For each Surface Hopping (SH) trajectory, we start our simulation from a single adiabatic PES (known as the active state). Next, we compute the probabilities for an electron to hop from this active state to all the other states (i.e., the adiabatic PESs). These computed probabilities are then compared with a uniformly generated random number. If the probability to jump from the active state to any other state is greater than the generated random number, the hop is accepted; otherwise, it will be rejected. This process is repeated for a swarm of trajectories, and the results are averaged over them to obtain the electron dynamics (as further explained in the next section). Among the variants of surface hopping approaches, the FSSH method is the most successful. In this method, the number of hops (switches) between the states is minimized, as described in the following example. Let us assume that our system has only two states or PESs (namely, ground and excited states), and we are running 10 surface hopping trajectories. Let us also consider that at a particular instant of time (say, t = 0), 5 of these 10 trajectories are in the excited state, while the other 5 are in the ground state; i.e., [N GS , N Ex ] = [5, 5] (see Fig. ). Finally, let us assume that in the next time step (t = 1), there are six trajectories in the ground state (N GS = 6), and four in the excited state (N Ex = 4). We can obtain this configuration, [N GS , N Ex ] = [6, 4], in several ways including: (1) switching 3 trajectories from the excited state to the ground state, and only 2 trajectories from the ground to the excited state (Fig. ) or (2) allowing 2 trajectories to hop from the excited state to the ground state, and only 1 trajectory to hop from the ground to the excited state, and so on. There will always be a way to achieve the desired configuration with the least number of switches/hops between the potential energy surfaces. To achieve the desired configuration with minimum switches in the present example, only 1 trajectory in the excited state should switch to the ground state, and zero trajectories should switch from the ground state to the excited state (Fig. ). This criterion is known as the fewest-switches (FS) criterion, and using this constraint, Tully proposed the FSSH mechanism to capture nonadiabatic electron dynamics phenomena. We use the CPA version of FSSH in conjunction with DFTB (as the underlying electronic structure method), as explained below.