A model Hamiltonian to simulate the complex photochemistry of benzene

The photochemistry and photophysics of benzene is a classic example of the richness of competing pathways available to a molecule after photo-excitation. Computer simulations are one way to provide a molecular picture for the dynamics behind the experimental observables. In this paper a first step is made towards quantum dynamics simulations of benzene by setting up a vibronic coupling model Hamiltonian to provide the potential energy functions that drive the nuclear motion. Seven coupled states and all vibrational modes are included and the parameters for the model obtained by fitting to points provided by quantum chemistry calculations. The model is shown to be a good fit of the adiabatic surfaces. A full vibronic analysis is made revealing the complexity of the features of this manifold of states. Low-level CASSCF calculations have been used here, and future calculations using higher level methods will build on this work to make the model suitable for the correct description of benzene.