The confined variational method is applied to investigate the low-energy elastic scattering of orthopositronium from H2 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 AS=2.02a0 and the zero-energy value of the pickoff annihilation parameter, 1Zeff=0.1839. The obtained 1Zeff agrees well with the precise experimental value of 0.186(1) [G. L. Wright, J. Phys. B 16, 4065 (1983)10.1088/0022-3700/16/21/027] and the obtained AS agrees well with the value of 2.1(2)a0 estimated from the average experimental momentum-transfer cross section for positronium energy below 0.3 eV [F. Saito, J. Phys. B 36, 4191 (2003)10.1088/0953-4075/36/20/011].
Bibliographical noteKAUST Repository Item: Exported on 2020-10-01
Acknowledgements: J.-Y.Z. was supported by the Hundred Talents Program of the Chinese Academy of Sciences. Z.-C.Y. was supported by the NSERC of Canada and in part by the CAS/SAFEA International Partnership Program for Creative Research Teams. The research reported in this paper was supported by funding from King Abdullah University of Science and Technology. The authors are thankful for the computational resources provided by the ACEnet of Canada.