Jiménez-Hoyos, Carlos A.Rodríguez-Guzmán, R.Scuseria, Gustavo E.2017-05-042017-05-042013Jiménez-Hoyos, Carlos A., Rodríguez-Guzmán, R. and Scuseria, Gustavo E.. "Excited electronic states from a variational approach based on symmetry-projected Hartree–Fock configurations." <i>The Journal of Chemical Physics,</i> 139, no. 22 (2013) American Institute of Physics: https://doi.org/10.1063/1.4840097.https://hdl.handle.net/1911/94186Recent 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.engArticle is made available in accordance with the publisher's policy and may be subject to US copyright law. Please refer to the publisher's site for terms of use.Journal articlehttps://doi.org/10.1063/1.4840097