These AO overlap integrals are explicitly evaluated with our in-house version of the DFTB+ code. After obtaining the eigenvectors, eigenvalues, wavefunction expansion coefficients, and the coupling elements at each time step, the surface hopping simulation is carried out using the CPA-FSSH scheme proposed by Akimov and Prezhdo, which is an adaptation of the original FSSH scheme proposed by Tully and Hammes-Schiffer (with some modifications). In CPA-FSSH, an instantaneous active state is assigned and the probability of switching from the current state, |Φ i (r, R(t)) , to any other state, |Φ j (r, R(t)) , during a small time interval, t ∈ [t, t + δ] is calculated as