1ST PRINCIPLES COMPUTATION OF THERMOCHEMICAL PROPERTIES BEYOND THE HARMONIC APPROXIMATION .1. METHOD AND APPLICATION TO THE WATER MOLECULE AND ITS ISOTOPOMERS

The anharmonic potential energy surface of water has been computed ab initio using an augmented coupled cluster method and various basis sets. Whereas the Pople 6-311 G family is manifestly unsatisfactory, Huzinaga-Dunning basis sets perform quite well. The [5s4p2d 1f,3s2p] surface reproduces harmonic frequencies and anharmonicity constants to better than about 2 and 1 cm-1, respectively. For quantitative agreement with experiment, both f functions on oxygen and inclusion of core correlation seem to be prerequisite. Comparison with various experimentally derived force fields reveals that the ab initio force field is of comparable quality. From the best computed force field, a set of spectroscopic constants has been derived for all important isotopomers of water. Using a hybrid analytic/direct summation method recently developed by the present authors, the thermodynamic functions gef(T), hcf(T), S0, and C(p) are computed including exact account of anharmonicity and rovibrational coupling, and very good analytical approximations to centrifugal distortion and quantum rotation effects. The computed functions substantially revise previous literature results at high temperatures. Differences between thermodynamic functions from various computed force fields are an order of magnitude smaller than these errors. Thermodynamic tables in JANAF style from 100 to 3000 K, as well as a full set of rovibrational spectroscopic constants, are presented. It is concluded that obtaining force fields of near-spectroscopic accuracy, and thermodynamic tables of very high accuracy, is presently feasible for small polyatomic molecules.