GAUSSIAN-2 THEORY - USE OF HIGHER-LEVEL CORRELATION METHODS, QUADRATIC CONFIGURATION-INTERACTION GEOMETRIES, AND 2ND-ORDER MOLLER-PLESSET ZERO-POINT ENERGIES

The performance of Gaussian-2 theory is investigated when higher level theoretical methods are included for correlation effects, geometries, and zero-point energies. A higher level of correlation treatment is e xamined using Brueckner doubles [BD(T)] and coupled cluster [CCSD(T)] methods rather than quadratic configuration interaction [QCISD(T)]. Th e use of geometries optimized at the QCISD level rather than the secon d-order Moller-Plesset level (MP2) and the use of scaled MP2 zero-poin t energies rather than scaled Hartree-Fock (HF) zero-point energies ha ve also been examined. The set of 125 energies used for validation of G2 theory [J. Chem. Phys, 94, 7221 (1991)] is used to test out these v ariations of G2 theory. Inclusion of higher levels of correlation trea tment has little effect except in the cases of multiply-bonded systems . In these cases better agreement is obtained in some cases and poorer agreement in others so that there is no improvement in overall perfor mance. The use of QCISD geometries yields significantly better agreeme nt with experiment for several cases including the ionization potentia ls of CS and O-2, electron affinity of CN, and dissociation energies o f N-2, O-2, CN, and SO2. This leads to a slightly better agreement wit h experiment overall. The MP2 zero-point energies gives no overall imp rovement. These methods may be useful for specific systems. (C) 1995 A merican Institute of Physics.