We next consider construction of the PES. The PES will be based on a fit to the data set, so if atoms never come close to one another in the data set, then the PES will never be able to calculate a proper energy for such close encounters; we may as well not try. Thus, the next step in the process is to use the data set to evaluate which atoms come closest to one another. We accomplish this evaluation by calculating Morse variables for the geometries in the data set and recording, for all N (N -1)/2 internuclear distances, the largest Morse variables, i.e., those corresponding to the shortest internuclear distances.