Positronium is just like a H-atom with the proton replaced by the positively charged antiparticle of the electron (called the positron which is as the electron). The ground state energy of positronium is (Hint: ${\mathrm{E}}_{\mathrm{n}}=-\frac{{\mathrm{me}}^4}{8{\mathrm{\epsilon }}_0^2{\mathrm{n}}^2{\mathrm{h}}^2}$ for H-atom reduced mass m = m_e. For positronium m = $\frac{{\mathrm{m}}_{\mathrm{e}}}{2}$)