Direct derivation of anisotropic atomic displacement parameters from molecular dynamics simulations in extended solids with substitutional disorder using a neural network potential

TL;DR

This paper presents a neural network-based method to derive anisotropic atomic displacement parameters directly from molecular dynamics simulations, applicable to extended solids with substitutional disorder at finite temperatures.

Contribution

It introduces a novel approach using machine learning to extract ADPs from MD simulations, enabling analysis of disordered crystals at finite temperatures unlike traditional lattice dynamics methods.

Findings

ADPs decrease to zero as temperature approaches zero.

ADPs are proportional to temperature in harmonic potentials.

Method successfully applied to MgO and thermoelectric materials.

Abstract

Atomic displacement parameters (ADPs) are crystallographic information that describe the statistical distribution of atoms around an atom site. Anisotropic ADPs by atom were directly derived from classical molecular dynamics (MD) simulations using a universal machine-learned potential. The (co)valences of atom positions were taken over recordings at different time steps in a single MD simulation. The procedure was demonstrated on extended solids, namely rocksalt structure MgO and three thermoelectric materials, Ag8SnSe6, Na2In2Sn4, and BaCu1.14In0.86P2. Unlike the very frequently used lattice dynamics approach, the MD approach can obtain ADPs in crystals with substitutional disorder and explicitly at finite temperature, but not under conditions where atoms migrate in the crystal. The calculated ADP becomes ->0 at temperature ->0 and the ADP is proportional to the temperature when the atom is in a harmonic potential and the sole contribution to the actual non-zero ADP is from the zero-point motion. The zero-point motion contribution can be estimated from the proportionality constant assuming this Einstein model. ADPs from MD simulations would act as a tool complementing experimental efforts to understand the crystal structure including the distribution of atoms around atom sites.