Gaussian-3 theory using coupled cluster energies

Variations of Gaussian-3 (G3) theory are presented in which the quadratic c onfiguration interaction (QCISD(T)) energy calculation is replaced by a cou pled cluster (CCSD(T)) energy calculation. This modification is made to fou r G3 methods that have been previously introduced. The replacement of the Q CISD(T) energy by the CCSD(T) energy results in little change in the accura cy of the methods as assessed on the G2/97 test set, although the maximum d eviations decrease slightly. These new G3 methods based on the coupled clus ter 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.