Water-water pair interactions and the second virial coefficient of steam

Recent ab initio calculations of the pair interaction between water molecules are claimed to be close to the Hartree-Fock limit. Second virial coefficient calculations for such a Hartree-Fock potential, together with a number of correlation energy corrections, are compared with results obtained from experiment. On the basis of this comparison it is shown that correlation energy terms are needed over the whole range of the interaction and it is not sufficient to directly add dispersion energy terms alone.