STRUCTURES AND VIBRATIONAL FREQUENCIES IN THE FULL CONFIGURATION-INTERACTION LIMIT - PREDICTIONS FOR 4 ELECTRONIC STATES OF METHYLENE USINGA TRIPLE-ZETA PLUS DOUBLE POLARIZATION (TZ2P) BASIS

Benchmark energies, geometries, dipole moments, and harmonic vibration al frequencies are determined for four states of methylene (CH2) by so lving the electronic Schrodinger equation exactly for a triple-zeta pl us double polarization (TZ2P) basis with the restrictions that dir cor e orbital remains doubly occupied and the highest-lying virtual orbita l is deleted, Approximate models of electron correlation are evaluated based on their ability to match the exact, full configuration interac tion results. Predictions from configuration interaction with all sing le, double, triple, and quadruple substitutions (CISDTQ) and coupled-c luster including singles, doubles, and triples (CCSDT) are virtually i dentical to the full CI results for all but the (c) over tilde (1)A(1) state, which is poorly described by a single-configuration reference. In agreement with previous work, the (c) over tilde state remains sli ghtly bent at the TZ2P full CI level of theory, with a bond angle of 1 70.1 degrees and a barrier to linearity of only 25 cm(-1). (C) 1998 Am erican Institute of Physics. [S0021-9606(98)02103-5].