Two exchange-correlation functionals compared for first-principles liquid water

TL;DR

This study compares two exchange-correlation functionals, BLYP and RPBE, in first-principles molecular dynamics simulations of liquid water, showing RPBE's improved structural and dynamical agreement with experiments.

Contribution

It provides a direct comparison of BLYP and RPBE functionals for liquid water, highlighting RPBE's closer match to experimental structure and dynamics.

Findings

RPBE yields less structured water with radial distribution functions closer to experiments.

Diffusivity with RPBE is 20% lower than experimental, but significantly better than BLYP.

Effective temperature deviations suggest weaker hydrogen bonds with RPBE.

Abstract

The first-principles description of liquid water using ab initio molecular dynamics (AIMD) based on Density Functional theory (DFT) has recently been found to require long equilibration times, giving too low diffusivities and a clear over-structuring of the liquid. In the light of these findings we compare here the room-temperature description offered by two different exchange correlation functionals: BLYP, the most popular for liquid water so far, and RPBE, a revision of the widely used PBE. We find for RPBE a less structured liquid with radial distribution functions closer to the experimental ones than the ones of BLYP. The diffusivity obtained with RPBE for heavy water is still 20% lower than the corresponding experimental value, but it represents a substantial improvement on the BLYP value, one order of magnitude lower than experiment. These characteristics and the hydrogen-bond (HB) network imperfection point to an effective temperature ~3% lower than the actual simulation temperature for the RPBE liquid, as compared with BLYP's ~17% deviation. The too long O--O average nearest-neighbor distance observed points to an excessively weak HB, possibly compensating more fundamental errors in the DFT description.