AB-INITIO CALCULATION OF DIPOLE-MOMENT FUNCTIONS - APPLICATION TO VIBRATIONAL BAND INTENSITIES OF H2O

The dipole moment function of H2O has been calculated at different levels of ab initio theory. The ab initio calculations range from Hartree-Fock to perturbative, limited configuration interaction, and coupled cluster methods, and cover a wide range of standard basis sets. The dipole moment function is taken as a series expansion in the bond coordinates, with expansion coefficients found from the ab initio results. These different dipole moment functions are used to calculate fundamental and overtone vibrational band intensities of H2O. A simple three-dimensional harmonically coupled anharmonic oscillator local mode description is employed to obtain the vibrational energies and wavefunctions. The results indicate that useful vibrational band intensities can be obtained from a limited ab initio approach. Fundamental intensities are more sensitive to electron correlation than overtone intensities. This provides an insight into the quality of dipole moment function necessary to calculate reasonable values for fundamental and overtone intensities, an insight that is important for studies with larger molecules where the highest levels of theory are not computationally feasible.