, the proposed approach provides a fundamental yet intuitive way to evaluate two-body potentials against ab initio calculations, thereby offering an efficient way to guide the development of classical force fields.