The threshold algorithm was developed as a method for finding the energy barrier between structures without the requirement of energy gradient and Hessian matrix calculations . Initiated from a local minimum on the energy landscape, a Monte Carlo trial is generated by random local perturbations with the restriction that the single point (i.e. unminimized) energy of the perturbed structure is below a defined threshold energy, called the lid energy. All attempted moves that remain below the current lid energy are accepted and all moves that increase the energy above the lid energy are rejected. Thus, the trajectory can only explore a local pocket on the lattice energy surface and can never reach regions with energy higher than the lid. If the energy barrier between the current structure and another is higher than the lid, the transition between the two energy basins in which these structures are located cannot be sampled.