A generalized hybrid scheme for multireference methods

摘要

A generalization of the hybrid scheme for multireference methods as recently put forward by Saitow and Yanai [J. Chem. Phys. 152, 114111 (2020)] is presented. The hybrid methods are constructed by defining internal and external excitation spaces and evaluating these two subsets of excitations at different levels of theory. New hybrids that use the mix of internally contracted multireference coupled-cluster, unshifted multireference coupled electron pair, and multireference perturbation methods are derived and benchmarked. A new separation of the excitation space, which combines all singles and doubles excitations to the virtual orbitals into the external space, is also presented and tested. In general, the hybrid methods improve upon their non-hybrid parent method and offer a good compromise between computational complexity and numerical accuracy. Published under an exclusive license by AIP Publishing.