As a minimal model of the chromophore of rhodopsin proteins, the penta-2,4-dieniminium cation (PSB3) poses a challenging test system for the assessment of electronic-structure methods for the exploration of ground- and excited-state potential-energy surfaces, the topography of conical intersections, and the dimensionality (topology) of the branching space. Herein, we report on the performance of the approximate linear-response coupled-cluster method of second order (CC2) and the algebraic-diagrammatic-construction scheme of the polarization propagator of second and third orders (ADC(2) and ADC(3)). For the ADC(2) method, we considered both the strict and extended variants (ADC(2)-s and ADC(2)-x). For both CC2 and ADC methods, we also tested the spin-component-scaled (SCS) and spin-opposite-scaled (SOS) variants. We have explored several ground- and excited-state reaction paths, a circular path centered around the S₁/S₀ surface crossing, and a 2D scan of the potential-energy surfaces along the branching space. We find that the CC2 and ADC methods yield a different dimensionality of the intersection space. While the ADC methods yield a linear intersection topology, we find a conical intersection topology for the CC2 method. We present computational evidence showing that the linear-response CC2 method yields a surface crossing between the reference state and the first response state featuring characteristics that are expected for a true conical intersection. Finally, we test the performance of these methods for the approximate geometry optimization of the S₁/S₀ minimum-energy conical intersection and compare the geometries with available data from multireference methods. The present study provides new insight into the performance of linear-response CC2 and polarization-propagator ADC methods for molecular electronic spectroscopy and applications in computational photochemistry.

The semiempirical orthogonalization-corrected OMx methods have recently been shown to perform well in extensive ground-state benchmarks. They can also be applied to the computation of electronically excited states when combined with a suitable multireference configuration interaction (MRCI) treatment. We report on a comprehensive evaluation of the performance of the OMx/MRCI methods for electronically excited states. The present benchmarks cover vertical excitation energies, excited-state equilibrium geometries (including an analysis of significant changes between ground- and excited-state geometries), minimum-energy conical intersections, ground- and excited-state zero-point vibrational energies, and 0–0 transition energies for a total of 520 molecular structures and 412 excited states. For comparison, we evaluate the TDDFT/B3LYP method for all benchmark sets, and the CC2, MRCISD, and CASPT2 methods for some of them. We find that the current OMx/MRCI methods perform reasonably well for many of the excited-state properties. However, in comparison to the first-principles methods, there are also a number of shortcomings that should be addressed in future developments.

8-Oxoguanine is the most abundant oxidation product found in oxidatively damaged DNA. The study of the excited-state properties of the corresponding deoxyribonucleoside 8-oxo-deoxyguanosine is thus of important biological relevance. Herein, we present an ADC(2)-s ab initio study of the neutral and the anionic form of 8-oxo-deoxyguanosine, for each of which we have considered the intramolecularly 5′-O–H⋯N3 hydrogen-bonded syn conformer. We present energy profiles for a radiationless deactivation mechanism via intramolecular excited-state proton transfer. This mechanism is accessible in the neutral form, but it is unavailable in the anion. We present optimized structures for the proton-transfer conical intersection of the neutral form as well as the ring-puckered conical intersections inherent to the 8-oxoguanine moiety for the neutral and anionic forms. We highlight the possible relevance of the proton-transfer mechanism for the neutral form and discuss our results in light of several recently published computational and spectroscopic studies. Our results provide new insight into the photophysics of this biologically relevant nucleoside and pave the way for future nonadiabatic dynamics studies and for further spectroscopic investigations.