Using a basis of products of contracted intra-molecular and contracted inter-molecular functions to compute the rovibrational spectrum of H2O–HF

In this paper, we report the J = 0-2 energy levels of H2O-HF calculated using our previous 9-D PES and compare the results with those of the experiment. The Schrodinger equation is solved variationally using a product contracted (PC) basis. The contracted basis functions are computed with the symmetry-adapted Lanczos method, and full-dimensional states are obtained from the PC basis by direct diagonalization. We compare the results with those of a previous calculation in which the monomers were constrained to be rigid. Allowing the monomers to be flexible has a significant effect on the low-lying inter-molecular vibrational levels due to coupling with intra-molecular coordinates. The R(1) transition frequencies, computed with flexible monomers, in different inter-molecular vibrational states, agree better with experiment than their rigid monomer counterparts. We calculate, for the first time, monomer excited states, vibrational shifts, and tunneling splittings. Although H2O-HCl and H2O-HF are similar, the number of contracted basis functions required for accurate predictions of intra- and inter-molecular levels is much larger for the H2O-HF dimer. This is the consequence of strong hydrogen bonding and important resonances between vibrational modes.