Ab initio discrete variable representation calculation of vibrational energy levels

A technique to calculate vibrational energy levels of a triatomic molecule without any explicit functional form for the potential energy surface (PES) is presented. The approach uses potential optimized discrete variable representation (DVR) to calculate the vibrational energy levels while ab initio electronic structure calculations are used to evaluate the potential energy at the nuclear configurations needed in the DVR calculation. The approach is called the ab initio-discrete variable representation or ABI-DVR technique. Example calculations for the water molecule are performed. Vibrational energy levels of H-2 O-16 are calculated up to 14 000 cm(-1) above the ground vibrational state within convergence better than 1 cm(-1). The potential energy is evaluated using GAUSSIAN 92 program suite. The 6-311+G** Gaussian basis set is used and the electron correlation is taken into account by second-order Moller-Plesset perturbation theory. The ABI-DVR results are compared with results of calculations in which some analytic form for the PES is used to represent the ab initio calculated potential energies and some aspects of how to construct accurate analytic PESs are discussed. (C) 1996 American Institute of Physics.