Abstract
The confined variational method is applied to investigate the low-energy elastic scattering of orthopositronium from H-2 by first-principles quantum mechanics. Describing the correlations with explicitly correlated Gaussians, we obtain accurate s-wave phase shifts and pickoff annihilation parameters for different incident momenta. By a least-squares fit of the data to the effective-range theory, we determine the s-wave scattering length A(s) = 2.02a(0) and the zero-energy value of the pickoff annihilation parameter, (1)Z(eff) = 0.1839. The obtained (1)Z(eff) agrees well with the precise experimental value of 0.186(1) [G. L. Wright et al., J. Phys. B 16, 4065(1983)] and the obtained A s agrees well with the value of 2.1(2)a(0) estimated from the average experimental momentum-transfer cross section for positronium energy below 0.3 eV [F. Saito et al., J. Phys. B 36, 4191 (2003)].