As the fitted PE surfaces are available globally, gradients can be numerically calculated from the fitted PE surfaces by a finite difference scheme, as explained above, leading to the so-called energy-only simulations. In Fig. the dotted lines show the population dynamics of 1000 trajectories of an energy-only simulation, using the same initial conditions and random seed as for the reference calculations. The populations of the energy-only simulations coincide very well with the populations of the simulation, where also the gradients were fitted. It