Positronium energy levels at order $m{\alpha }^7$: Product contributions in the two-photon-annihilation channel

Ongoing improvements in the measurement of positronium transition intervals motivate the calculation of the $O(m{\alpha }^7)$ corrections to these intervals. In this work we focus on corrections to the spin-singlet parapositronium energies involving virtual annihilation to two photons in an intermediate state. We have evaluated all contributions to the positronium S-state energy levels that can be written as the product of a one-loop correction on one side of the annihilation event and another one-loop correction on the other side. These effects contribute $\mathrm{\Delta }E=-0.561971(25)m{\alpha }^7\mathrm{/}{\pi }^3$ to the parapositronium ground state energy.