The electron configurations, |22...n 1 n 2 ...n L 00...⟩, cover all possible combinations of N active electrons distributed in L active orbitals, with orbital occupations, n i , of 0, 1, or 2. C n 1 n 2 ...n L are the coefficients associated with each configuration. Multiple states can be treated by optimizing the state-averaged (SA) energy over a model space of states with different symmetries and excitation numbers. An effective active space can sufficiently capture a large portion of correlation in all states at a greatly reduced cost relative to full configuration interaction (FCI).