Excited electronic states from a variational approach based on symmetry-projected Hartree–Fock configurations
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Recent work from our research group has demonstrated that symmetry-projected Hartree–Fock (HF) methods provide a compact representation of molecular ground state wavefunctions based on a superposition of non-orthogonal Slater determinants. The symmetry-projected ansatz can account for static correlations in a computationally efficient way. Here we present a variational extension of this methodology applicable to excited states of the same symmetry as the ground state. Benchmark calculations on the C2 dimer with a modest basis set, which allows comparison with full configuration interaction results, indicate that this extension provides a high quality description of the low-lying spectrum for the entire dissociation profile. We apply the same methodology to obtain the full low-lying vertical excitation spectrum of formaldehyde, in good agreement with available theoretical and experimental data, as well as to a challenging model C2v insertion pathway for BeH2. The variational excited state methodology developed in this work has two remarkable traits: it is fully black-box and will be applicable to fairly large systems thanks to its mean-field computational cost.
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Jiménez-Hoyos, Carlos A., Rodríguez-Guzmán, R. and Scuseria, Gustavo E.. "Excited electronic states from a variational approach based on symmetry-projected Hartree–Fock configurations." The Journal of Chemical Physics, 139, no. 22 (2013) American Institute of Physics: https://doi.org/10.1063/1.4840097.