Downfolded Configuration Interaction for Correlated Electronic States

Abstract

<p><em>I will present our recent progress in developing a deterministic variant of selected configuration interaction (CI) method by downfolding external determinant configurations into a compact internal subspace. The Löwdin-type effective Hamiltonian within a small internal subspace can be constructed for direct diagonalizations to yield chemically accurate total energies and compressed eigenstates. This method is state-specific and termed the downfolded CI (dCI), exhibiting a salient advantage of selecting most important external determinants that are based on the quadratic criterion of internal-external coupling integrals, which is different from traditional selected CI approaches that usually adopt the linear coupling scheme. I will discuss two implementations: the recursive dCI and perturbative dCI algorithms, as well as the parallelization scheme. As a result, the sub-mhartree accuracy of the dCI ground, excited states and excitation energies will be demonstrated to a few challenging molecules including benzene with 30 valence electrons and 258 molecular orbitals.</em></p>