The vibrations of H2O2, studied by "multimode," with a large amplitude motion

Recently Carter and Bowman [J. Chem. Phys. 108, 4397 (1998)] have introduced a variational scheme ("multimode") for the calculation of rovibrational energy levels of polyatomic molecules using normal coordinates with the Watson Hamiltonian [Mol. Phys. 15, 479 (1968)]. The key to their algorithm is that at most four-mode coupling is allowed in matrix element evaluation. However nearly all larger molecules have one or more "large amplitude" motions, which are not treatable using normal coordinates. Here we extend multimode to include one large amplitude motion, using the theory of the reaction path Hamiltonian [J. Chem. Phys. 72, 99 (1980)], which is an almost identical problem. Essentially exact variational calculations are possible, and the approach is applied to the vibrations and tunneling motion of hydrogen peroxide. (C) 2000 American Institute of Physics. [S0021-9606(00)30327-0].