AB-INITIO DISCRETE VARIABLE REPRESENTATION CALCULATION OF VIBRATIONAL-ENERGY LEVELS

A technique to calculate vibrational energy levels of a triatomic mole cule without any explicit functional form for the potential energy sur face (PES) is presented. The approach uses potential optimized discret e variable representation (DVR) to calculate the vibrational energy le vels while ab initio electronic structure calculations are used to eva luate the potential energy at the nuclear configurations needed in the DVR calculation. The approach is called the ab initio-discrete variab le representation or ABI-DVR technique. Example calculations for the w ater molecule are performed. Vibrational energy levels of H-2 O-16 are calculated up to 14 000 cm(-1) above the ground vibrational state wit hin convergence better than 1 cm(-1). The potential energy is evaluate d using GAUSSIAN 92 program suite. The 6-311+G* Gaussian basis set is used and the electron correlation is taken into account by second-ord er Moller-Plesset perturbation theory. The ABI-DVR results are compare d 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.