  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.
