Towards an assessment of the accuracy of density functional theory for first principles simulations of water II

TL;DR

This study compares Car-Parrinello and Born-Oppenheimer molecular dynamics for simulating water, revealing their agreement at 300K but discrepancies at higher temperatures, and highlights the significant role of proton quantum effects.

Contribution

It provides a detailed comparison of CP and BO methods for water simulations and investigates proton quantum effects on structure and diffusion.

Findings

Excellent agreement between CP and BO at 300K with proper parameters

CP diffusion coefficients are higher than BO at T>400K

Proton quantum effects significantly influence water's structure and dynamics

Abstract

A series of 20 ps ab initio molecular dynamic simulations of water at ambient density and temperatures ranging from 300 to 450K are presented. Both Car-Parrinello (CP) and Born-Oppenheimer (BO) molecular dynamics techniques are compared for systems containing 54 and 64 water molecules. At 300K, excellent agreement is found between radial distribution functions (RDFs) obtained with BO and CP dynamics, provided an appropriately small value of the fictitious mass parameter is used in the CP simulation. However, we find that the diffusion coefficients computed from CP dynamics are approximately two times larger than the corresponding BO simulations for T>400K, where statistically meaningful comparisons can be made. Overall, both BO and CP dynamics at 300 K yield overstructured RDFs and slow diffusion as compared to experiment. In order to understand these discrepancies, the effect of proton quantum motion is considered with the use of empirical interaction potentials. We find strong evidence that proton quantum effects may have a larger impact than previously thought on structure and diffusion of the liquid.