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.
Highlights
Over the last decade, a great deal of work has been done and yet plenty of controversy still surrounds the characterization of the main intrinsic photochemical mechanisms governing the photoinduced processes occurring in the DNA nucleobases [1,2,3,4,5,6,7,8,9,10,11]
In this paper we focus on the assessment of different multiconfigurational electronic structure theory approaches and geometry optimization techniques to benchmark the potential energy hypersurfaces (PEHs) of the lowest-lying excited states in thymine and calibrate the extent to which the theoretical method employed can influence the model put forth to explain the photoinduced phenomena in DNA
The geometries are optimized with the complete active space self-consistent field (CASSCF) method and the protocol, the geometries are optimized with the CASSCF method and the energies are computed energies are computed with the CASPT2 method at the optimized structures
Summary
A great deal of work has been done and yet plenty of controversy still surrounds the characterization of the main intrinsic photochemical mechanisms governing the photoinduced processes occurring in the DNA nucleobases [1,2,3,4,5,6,7,8,9,10,11] Insight into their photochemical pathways provides unique information to ascertain the effect of light irradiation on the cellular system and is tightly related to the ability possessed by the genetic material to dissipate the excess of energy gained upon absorption in a harmless manner. Several techniques have been employed to study the photoinduced phenomena of DNA nucleobases, ranging from pump-probe [4,28,53,54,55], time-resolved infrared [56,57,58,59], photoelectron [17,41,60,61] and recently even Auger spectroscopy [16]
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