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Ab initio calculation of vibrational dipole moment matrix elements. II. the water molecule as a polyatomic test case

This paper is the second part of a series devoted to the ab initio calculation of vibrational properties, like transition energies, dipole moment matrix elements and infrared intensities. The method of calculation has been detailed in the first paper, where test calculations on diatomic systems were also analyzed. The present paper applies the method to a simple polyatomic system, the water molecule in its ground electronic state. The purely polyatomic features of the method are discussed, as the determination of the potential energy and dipole moment surfaces and the description of vibrational mode couplings. The results concern vibrational energies of levels in the range 0 ⩽ v1 + v2 + v3 ⩽ 3 and vibrational transition moments and intensities of cold and hot bands. Many methodological tests and comparisons to the experimental and theoretical literature are reported, showing that theoretical values of transition energies and dipole moments converge to the experimental ones; at the highest level of approximation (TZ + 2P/SD-CI; vibrational SCF/CI and FCI) the precision obtained is sufficient to provide useful complementary spectroscopical data. This convergence is however not achieved for diagonal dipole moment matrix elements, which are shown to be very sensitive to the mechanical and electrical anharmonicities.