An extended coupled-cluster functional for molecular properties: study of an analytical and numerical approach

A stationary coupled-cluster method using an extended coupled-cluster functional for molecular properties is tested. A recently developed analytical derivative code using non-relaxed orbitals in an approximate singles and doubles scheme is compared with the numerical finite-field results incorporating the relaxation of the orbitals. For this study, the dipole moment and polarizability values of HF at different internuclear distances and CH+ are compared in the two schemes. A critical discussion of the relaxation effects incorporated in the numerical method and the approximations in the functional is presented. (C) 1998 Elsevier Science B.V. All rights reserved.