FERMIONIUM IN AN EXTERNAL POTENTIAL

The set of 16 first-order relativistic equations derived from the two- body Dirac equation is discussed for a system of two spin-1/2 particle s moving in an external potential. Scalar and vector wave-function com ponents are used in place of double bispinor components. In general, t he case of different masses is considered. Iii the case of equal masse s, this set is explicitly reduced to four second-order equations invol ving ''large-large'' components, if terms quartic in momenta are negle cted.