We have validated eFF on a variety of ground state minima; now, we examine the energetics of allowed and forbidden reactions, and the transition states that connect different minima. As examples, we compute the potential energy surface of , an allowed reaction, and , a forbidden reaction (Figure 4.13).
For , we assume that the atoms are colinear, and find a saddle point at the symmetric geometry with = 42 kcal/mol relative to separated . In comparison, quantum Monte Carlo calculations  find the transition state to be at with = 9.7 kcal/mol. Although the transition state energy is significantly too high, it is still smaller than the dissociation energy; that together with the shape of the potential energy curve indicates that the reaction is allowed.
For , we assume that the atoms are aligned in a square, and we find that there is no low energy path connecting product to reactant -- both of the molecules must break simultaneously in order for the reaction to happen. The potential energy surface is rather flat, and there exists a saddle point at with = 132 kcal/mol relative to separated , which makes the reaction forbidden. To compare, we evaluated a potential for due to Boothroyd  derived from MRD-CI calculations over a set of symmetric square geometries, and found a saddle point at with = 147 kcal/mol.