  We present a novel method that appropriately handles both dynamical and
static electron correlation in a balanced manner, using a perturbation theory
on a spin-extended Hartree-Fock (EHF) wave function reference. While EHF is a
suitable candidate for degenerate systems where static correlation is
ubiquitous, it is known that most of dynamical correlation is neglected in EHF.
In this work, we derive a perturbative correction to a fully spin-projected
self-consistent wave function based on second-order M{\o}ller-Plesset
perturbation theory (MP2). The proposed method efficiently captures the ability
of EHF to describe static correlation in degeneracy, combined with MP2's
ability to treat dynamical correlation effects. We demonstrate drastic
improvements on molecular ground state and excited state potential energy
curves and singlet-triplet splitting energies over both EHF and MP2 with
similar computational effort to the latter.
