The predictive power of molecular dynamics simulations crucially relies on the accuracy of trajectory-based representations of molecular systems to determine their structural and dynamical properties at a reasonable computational cost. A most common use of trajectorybased methods is to mimic the evolution of the nuclei by driving them with classical, quasiclassical, semiclassical, or quantum forces that represent the effect of the electrons, either in the ground state (i.e., for adiabatic dynamics), or including the effect of the excited states (i.e., for nonadiabatic dynamics). On the least computationally demanding end, purely classical trajectory simulations offer access to complex systems consisting of hundreds of atoms.