  A comparative study of two earlier three-state and fifteen-state [Chaudhuri
et al., Pramana- J. Phys., 43, 175 (1994); ibid., Phys. Rev. A, 52, 1137
(1995)] close-coupled treatments of He2+-He single and double charge transfer
collisions is made in this paper with a larger, 27-state close-coupled
calculation. The calculations have been done using the diabatic molecular basis
set used in the earlier work extended by adding excited orbitals leading to
higher excitation channels up to 3s and 3p. For such molecular basis functions
that go to the correct separated-atom limits used in this work, the present
results show that without the inclusion of the electron translation factors
(ETFs) the quantitative cross-section calculations up to velocity ~1.1 a.u.
(~30 keV/amu) are in good agreement with both experiment and other
calculations. This suggests that if ETFs are properly incorporated into the
charge transfer collision studies at low energies this diabatic molecular basis
can be used for benchmark calculations. With the aid of the Wannier's picture
of the ground state correlations, a combined detailed analysis and comparison
has been carried out to find a connection with the dynamic two-electron
correlation picture in charge transfer collision processes which involve
sequential/simultaneous two-electron exchange. If ETFs are included, the model
approach of the present work may open up opportunities to investigate dynamic
two-electron correlation effect in charge transfer ion-atom collision processes
with benchmark accuracy.
  PACS codes: 34.70.+e, 34.20.-b
