Excitation energies and transition moments by the multireference averaged quadratic coupled cluster (MR-AQCC) method

The MR-AQCC (multireference averaged quadratic coupled cluster) and the MR-ACPF (multireference averaged coupled pair functional) methods can be considered as size-extensivity corrected MR-CISD procedures. So far, they have been applied mostly to ground state calculations of a given symmetry. By formulating MR-AQCC and MR-ACPF in terms of a diagonal shift to the Hamiltonian this restriction is removed. In order to compute transition moments, linear response theory (LRT) is used since there is no wavefunction defined within these methods. A LRT-compliant variant of MR-AQCC and MR-ACPF is developed which allows a consistent formulation of transition energies and moments. In calculations on CH₂, CH⁺ and C₂ (where full CI results are available) it is shown that MR-AQCC brings about a substantial improvement for excitation energies and transition moments as compared to MR-CISD.