Water pair potential of near spectroscopic accuracy. I. Analysis of potential surface and virial coefficients

A new ab initio pair potential for water was generated by fitting 2510 inte raction energies computed by the use of symmetry-adapted perturbation theor y (SAPT). The new site-site functional form, named SAPT-5s, is simple enoug h to be applied in molecular simulations of condensed phases and at the sam e time reproduces the computed points with accuracy exceeding that of the e laborate SAPT-pp functional form used earlier [J. Chem. Phys. 107, 4207 (19 97)]. SAPT-5s has been shown to quantitatively predict the water dimer spec tra, see the following paper (paper II). It also gives the second virial co efficient in excellent agreement with experiment. Features of the water dim er potential energy surface have been analyzed using SAPT-5s. Average value s of powers of the intermolecular separation-obtained from the ground-state rovibrational wave function computed in the SAPT-5s potential-have been co mbined with measured values to obtain a new empirical estimate of the equil ibrium O-O separation equal to 5.50 +/-0.01 bohr, significantly shorter tha n the previously accepted value. The residual errors in the SAPT-5s potenti al have been estimated by comparison to recent large-scale extrapolated ab initio calculations for water dimer. This estimate-together with the dissoc iation energy D-0 computed from SAPT-5s-leads to a new prediction of the li mit value of D-0 equal to 1165 +/- 54 cm(-1), close to but significantly mo re accurate than the best empirical value. (C) 2000 American Institute of P hysics. [S0021-9606(00)30140-4].