A time averaged semiclassical approach to the computation of nonadiabatic vibronic absorption spectra

In this work, we propose a method to compute semiclassical absorption spectra of nonadiabatic systems. We first report the working formula for the absorption cross section for diabatically coupled systems and review the main features of the Meyer-Miller-Stock-Thoss Hamiltonian and its semiclassical propagation. Then, by combining the mapped Hamiltonian and the initial value representation formalism, we introduce a time-averaged semiclassical method for the calculation of the absorption spectrum, which also accounts for nonadiabatic effects in vibronic spectroscopy. After improving an already existing symplectic algorithm for the symplectic phase space propagation, we consider a model system to benchmark our semiclassical approach against exact quantum mechanical calculations. Finally, we test our method on the four mode model of pyrazine-a fundamental benchmark in the field of nonadiabatic vibronic spectroscopy-for understanding the potentiality and limitations of our semiclassical approach.