Ab initio calculations using Hartree-Fock (HF) and second order Moller-Plesset perturbation (MP2) theoretic methods using the 6-31G basis set have been used to study the interaction between H+, H, He, Li+ and H-2 with C-60 fullerene. The barrier for penetration of the guest species through the center of the hexagon of the cage is reported. There is a substantial change in the HOMO-LUMO energy gap for the endohedral complex of C-60 fullerene when the proton or hydride ion is encapsulated. The calculated HOMO-LUMO energy gap for the endohedral complex is correlated with the orbital energy of the guest species. The interaction of the guest species with the host is examined by a critical point analysis using Bader's theory of atoms in molecules. The effect of the guest species on the electrostatic potential inside and outside of the C-60 cage is also discussed. (C) 2008 Elsevier B.V. All rights reserved.