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24.5 Quasi-variational coupled cluster, QVCCD

Performs closed-shell quasi-variational coupled cluster, QVCCD (J. B. Robinson and P. J. Knowles, J. Chem. Phys. 136, 054114 (2012), doi:10.1063/1.3680560). Normally the effect of single excitations needs to be included through orbital optimisation, and this can be done either through the Brueckner condition (BQVCCD), or through variational minimisation of the energy functional with respect to orbital rotations (OQVCCD). The effects of triple excitations can be included using the standard perturbation theory, BQVCCD(T) or OQVCCD(T) (J. B. Robinson and P. J. Knowles, Phys. Chem. Chem. Phys. 14, 6729-6732 (2012), doi:10.1039/C2CP40698E). 2018-07-21