Coupled cluster benchmarks of water monomers and dimers extractedfrom density-functional theory liquid water: The importance of monomerdeformations

摘要

To understand the performance of popular density-functional theory exchange-correlation (xc)functionals in simulations of liquid water, water monomers and dimers were extracted from a PBEsimulation of liquid water and examined with coupled cluster with single and double excitationsplus a perturbative correction for connected triples [CCSD(T)]. CCSD(T) reveals that most of thedimers are unbound compared to two gas phase equilibrium water monomers, largely becausemonomers within the liquid have distorted geometries. Of the three xc functionals tested, PBE andBLYP tend to predict too large dissociation energies between monomers within the dimers. We showthat this is because the cost to distort the monomers to the geometries they adopt in the liquid issystematically underestimated with these functionals. PBEO reproduces the CCSD(T) monomerdeformation energies very well and consequently the dimer dissociation energies much moreaccurately than PBE and BLYP. Although this study is limited to water monomers and dimers, theresults reported here may provide an explanation for the overstructured radial distribution functionsroutinely observed in BLYP and PBE simulations of liquid water and are of relevance to water inother phases and to other associated molecular liquids.