MOLECULAR-GEOMETRY AND VIBRATIONAL FREQUENCIES OF OZONE FROM COMPACT VARIATIONAL WAVE-FUNCTIONS EXPLICITLY INCLUDING TRIPLE AND QUADRUPLE SUBSTITUTIONS

The performance of a multireference CISD method, CISD[TQ], is compared to that of other approaches which include a large degree of electron correlation, including Brueckner methods. The CISD[TQ] method selects as references all single and double substitutions within an active orb ital space. Certain triple and quadruple substitutions from the Hartre e-Fock reference are included in the CISD[TQ] wave function as singles and doubles from the selected reference set. This wave function has p reviously been shown in simpler cases to provide results near to those predicted by the configuration interaction wave function, including a ll single, double, triple, and quadruple substitutions (CISDTQ). For t he challenging multireference case of ozone, the CISD[TQ] wa ve functi on yields geometries and harmonic vibrational frequencies with an accu racy similar to the full CCSDT method. These promising results suggest that for difficult multireference problems the CISD[TQ] wave function provides an efficient and accurate approach for approximating the com plete CISDTQ. (C) 1997 American Institute of Physics.