Six-dimensional variational calculation of the bending energy levels of HFtrimer and DF trimer

The bending energy levels of HF and DF trimer were calculated variationally using a six-dimensional kinetic-energy operator derived by constraining th e stretch degrees of freedom to their equilibrium values. A basis of direct products of spherical harmonics is used to represent the wave function. Th e symmetry adapted Lanczos method is used to calculate levels of all symmet ries from one sequence of matrix-vector products. Two efficient ways of imp lementing the symmetry-adapted Lanczos method are presented. To determine t he concerted hydrogen bond breaking tunneling splitting in HF and DF trimer one needs to use the D-3h point group. By doing D-3h calculations, we find that the splitting is negligible for the low-lying levels. If the tunnelin g is unfeasible, the appropriate group is C-3h. Our calculation using the m ost accurate SO-3+HF3BG potential developed by Quack, Stohner, and Suhm ind icates that two gas-phase overtone bending bands of HF trimer should be rea ssigned, and that the noble gas matrix spectra of the bending fundamental b ands are significantly shifted from their gas-phase counterparts. (C) 2001 American Institute of Physics.