Evolution and crossing of the ground and ionic states in an atom transfer reaction. (a) The ionic state evolution in the far field is essentially coulombic (IP-EA-e^2/r). Far field evolution is modeled without distorting the reactants. Perturbations account for multipole charge distribution, polarizabilities, and variations in the reactant distance at the onset of significant overlap and distortion, which initiates the atom transfer curve crossing. (b) The curve crossing energy and location is goverened by the excited state gaps in the reactants and products. The energy is dominated by the lower energy gap, while the location is controled by the energy difference of the two gaps. The actual transition state energy is determined by the crossing energy and the coupling strength of the two states.