Development, benchmarking and application of improved quantum-classical trajectory surface-hopping methods for the ab initio simulation of photoinduced hydrogen-atom transfer reactions and the computation of time-resolved spectroscopic signals

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The computational treatment of photochemical reaction dynamics involving nonadiabatically coupled electronic potential-energy surfaces in polyatomic molecules is challenging and has only recently become possible due to the development of efficient computational methods and the availability of powerful computing resources. Because the computational cost of classical nuclear dynamics on electronic potential-energy surfaces increases only linearly with the number of degrees of freedom, mixed quantum-classical computational methods, in which the electrons are described quantum mechanically and the nuclei classically, are attractive for the simulation of the photochemical dynamics of complex systems. The goal of this project is the further development, benchmarking and application of improved quasi-classical trajectory surface-hopping methods for the computational simulation of nonadiabatic photochemical processes and the calculation of time-resolved spectroscopic signals. In the first funding period, we developed and implemented a novel robust and efficient surface-hopping algorithm which is based on the Landau-Zener formula for nonadiabatic transition probabilities at conical intersections. The accuracy of this quasi-classical method was critically examined by comparison with exact quantum dynamics calculation for multi-state multi-mode models of photophysical and photochemical dynamics. We developed a computer program for full-dimensional on-the-fly simulations of photochemical dynamics with the Landau-Zener-based surface-hopping algorithm, using the second-order algebraic-diagrammatic construction (ADC(2)) electronic-structure method for the ab initio calculation of energies and gradients. In the second funding period, we will move beyond the computation of nonequilibrium electronic population probabilities and explore the possibility of accurate quasi-classical simulations of time and frequency resolved spectroscopic signals for systems exhibiting ultrafast nonadiabatic dynamics. The main chemical application of the improved surface-hopping method will be the ab initio exploration of excited-state proton-coupled electron transfer (PCET) reactions involved in the photocatalytic oxidation of water with carbon nitride materials, which currently is a hot topic in solar energy conversion research.