title: Coupled-cluster theory in a projected atomic orbital basis.
abstract: We present a biorthogonal formulation of coupled-cluster (CC) theory using a redundant projected atomic orbital (PAO) basis. The biorthogonal formulation provides simple equations, where the projectors involved in the definition of the PAO basis are absorbed in the integrals. Explicit expressions for the coupled-cluster singles and doubles equations are derived in the PAO basis. The PAO CC equations can be written in a form identical to the standard molecular orbital CC equations, only with integrals that are related to the atomic orbital integrals through different transformation matrices. The dependence of cluster amplitudes, integrals, and correlation energy contributions on the distance between the participating atomic centers and on the number of involved atomic centers is illustrated in numerical case studies. It is also discussed how the present reformulation of the CC equations opens new possibilities for reducing the number of involved parameters and thereby the computational cost.
authors.affiliation: Center for Theoretical Chemistry, Department of Chemistry, Langelandsgade 140, DK-8000 Aarhus C, Denmark. ove@chem.au.dk
authors.forename: Ove
authors.initials: O
authors.lastname: Christiansen
authors.forename: Pekka
authors.initials: P
authors.lastname: Manninen
authors.forename: Poul
authors.initials: P
authors.lastname: Jorgensen
authors.forename: Jeppe
authors.initials: J
authors.lastname: Olsen