The "solvation" of a H3+ molecule-ion by H2 molecule(s) and/or a He atom was examined in ab initio electronic structure calculations. Binding energies and geometrical parameters were determined for the ground electronic states of H"+ ( = 3,5,7,9,11) and HeH"+ ( = 3, 5,7,9) cluster ions. Floating spherical Slater orbitals (FSSO) were employed with full single and double configuration interaction (Cl). All of the clusters studied were found to be weakly bound (with respect to removal of "solvent" species). Inclusion of configuration interaction had a major effect; for > 5, it increased the energy required to remove an H2 molecule from H"+ by ~35% and that to remove the He atom from HeH"+ by ~300-400%. The net solvation energy per H2 (~3.5 kcal mol"1 for the first three solute molecules, ~1.5 kcal mol'1 for the fourth) was found to be about threefold larger than for He.

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