Next, we examined the potential energy surface along the dissociation of a uracil dimer pair, from the S66x8 data set of Rezac et al. The dimer is displaced along an axis parallel to the two hydrogen bonds, at 0.9, 0.95, 1.0, 1.05, 1.1, 1.25, 1.5, and 2.0 times the average distance of the two hydrogen bonds (R eq = 1.805 Å). As shown in Figure