Solid-liquid coexistence of the noble elements. I. Theory illustrated by the case of argon

TL;DR

This paper uses molecular dynamics with a simplified ab initio potential to accurately model the solid-liquid coexistence and phase transitions of noble gases, specifically argon, validating theoretical predictions with experimental data.

Contribution

It introduces and applies the SAAP potential to predict phase boundaries and melting properties of argon, demonstrating the validity of isomorph theory for noble gases.

Findings

Accurate prediction of argon’s fcc freezing line.

Validation of hidden scale invariance and isomorphs in noble gases.

Prediction of bcc phase stability at high pressures.

Abstract

The noble elements constitute the simplest group of atoms. At low temperatures or high pressures they freeze into the face-centered cubic (fcc) crystal structure (except helium). We perform molecular dynamics using the recently proposed simplified ab initio atomic (SAAP) potential [Deiters and Sadus, J. Chem. Phys. 150, 134504 (2019)] . This potential is parameterized using data from accurate ab initio quantum mechanical calculations by the coupled-cluster approach on the CCSD(T) level. We compute the fcc freezing lines for Argon and find a great agreement with the experimental values. At low pressures, this agreement is further enhanced by using many-body corrections. Hidden scale invariance of the potential energy function is validated by computing lines of constant excess entropy (configurational adiabats) and shows that mean square displacement and the static structure factor are invariant. These lines (isomorphs) can be generated from simulations at a single state-point by having knowledge of the pair potential. The isomorph theory for the solid-liquid transition is used to accurately predict the shape of the freezing line in the pressure-temperature plane, the shape in the density-temperature plane, the entropy of melting and the Lindemann parameters along the melting line. We finally predict that the body-centered cubic (bcc) crystal is stable at high pressures.