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 an alytical 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 intern uclear distances and CH+ are compared in the two schemes. A critical d iscussion of the relaxation effects incorporated in the numerical meth od and the approximations in the functional is presented. (C) 1998 Els evier Science B.V. All rights reserved.