The G2(B3LYP/MP2/CC) Approach: A Modification of the G2(MP2) Approach and a Comparison with B3LYP Results

The quadratic configuration interaction calculation in the G2(MP2) approach is replaced by a coupled-cluster singles and doubles calculation including a perturbational estimate of the triples excitations. In addition, the SCF and MP2 geometry optimizations and SCF frequency calculation in the G2(MP2) approach are replaced by a B3LYP geometry optimization and frequency calculation in the proposed G2(B3LYP/MP2/CC) approach. This simplification does not affect the average absolute deviation from experiment, but decreases the maximum error compared with the G2(MP2) approach. The G2(B3LYP/MP2/CC) results are compared with those obtained using the B3LYP approach, and the G2(B3LYP/MP2/CC) model is found to be more reliable, even if the B3LYP calculations are performed using a large basis set.