Extrapolated Born–Oppenheimer energy for the ground state of the hydrogen molecule

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Abstract
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 H2 and HD. For D2 the disagreement between theoretical and experimental values of the ionization potential seems to be significant.