High torsional vibrational energies of H2O2 and CH3OH studied by MULTIMODE with a large amplitude motion coupled to two effective contraction schemes

We have introduced the theory of the Reaction Path Hamiltonian into the variational scheme MULTIMODE, for the calculation of vibrational energy levels of polyatomic molecules which have a single large amplitude motion (for which the classic example is hydrogen peroxide). As with all MULTIMODE calculations, the greatest difficulty is the size of the CI matrix. The algorithm is now enhanced to include user-defined contraction schemes in order to probe high-energy regions of the potential energy surface. Furthermore an increased efficiency in matrix element evaluation is reported. High torsional levels of hydrogen peroxide and methanol are reported; those for hydrogen peroxide are consistent with an ‘exact’ variational procedure in valence coordinates, whilst those for methanol are predictions based on a recently-derived potential energy surface. The coupling between the torsional mode and the remaining normal modes is highlighted. These ‘technical’ advances open up the use of MULTIMODE for the study of vibr...