S
-wave elastic scattering of 
o
-Ps from 
H2
 at low energy
The confined variational method is applied to investigate the low-energy elastic scattering of ortho-positronium from $\text{H}_2$ by first-principles quantum mechanics. Describing the correlation effect with explicitly correlated Gaussians, we obtain accurate $S$-wave phase shifts and pick-off 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, $A_s=2.06a_0$, and the zero-energy value of the pick-off annihilation parameter, $^1\!\text{Z}_\text{eff}=0.1858$. The obtained $^1\!\text{Z}_\text{eff}$ agrees well with the precise experimental value of $0.186(1)$ (J.\ Phys.\ B \textbf{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 Ps energy below 0.3 eV (J.\ Phys.\ B \textbf{36}, 4191 (2003)).
Confined variational calculation of positronium-hydrogen scattering below the positronium excitation threshold
The confined variational method is used to study $S$-, $P$-, and $D$-wave positronium-hydrogen scattering with the scattering energy between 0.068 and 3.333 eV. Accurate phase shifts and $S$-wave scattering lengths are calculated and compared with other theoretical methods. Existing discrepancies for the $D$-wave phase shifts are resolved. According to collision momentum, total orbital angular momentum, and spin configuration, the distortion effects of positronium are quantitatively studied. Finally, the finite nuclear mass effects are investigated.
Annihilation during positron-hydrogen collisions: Addendum and erratum
The $p$-wave contribution to the positron-hydrogen annihilation rate, in terms of ${Z}_{\mathrm{eff}}$, is corrected from a previous calculation of the authors, thereby removing the discrepancy with later calculations of Chan and McEachran. The present results should be the most accurate available. This opportunity is taken to give thermally averaged positron-hydrogen annihilation coefficients $\overline{Z}(T)$, which are of use in the analysis of solar-flare observations of the positron-electron annihilation line. For reference and experimental purposes we also present curves of the integrated elastic and momentum-transfer (i.e., diffusion) cross sections, as well as effective cross sections which account for the finite acceptance angle of the detector.