Ground state energy of the hydrogen molecule, at the equilibrium internuclear separation, has been computed in the Born-Oppenheimer approximation. The generalized James-Coolidge expansion has been used for the variational wave function. The expansion length has been systematically extended, and from the results a limiting value of the energy has been obtained which is by 0.009 cm(-1) lower than the best variational value. Improved agreement is obtained with experimental values of the ionization potential for H-2 and HD. For D-2 the disagreement between theoretical and experimental values of the ionization potential seems to be significant.