Spin-flip (SF) methods, pioneered by Anna Krylov, are a way of naturally identifying the appropriate active space for calculations on molecules that contain strong electron correlations.  The presence of strong correlations in a low-spin state is indicated by the presence of a low-lying high spin state -- the number of unpaired spins in this high spin reference becomes the active space.  For problems where multiple spin flips are required to describe the strong correlations, a complete active space approach is a suitable starting point.  The restricted active space space spin flip (RAS-SF) method [1,2] is such an approach.In the work described here, we increase the efficiency of the RAS-SF method by treating the hole (h) and particle (p) excitations out of the active space by perturbation theory [3].  Speedups of two or three orders of magnitude are attainable.  We also demonstrate the feasibility of extending the method to treat additional classes of excitations, such as single excitations into and out of the active space (hp excitations).  This leads to recovery of part of the missing dynamical correlation effects.A variety of spectroscopic applications of the SF-CAS methods will be discussed, with a focus on transition metal complexes since they often exhibit strong correlations.  Examples will include the term energies of transition metal complexes with a singlet metal center, and the Heisenberg spin-coupling constants associated with a variety of bimetallic transition metal complexes.[1] D. Casanova and M. Head-Gordon, Phys. Chem. Chem. Phys. 11, 9779-9790 (2009).[2] F. Bell, P.M. Zimmerman, D. Casanova, M. Goldey, and M. Head-Gordon, Phys. Chem. Chem. Phys. 15, 358 - 366 (2013).[3] N.J. Mayhall, M. Goldey, and M. Head-Gordon, J. Chem. Theory Comput. 10, 589-599 (2014).

New developments in complete active space spin flip methods: Theory and applications

248th National Meeting (2014)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.