Water pair potential of near spectroscopic accuracy. II. Vibration-rotation-tunneling levels of the water dimer

Nearly exact six-dimensional quantum calculations of the vibration-rotation -tunneling (VRT) levels of the water dimer for values of the rotational qua ntum numbers J and K less than or equal to2 show that the SAPT-5s water pai r potential presented in the preceding paper (paper I) gives a good represe ntation of the experimental high-resolution far-infrared spectrum of the wa ter dimer. After analyzing the sensitivity of the transition frequencies wi th respect to the linear parameters in the potential we could further impro ve this potential by using only one of the experimentally determined tunnel ing splittings of the ground state in (H2O)(2). The accuracy of the resulti ng water pair potential, SAPT-5st, is established by comparison with the sp ectroscopic data of both (H2O)(2) and (D2O)(2): ground and excited state tu nneling splittings and rotational constants, as well as the frequencies of the intermolecular vibrations. (C) 2000 American Institute of Physics. [S00 21-9606(00)30240-9].