A MODIFICATION OF THE GAUSSIAN-2 APPROACH USING DENSITY-FUNCTIONAL THEORY

The quadratic configuration interaction calculation in the Gaussian-2 second-order Moller-Plesset perturbation theory approach, G2(MP2), is replaced by a coupled-cluster (CC) singles and doubles calculation inc luding a perturbational estimate of the triples excitations. In additi on, the self-consistent-held (SCF) and MP2 geometry optimizations and SCF frequency calculation in the G2(MP2) approach are replaced by a de nsity functional theory geometry optimization and frequency calculatio n [using the Becke three parameter hybrid functional with the Lee-Yang -Parr non-local correlation functional (B3LYP)] in the proposed G2(B3L YP/MP2/CC) approach. This simplification does not affect the average a bsolute deviation from experiment, but decreases the maximum error com pared with the G2(MP2) approach. The G2(B3LYP/MP2/CC) atomization ener gies are compared with those obtained using the B3LYP approach, and th e G2(B3LYP/MP2/CC) model is found to be more reliable, even if the B3L YP calculations are performed using a large basis set.