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

Abstract

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.