APPROXIMATE FACTORIZATION OF MOLECULAR-POTENTIAL SURFACES .1. BASIC APPROACH

The intermediate vibrational energy regime-well above the zero point, yet below the first dissociation limit-plays an important role in many molecular processes, such as radiationless transitions or intramolecu lar vibrational energy redistribution (IVR). For molecules with N>6 vi brational degrees of freedom, the calculation of energy levels and spe ctra of the full anharmonic vibrational Hamiltonian at high energies p resents a formidable problem. Here we derive an asymptotic factorizati on of the potential energy surface (PES) based on the global topograph y of molecular potentials. The resulting approximate models require on ly a few input parameters such as vibrational frequencies and cubic an harmonicities. They are compared to benchmark PES derived from curvili near model potentials and Monte Carlo sampled ab initio calculations f or several small molecules. Higher order couplings are found to make a significant contribution to IVR decays at long times. (C) 1997 Americ an Institute of Physics.