We here consider the fundamental interactions which drive barrier evolution in atom transfer reactions. By applying the functional behavior predicted by second-order nondegenerate perturbation theory to a symmetric linear curve crossing model, we are able to derive a simple formula for two-state mixing in the near-field. It becomes readily apparent that the system property most critically responsible for governing the magnitude of coupling is the diabatic state-to-state overlap. In order to explore this parameter in depth, we outline a basic strategy for diabatic state construction in the near-field and use the imposed symmetry requirements and phase relations to derive a functional form which relates the state-to-state overlap to molecular orbital properties of the isolated reactants and the interatomic overlap matrix. Finally, we show how trends in barrier heights may be analyzed in the context of combined far-field and near-field effects and how these effects may be separated in order to provide insight into the underlying physics and broadly applicable mechanistic information.