Non-Born-Oppenheimer confined variational calculation of low-energy Ps-H-2 scattering

Using the confined variational method, the low-energy S-wave elastic scattering of positronium from molecular hydrogen is studied nonadiabatically at incident energy up to 0.13 eV. Accurate S-wave phase shifts, scattering lengths, and cross sections are calculated and compared with other theoretical and experimental values. Using the modified effective-range formula, the S-wave scattering length is determined to be 1.97 Bohr radii, which is about 2.5% smaller than that of the confined variational calculation under the fixed-nucleus approximation. The consistency of the present cross sections with those from the angular correlation of annihilation radiation experiment indicates that the results of Doppler broadening spectroscopy might be problematic. We also discuss the distortion effects of positronium and molecular hydrogen.