Gaussian-3 theory using coupled cluster energies

Variations of Gaussian-3 (G3) theory are presented in which the quadratic configuration interaction (QCISD(T)) energy calculation is replaced by a coupled cluster (CCSD(T)) energy calculation. This modification is made to four G3 methods that have been previously introduced. The replacement of the QCISD(T) energy by the CCSD(T) energy results in little change in the accuracy of the methods as assessed on the G2/97 test set, although the maximum deviations decrease slightly. These new G3 methods based on the coupled cluster technique are a useful alternative to the G3 methods based on quadratic configuration interaction. (C) 1999 Published by Elsevier Science B.V. All rights reserved.