Conical intersections and avoided crossings result from the (near) degeneracy of electronic states and are important for modeling photochemical reactions, as well as geometric phase effects. The prevalence of such topological structures requires that computational methods to describe excited states or dynamics must be able to account for their presence. Unfortunately, the most commonly used approach, time-dependent density functional theory, is unable to correctly account for degeneracies with the ground state, while the multi-reference approaches that are typically employed are frequently too expensive for application to chemically relevant systems. Recently, there has been interest in constructing models from nonorthogonal configuration interaction approaches through coupling independently nonlinearly optimized solutions of mean-field models, for example Hartree-Fock or constrained density functional theory solutions. However, such models require significant insight or advanced computational approaches in order to properly determine a suitable determinant set. In order to attempt to build a more black-box approach, we have recently considered reoptimization of orbitals under coupling of determinants in the nonorthogonal configuration interaction problem. The question then arises as to whether an automated approach for the initial selection of the determinant basis can be used — for example, an orthogonal expansion built from orbital substitutions of a single determinant. In this presentation, I will examine the application of an orbital-reoptimized nonorthogonal configuration interaction model to predicting geometries of conical intersections and avoided crossings. Through computations of molecular systems, I will compare the performance of the model to other computational approaches and attempt to establish the extent to which different orbitals for different configurations can i) compress the wavefunction and ii) capture dynamic correlation relative to orthogonal determinant expansions.

Description of conical intersections and avoided crossings in the resonating Hartree-Fock approach

ACS Spring Meeting 2021

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