title: Assessment of the Potential Energy Hypersurfaces in Thymine within Multiconfigurational Theory: CASSCF vs. CASPT2.
abstract: The present study provides new insights into the topography of the potential energy hypersurfaces (PEHs) of the thymine nucleobase in order to rationalize its main ultrafast photochemical decay paths by employing two methodologies based on the complete active space self-consistent field (CASSCF) and the complete active space second-order perturbation theory (CASPT2) methods: (i) CASSCF optimized structures and energies corrected with the CASPT2 method at the CASSCF geometries and (ii) CASPT2 optimized geometries and energies. A direct comparison between these strategies is drawn, yielding qualitatively similar results within a static framework. A number of analyses are performed to assess the accuracy of these different computational strategies under study based on a variety of numerical thresholds and optimization methods. Several basis sets and active spaces have also been calibrated to understand to what extent they can influence the resulting geometries and subsequent interpretation of the photochemical decay channels. The study shows small discrepancies between CASSCF and CASPT2 PEHs, displaying a shallow planar or twisted ¹(ππ*) minimum, respectively, and thus featuring a qualitatively similar scenario for supporting the ultrafast bi-exponential deactivation registered in thymine upon UV-light exposure. A deeper knowledge of the PEHs at different levels of theory provides useful insight into its correct characterization and subsequent interpretation of the experimental observations. The discrepancies displayed by the different methods studied here are then discussed and framed within their potential consequences in on-the-fly non-adiabatic molecular dynamics simulations, where qualitatively diverse outcomes are expected.
authors.affiliation: Instituto de Ciencia Molecular, Universitat de València, P. O. Box 22085, ES-46071 Valencia, Spain. Javier.segarra@uv.es.|Present Address: Laboratoire de Chimie UMR 5182, École Normale Supérieure de Lyon, CNRS, Université de Lyon, 46 Allée d'Italie, F-69364 Lyon Cedex 07, France. Javier.segarra@uv.es.
authors.forename: Javier
authors.initials: J
authors.lastname: Segarra-Martí
authors.affiliation: Instituto de Ciencia Molecular, Universitat de València, P. O. Box 22085, ES-46071 Valencia, Spain. Antonio.Frances@uv.es.
authors.forename: Antonio
authors.initials: A
authors.lastname: Francés-Monerris
authors.affiliation: Instituto de Ciencia Molecular, Universitat de València, P. O. Box 22085, ES-46071 Valencia, Spain. Daniel.Roca@uv.es.
authors.forename: Daniel
authors.initials: D
authors.lastname: Roca-Sanjuán
authors.affiliation: Instituto de Ciencia Molecular, Universitat de València, P. O. Box 22085, ES-46071 Valencia, Spain. Manuela.Merchan@uv.es.
authors.forename: Manuela
authors.initials: M
authors.lastname: Merchán
mesh_terms: D008956:Models, Chemical; D056004:Molecular Dynamics Simulation; D055668:Photochemical Processes; D013816:Thermodynamics; D013941:Thymine / Q000737:chemistry*
chemical_list: D013941:Thymine
keywords: CASSCF/CASPT2; DNA; photochemistry; photostability; thymine