Variational calculation of positronium-helium-atom scattering length

We present a variational basis-set calculational scheme for elastic scattering of positronium atom by helium atom in S wave and apply it to the calculation of the scattering length. Highly correlated trial functions with appropriate symmetry are used in this calculation. We report numerical result for the scattering length in atomic unit: $(1.0\pm 0.1)a_0$. This corresponds to a zero-energy elastic cross section of $(4.0\pm 0.8)\pi a_0^2$.