title: Restricted active space spin-flip configuration interaction: theory and examples for multiple spin flips with odd numbers of electrons.
abstract: The restricted active space spin flip (RAS-SF) method is extended to allow ground and excited states of molecular radicals to be described at low cost (for small numbers of spin flips). RAS-SF allows for any number of spin flips and a flexible active space while maintaining pure spin eigenfunctions for all states by maintaining a spin complete set of determinants and using spin-restricted orbitals. The implementation supports both even and odd numbers of electrons, while use of resolution of the identity integrals and a shared memory parallel implementation allow for fast computation. Examples of multiple-bond dissociation, excited states in triradicals, spin conversions in organic multi-radicals, and mixed-valence metal coordination complexes demonstrate the broad usefulness of RAS-SF.
authors.affiliation: Kenneth S. Pitzer Center for Theoretical Chemistry, Department of Chemistry, University of California, Berkeley, California 94720, USA.
authors.forename: Paul M
authors.initials: PM
authors.lastname: Zimmerman
authors.forename: Franziska
authors.initials: F
authors.lastname: Bell
authors.forename: Matthew
authors.initials: M
authors.lastname: Goldey
authors.forename: Alexis T
authors.initials: AT
authors.lastname: Bell
authors.forename: Martin
authors.initials: M
authors.lastname: Head-Gordon