Slow collisions between heavy particles

Certain coupling terms neglected in the pertubed stationary state and related approximations for treating slow collisions between heavy particles are discussed. It is pointed out that their inclusion would make the group of approximations in question tend to the Born approximation at high impact energies and weak interactions. One member of the group differs from the Born approximation only in the coupling terms. Information on the importance of these terms can hence be obtained by comparing the values of cross-sections given by this member with the corresponding values given by the Born approximation. Calculations are carried out on the processes H+ + H(1s)→ H+ + H(2p) and H(1s) + H(1s)→ H(1s) + H(2p).