However, if we admit muons, the process: H + µ → e +( µp ), which describes the formation of muonic Hydrogen, can be shown to be Thomas-forbidden. Molecular processes are simpler to come by, e.g. Na + I$_{2}$ → NaI + I . These and other processes are discussed below. Our study reveals interesting structure in the forbidden regions of the Lieber diagram. We have not determined the quantum mechanical behavior of the cross section for these processes, which would correspond to third and higher order terms in the Born series. In this situation there is no on-shell pole in the second Born term, but may be one in the third or higher term. It is not clear whether such a pole would lead to dominance of this term in the high energy cross section. In section II we briefly review the kinematics of the Thomas double collision processes. This will establish the notation and methods to be used for exploring the forbidden processes. 4