The time evolution of a (non-relativistic) molecular system is determined by the time-dependent Schrödinger equation (TDSE). Unfortunately, solving the TDSE and simulating quantum dynamics in molecules, especially when electronically excited states are involved, remains a fundamentally difficult problem, due to the non-local nature of quantum mechanics and the exponential scaling of the computational effort with the number of degrees of freedom. One popular and effective approximation to circumvent these issues is to describe the nuclear motion with classical mechanics instead of quantum mechanics .