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 lev els 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 di pole moment function is taken as a series expansion in the bond coordi nates, with expansion coefficients found from the ab initio results. T hese different dipole moment functions are used to calculate fundament al and overtone vibrational band intensities of H2O. A simple three-di mensional harmonically coupled anharmonic oscillator local mode descri ption 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 a re more sensitive to electron correlation than overtone intensities. T his provides an insight into the quality of dipole moment function nec essary to calculate reasonable values for fundamental and overtone int ensities, an insight that is important for studies with larger molecul es where the highest levels of theory are not computationally feasible .