Coupling Natural Orbital Functional Theory and Many-Body Perturbation Theory by Using Nondynamically Correlated Canonical Orbitals

We develop a new family of electronic
structure methods for capturing
at the same time the dynamic and nondynamic correlation effects. We
combine the natural orbital functional theory (NOFT) and many-body
perturbation theory (MBPT) through a canonicalization procedure applied
to the natural orbitals to gain access to any MBPT approximation.
We study three different scenarios: corrections based on second-order
Møller–Plesset (MP2), random-phase approximation (RPA),
and coupled-cluster singles doubles (CCSD). Several chemical problems
involving different types of electron correlation in singlet and multiplet
spin states have been considered. Our numerical tests reveal that
RPA-based and CCSD-based corrections provide similar relative errors
in molecular dissociation energies (De) to the results obtained using a MP2 correction. With respect to
the MP2 case, the CCSD-based correction improves the prediction, while
the RPA-based correction reduces the computational cost.