FOCK-SPACE COUPLED-CLUSTER METHOD - A STUDY OF THE ELECTRONIC-SPECTRA OF MODEL PI-ELECTRON SYSTEMS

A generalization of the single-reference coupled-cluster method, employing the algebraic properties of the fermionic Fock space, is presented. This Fock-space coupled-cluster (FSCC) method is capable of providing not only the ground-state energy of an N-electron system, but also an important fraction of system's excitation spectrum, including ionization potentials, electron affinities, and excitation energies corresponding to N-electron singlet and triplet states. The FSCC method is applied to study the electronic spectra corresponding to the Pariser-Parr-Pople model of butadiene, hexatriene, and benzene, with the full configuration-interaction results taken as the reference. The problem of intruder states is discussed.