Fock-space coupled-cluster method: The (1,2) sector

The coupled-cluster method with single and double excitations is applied to the (1,2) sector of the Fock space, the sector of one valence hole and two valence particles added to a closed shell. The coupled-cluster equations in the (1,2) sector are linear and are solved directly. The systems investigated are the two isoelectronic sequences B, C⁺ , N²⁺ and Al, Si⁺, P²⁺. The primary interest is in the excitation spectra of these systems. In addition, due to the hierarchical nature of the Fock-space approach, other results are obtained gratuitously in the lower sectors. Excellent results are obtained, with most calculated energies differing from experimental values by less than 0.1 eV.