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