Extension of the K-orbital method and its demonstration in conjunction with spin-adapted large-scale parallelized complete active space configuration interaction

Abstract Based on Davidson's K-orbital method, we present a straightforward approach to obtaining orbitals that effectively incorporate dynamical correlation through large-scale multi-reference configuration interaction calculations. The utility of the extended K-orbitals was evaluated on the most commonly studied molecule, CH2, and it was shown how effective the method is in achieving fast total energy convergence as well as singlet-triplet separation.