Coupled cluster benchmarks of water monomers and dimers extracted from DFT liquid water: the importance of monomer deformations

TL;DR

This study evaluates the accuracy of different density-functional theory functionals in modeling liquid water by comparing their predictions for water monomers and dimers against high-level coupled cluster calculations, highlighting the importance of accounting for monomer deformations.

Contribution

It demonstrates that PBE0 accurately captures monomer deformation energies and dimer dissociation energies, explaining discrepancies in liquid water simulations.

Findings

PBE and BLYP underestimate monomer deformation costs.

Most liquid water dimers are unbound compared to gas-phase monomers.

PBE0 provides a more accurate description of water dimers.

Abstract

To understand the performance of popular density-functional theory (DFT) exchange-correlation (xc) functionals in simulations of liquid water, water monomers and dimers were extracted from a PBE simulation of liquid water and examined with coupled cluster with single and double excitations plus a perturbative correction for connected triples [CCSD(T)]. CCSD(T) reveals that most of the dimers are unbound compared to two gas phase equilibrium water monomers, largely because monomers within the liquid have distorted geometries. Of the three xc functionals tested, PBE and BLYP systematically underestimate the cost of the monomer deformations and consequently predict too large dissociation energies between monomers within the dimers. This is in marked contrast to how these functionals perform for an equilibrium water dimer and other small water clusters in the gas phase, which only have moderately deformed monomers. PBE0 reproduces the CCSD(T) monomer deformation energies very well and consequently the dimer dissociation energies much more accurately than PBE and BLYP. Although this study is limited to water monomers and dimers, the results reported here may provide an explanation for the overstructured radial distribution functions routinely observed in BLYP and PBE simulations of liquid water and are of relevance to water in other phases and to other associated molecular liquids.