Positronic Water, ${\mathrm{Ps}}_{2}O$

We demonstrate the chemical stability of a compound containing two positrons, ${\mathrm{Ps}}_2\mathrm{O}$, which may be considered an extreme isotopic variant of ordinary water. We introduce a dynamical determination of the step length into the diffusion quantum Monte Carlo method, and find that ${\mathrm{Ps}}_2\mathrm{O}$ is more stable than its lowest dissociation threshold, $\mathrm{PsO}+\mathrm{Ps}$, by $1.27\pm 0.08$ eV. We estimate its annihilation rate to be $4.6{\mathrm{ns}}^{-1\mathrm{}}$. This paper is the first demonstration of the stability of a two-positron compound since Wheeler's work on diatomic positronium, ${\mathrm{Ps}}_2$ [Ann. N.Y. Acad. Sci. 48, 219 (1946)].