The binding energy and equilibrium constant for the endohedral He@C-60 compound have been determined from ab initio and density functional (DFT) calculations. Very large grids for the numerical integration are necessary to converge the DFT results to within 0.1 kcal/mol. Gradient-corrected DFT methods incorrectly predict He@C-60 to be less stable than He+C-60. At the highest ab initio level employed, i.e., second-order Moller-Plesset perturbation theory (MP2) with extended basis sets and counterpoise corrections, He@C-60 is bound by 2.0 kcal/mol. The equilibrium constant for He incorporation into C-60 has been evaluated from Hartree-Fock and DFT interaction potentials adjusted to reproduce the MP2 binding energy. Computed equilibrium yields at 3000 atm and 900 K exceed 10%, compared with 0.1% observed in the experiment, which indicates that suitable catalysts could increase the observed yield significantly.