Efficient Multi-Configurational Wavefunction Method with Dynamical Correlation Using Non-Orthogonal Configuration Interaction Singles and Doubles (NOCISD)

It is well known that inclusion of dynamical correlation is needed in order to reach quantitative agreement with experiment for molecular systems with multi-reference character. In this work, we start from a non-orthogonal configuration interaction (NOCI) framework that accounts for the static correlation and incorporate dynamical correlation by including singles and doubles excitations out of each reference determinant resulting in a NOCISD wavefunction. The equations defining the NOCISD wavefunction commonly require the solution a poorly condition generalized eigenvalue problem, which we avoid by projecting the equations to a small dimension space defined by the CISD eigenvectors of each reference determinant. We show that NOCISD results are in good qualitative agreement with other state-of-the-art method for challenging problems such as the electron transfer in the ethylene dimer radical cation and LiF, as well as the description of the Jahn-Teller distortion in the cyclopentadienyl and nitrogen trioxide radicals.