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Quasi-variational coupled-cluster theory

This modification of standard CCSD is capable of robustly describing chemical bond breaking with a single Hartree-Fock reference determinant (see J. B. Robinson and P. J. Knowles, J. Chem. Phys. 136, 054114 (2012), doi:10.1063/1.3680560). It is implemented for closed-shell systems, with either the Brueckner condition or energy optimisation for the determination of orbitals. Triple excitations can be included perturbatively, BQVCCD(T) or OQVCCD(T) (J. B. Robinson and P. J. Knowles, Phys. Chem. Chem. Phys. 14, 6729-6732 (2012), doi:10.1039/C2CP40698E; J. Chem. Theor. Comput. 8, 2653-2660 (2012), doi:10.1021/ct300416b).