A new ab initio pair potential for water was generated by fitting 2510 interaction energies computed by the use of symmetry-adapted perturbation theory ͑SAPT͒. 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 ͑1997͔͒. SAPT-5s has been shown to quantitatively predict the water dimer spectra, see the following paper ͑paper II͒. 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 5.50Ϯ0.01 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 estimatetogether with the dissociation energy D 0 computed from SAPT-5s-leads to a new prediction of the limit value of D 0 equal to 1165Ϯ54 cm Ϫ1 , close to but significantly more accurate than the best empirical value.

A range of basis sets differing in the location of basis functions has been explored from the point of view of the effectiveness of calculating the electrostatic, induction, dispersion, and exchange components of intermolecular interaction energies. Possible location strategies range from monomer-centered basis sets, through the dimer-centered ones, to sets with functions centered at the intermolecular bond. It is shown that the most effective approach is to use the so-called ‘‘monomer plus’’ basis sets containing, in addition to monomer-centered functions and bond functions, a small number of functions centered on the interacting partner. Using such basis sets for He2 and (H2O)2 the best values to date have been obtained for several interaction energy components. The conclusions from this work are relevant also for supermolecular calculations of interaction energies.

Nearly exact six-dimensional quantum calculations of the vibration-rotation-tunneling ͑VRT͒ levels of the water dimer for values of the rotational quantum numbers J and K р2 show that the SAPT-5s water pair potential presented in the preceding paper ͑paper I͒ gives a good representation of the experimental high-resolution far-infrared spectrum of the water dimer. After analyzing the sensitivity of the transition frequencies with respect to the linear parameters in the potential we could further improve this potential by using only one of the experimentally determined tunneling splittings of the ground state in (H 2 O) 2 . The accuracy of the resulting water pair potential, SAPT-5st, is established by comparison with the spectroscopic data of both (H 2 O) 2 and (D 2 O) 2 : ground and excited state tunneling splittings and rotational constants, as well as the frequencies of the intermolecular vibrations.