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)).
Effective range description of annihilating collisions
Effective range theory is applied to the description of annihilation processes in positron-atom scattering. It is found that the result Z(eff)(k)similar tok(2)l[a+o(k(2))] is obeyed for the lth partial wave irrespective of whether the scattering potential includes long-range polarization potentials or not.
S-wave positron scattering by hydrogen atoms
The S-wave scattering of positrons by hydrogen atoms is studied using the coupled static approximation with 26 correlation functions for energies up to the excitation threshold of hydrogen. Up to a certain energy, the formulation satisfies a bound principle. Below the positronium formation threshold, the calculated phase shifts are very close to those of Bhatia et al (1971). Above this threshold, the R matrix elements and the cross sections are very different from those of other earlier calculations. No resonances were detected in a search with energy spacing of about 0.02 eV over the region just below the excitation threshold.