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 interaction energies computed by the use of symmetry-adapted perturbation theory ~SAPT!. The new site–site functional form, named SAPT-5s, is simple enough to be applied in molecular simulations of condensed phases and at the same time reproduces the computed points with accuracy exceeding that of the elaborate SAPT-pp functional form used earlier @J. Chem. Phys. 107, 4207 ~1997!#. SAPT-5s has been shown to quantitatively predict the water dimer spectra, see the following paper ~paper II!. It also gives the second virial coefficient in excellent agreement with experiment. Features of the water dimer potential energy surface have been analyzed using SAPT-5s. Average values of powers of the intermolecular separation—obtained from the ground-state rovibrational wave function computed in the SAPT-5s potential—have been combined with measured values to obtain a new empirical estimate of the equilibrium O–O separation equal to 5.5060.01 bohr, significantly shorter than the previously accepted value. The residual errors in the SAPT-5s potential have been estimated by comparison to recent large-scale extrapolated ab initio calculations for water dimer. This estimate— together with the dissociation energy D0 computed from SAPT-5s—leads to a new prediction of the limit value of D0 equal to 1165654 cm , close to but significantly more accurate than the best empirical value. © 2000 American Institute of Physics. @S0021-9606~00!30140-4#