The rationale for this choice is the following. As mentioned, the semiclassical prescription implies that the correct kinetic energy reservoir should be restricted to atoms contributing to the nonadiabatic coupling vector [Eq. ( )]. Because this vector corresponds to one degree of freedom among NDF degrees, dividing the available kinetic by NDF should (statistically speaking) deliver a sensible reservoir, as it also refers only to one degree of freedom. In the case of pyrene with NDF = 72, this prescription significantly reduces the available kinetic energy, decreasing the number of back hoppings. It is triggered only to determine whether a back hopping will occur or will be frustrated; all the other features of the surface hopping propagation remain the same. An analogous reduction of the kinetic energy reservoir is typically used in QM/MM surface hopping dynamics, where only the kinetic energy of the QM atoms is considered. Figure illustrates the impact of the kinetic energy reservoir on a typical pyrene trajectory starting with the same initial conditions. In both cases, the molecule quickly relaxes to lower excited states within less than 100 fs. However, as expected, the conventional adjustment with the total kinetic energy [Figure (a)] has many more back hoppings than the adjustment with the reduced kinetic energy [Figure (b)].