The state-averaged complete active space self-consistent field (SA-CASSCF) method has long been successfully applied in the theoretical study of photochemistry because it can accurately describe the global shape of ground and excited state potential energy surfaces at a wide range of nuclear geometries. However, the SA-CASSCF procedure involves the simultaneous optimization of the orbitals and configuration interaction (CI) coefficients, which in some circumstances leads to undesirable behaviors, such as the existence of multiple solutions and spatial wavefunction symmetry breaking. Here we propose several alternative approaches in which a set of reasonable orbitals are determined in an initial step, and then a CAS-CI calculation based on these orbitals is performed in a subsequent step. Though variationally inferior to full SA-CASSCF, we demonstrate that these two-step approaches can offer other advantages, including lower computational cost, a continuous, symmetry-preserving description of the wavefunction, and size consistency.

Two-step multireference approaches to efficiently model electronic excited states

247th National Meeting (2014)

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