CALCULATION OF VIBRATIONAL FUNDAMENTAL AND OVERTONE BAND INTENSITIES OF H2O

Vibrational intensities are calculated for the fundamental and overton e transitions of H2O up to approximately 18000 cm(-1). The intensities are determined from a dipole moment function expanded in the three in ternal bond coordinates. The expansion coefficients are computed ab in itio at the second-order Moller-Plesset level of theory with a 6-311G basis set. Vibrational wave functions are calculated either from a t hree-dimensional harmonically coupled anharmonic oscillator (HCAO) mod el which uses Morse oscillators to represent both the stretches and th e bend of H2O, or from a variational calculation employing the best av ailable potential energy surface and an exact kinetic energy operator. To obtain the most meaningful vibrational intensities we define dipol e moment components using the Eckart embedding. Both the HCAO and the variational intensities agree quite well with the experimental results , which span eight orders of magnitude. From the calculations we predi ct that it may be possible to detect as yet unobserved vibrational tra nsitions of H2O.