The mass spectrum of the S-wave positronium is considered in the framework of a non-relativistic model. A good agreement is obtained with the experimental masses of the 1S0, singlet state, the para-positronium, and the 3S1 state, the ortho-positronium. The role of one-photon exchange potential (OPEP) and the confinement potential is discussed. The mass difference between ortho- and para-positronium is attributed to the spin-dependent terms of OPEP.
Shastrimath et al. (Mon,) studied this question.