The importance of an accurate target wavefunction in variational calculations for (e+-H2) scattering

Using the complex Kohn method, we have calculated variational values of phase shifts and the annihilation parameter, Zeff, for the elastic scattering of positrons by molecular hydrogen. Our results are sensitive to small changes in the accuracy of the wavefunction representing the target hydrogen molecule. We have developed a systematic approach to demonstrate that, at low positron energies, there are particular forms of the Kohn trial wavefunction for which the results of variational calculations are not reliable, even when the target wavefunction accounts for as much as 96.8% of the correlation energy of H2. We find that reliable results can be recovered if our calculations are extended to admit more sophisticated target wavefunctions accounting for 99.7% of the correlation energy. Remaining discrepancies between theory and experiment are briefly discussed.