The on-the-fly training scheme we propose here (the bottom panel of Figure ) is based on threshold Monte Carlo (MC) simulations for sampling the configurational space. This method has been applied to molecular crystals using empirical force fields and DFTB to characterize the global structure of crystal energy landscapes and to reduce overprediction of polymorphism. These simulations involve regular MC sampling of the configurational space, but with the distinguishing feature of an energy lid, which is defined relative to the energy of the initial configuration from which the simulation was initiated. During the simulation MC moves are accepted if and only if the energy of the resulting structure is below the current energy lid. Consequently, the energy lid effectively constrains the simulation to explore only regions accessible below the lid, thereby providing a high level of control over the trajectory.