  The $\Delta \text{NO}$ method for static correlation is combined with
second-order M{\o}ller-Plesset perturbation theory (MP2) and coupled-cluster
singles and doubles (CCSD) to account for dynamic correlation. The MP2 and CCSD
expressions are adapted from finite-temperature CCSD, which includes orbital
occupancies and vacancies, and expanded orbital summations. Correlation is
partitioned with the aid of damping factors incorporated into the MP2 and CCSD
residual equations. Potential energy curves for a selection of diatomics are in
good agreement with extrapolated full configuration interaction results
(exFCI), and on par with conventional multireference approaches.

Chemical Physics;Strongly Correlated Electrons;Computational Physics