Positron annihilation in positronium hydrides

The two-photon annihilation rate for positronium hydride is calculated with the use of extensive Hylleraas-type wave functions in which all six interparticle coordinates are used. The binding energy against dissociation into a positronium atom and a hydrogen atom is improved upon the previous best value by about 3.8%. The two-photon annihilation rate, calculated with a wave function that gives a positron-electron cusp value of -0.492$a_0^{\mathrm{-}1}$ (compared with the exact value of -0.5$a_0^{\mathrm{-}1}$), is determined as 2.459 ${\mathrm{nsec}}^{\mathrm{-}1}$.