Calculation of sum rule properties for H2O

Abstract

Accurate Hartree–Fock and valence-bond calculations on the sum rule moments Sk(r) of the electric dipole photoabsorption cross section, with k=−1, 1, and 2, are presented for the water molecule. Our values averaged over all molecular orientations are in good agreement with experiment, the discrepancies being on the order of 10% for S−1, 5% for S1, and within experimental error for S2. The errors in S−1 and S1 are due to the effect of electron correlation on the two-electron terms in the sum rule formulas. The sum rule moments are used in conjunction with moment theory to calculate parameters characterizing a variety of optical properties, long-range intermolecular attractions, and the energy loss of fast charged particles passing through the vapor. Although the results are generally not as good as those obtained for most of the noble gases, they are semiquantitative. The agreement with experiment is, typically, within 10%. Of particular interest are the pure rotational Rayleigh plus Raman scattering and the orientation dependence of the van der Waals dispersion energy, since these depend upon the anisotropy of the moments which is (otherwise) experimentally inaccessible.