Water hexamer: Self-consistent phonons versus reversible scaling versus replica exchange molecular dynamics

TL;DR

This paper compares different computational methods for calculating free energies of water hexamer isomers, highlighting the strengths and limitations of each approach and questioning the convergence of replica exchange molecular dynamics results.

Contribution

It provides a detailed comparison of self-consistent phonons, reversible scaling, and replica exchange methods for water clusters, including new insights into their accuracy and convergence issues.

Findings

SCP and RS methods agree over a broad temperature range.

Discrepancies with REMD results raise questions about convergence.

Rotational corrections improve accuracy for water clusters.

Abstract

Classical free energies for the cage and prism isomers of water hexamer computed by the self- consistent phonons (SCP) method and reversible scaling (RS) method are presented for several flexible water potentials. Both methods have been augmented with a rotational correction for improved accuracy when working with clusters. Comparison of the SCP results with the RS results suggests a fairly broad temperature range over which the SCP approximation can be expected to give accurate results for systems of water clusters, and complements a previously reported assessment of SCP. Discrepancies between the SCP and RS results presented here, and recently published replica exchange molecular dynamics (REMD) results bring into question the convergence of the REMD and accompanying replica exchange path integral molecular dynamics results. In addition to the ever-present specter of unconverged results, several possible sources for the discrepancy are explored based on inherent characteristics of the methods used.