The quartic force field of H2O determined by many-body methods that include quadruple excitation effects

Many-body perturbation theory (MBPT) and coupled cluster methods are employed in an investigation of the potential energy surface of H2O in the vicinity of its equilibrium geometry. The basis set of 39 Slater-type orbitals is the same as that previously used in a configuration interaction study (limited to all single and double excitations, SD-CI) of this surface, and is capable of accounting for 80% of the total correlation energy of the molecule. Detailed comparisons among the results of the various methods are presented, with particular reference to the role of size extensivity in providing a reliable model for the prediction of the shape of the surface. The predicted quartic force field obtained by the coupled cluster doubles (CCD) and by several MBPT models is in very good agreement with experiment. The inclusion of quadruple excitations, which account for about 5% of the correlation energy, is found to have a significant effect on the shape of the surface, bringing the predicted force field into substantially better agreement with experiment than that obtained with SD-CI.