Elastic scattering of ${\it o}$-Ps by $\text{H}_2$ at low energies

We investigate the low-energy scattering of orth-positronium by $\text{H}_2$ from first principles of quantum mechanics using explicitly correlated Gaussians as basis functions. Using the confined variational method, we obtain highly accurate $S$-wave phase shifts and pick-off annihilation parameters for different incident momenta. With the least-squares fit of these data to the effective-range theory expansions, we determine the $S$-wave scattering length $A_s = 2.09 a_0$ and the zero-energy value of pick-off annihilation parameters $^1\!\text{Z}_\text{eff}=0.186$. The present value of $^1\!\text{Z}_\text{eff}$ is in good agreement with the experimental values $0.194(4)$ (PRA ${\textbf 20}$, 347 (1979)) and $0.186(1)$ (JPB ${\textbf 16}$, 4065 (1983)). In addition, the present value of $S$-wave scattering length $2.09a_0$ agrees well with the experimental value $2.1(2)a_0$ estimated with the experimental momentum-transfer cross section (JPB ${\textbf 36}$, 4191 (2003)).