Many important chemical processes (bond breaking, photochemistry, molecular magnetism, catalysis, etc) involve interactions of weakly coupled electrons. However, these also represent some of the most difficult processes to study computationally as static electron correlation renders the Hartree-Fock starting point unsuitable. One must often resort to multireference methods to obtain meaningful results.Although true multireference methods are still not yet at the 'black-box' level, the Spin-Flip family of methods offer a single reference solution to traditionally multireference problems.Using a reference optimized for a high-spin state (which is single configurational), a spin-flipping CI operator is used to move to a lower ms block of the Hamiltonian to describe the states of interest.When multiple spin-flips are needed to treat several strongly correlated electrons, the recently developed restricted active-space spin-flip (RAS-nSF) method provides an attractive qualitative description. We have recently presented a perturbative approximation to the RAS-nSF method, called spin-flip complete active-space (SF-CAS(h,p)1) which uses second-order perturbation theory to describe the hole and particle excitations in the wavefunction. The decreased cost of this approximation makes it possible augment the SF-CAS(h,p)1 model with additional excitations which capture dynamical electron correlations.In this talk I will discuss our recent developments which involve the inclusion of all excitations that can couple at second-order through the active-space. This includes the subset of double excitations which involve at least one active-space orbital. Results for a variety of systems will be presented, as well as general strategies for computing exchange coupling constants in spin-coupled bimetallic complexes.

Toward accurate single-reference descriptions of strongly correlated systems: Improving active-space spin-flip configuration interaction with perturbation theory

248th National Meeting (2014)

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