Study of the Born—Oppenheimer Approximation with Application to H2

A method for evaluating energy corrections dur to breakdown of the Born—Oppenheimer (BO) approximation in ordinary diatomic molecules has been developed. It is shown that the effective potential for nuclear motion depends upon both the vibrational momentum and the internuclear distance. Energy levels for H2 have been calculated by numerical solution of the Schrödinger equation for motion in this effective potential. The v=0 level is shifted ∼0.2 cm—1 by non-BO terms as compared to the total anharmonicity correction of about 30 cm—1. This shift is an order of magnitude too small to explain the difference between the best theoretical and experimental values of D00.