title: A new internally contracted multi-reference configuration interaction method.
abstract: We present a new internally contracted multi-reference configuration interaction (MRCI) method which, at the same time, efficiently handles large active orbital spaces, long configuration expansions, and many closed-shell orbitals in the reference function. This is achieved by treating the closed-shell orbitals explicitly, so that all required coupling coefficients and density matrices only depend on active orbital labels. As a result, closed-shell orbitals are handled as efficiently as in a closed-shell single-reference program, and this opens up the possibility to perform high-accuracy MRCI calculations for much larger molecules than before. The enormously complex equations are derived using a new domain-specific computer algebra system and semi-automatically implemented using a newly developed integrated tensor framework. The accuracy and efficiency of the MRCI method is demonstrated with applications to dioxygen-copper complexes with different ligands, some of which involve more than 30 atoms, and to spin-state splittings of ferrocene.
authors.affiliation: Institut für Theoretische Chemie, Universität Stuttgart, Stuttgart, Germany.
authors.forename: K R
authors.initials: KR
authors.lastname: Shamasundar
authors.forename: Gerald
authors.initials: G
authors.lastname: Knizia
authors.forename: Hans-Joachim
authors.initials: HJ
authors.lastname: Werner