Denominator Correction to the Van Vleck Transformation: Internal Rotation Problem

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Abstract
A method is given for obtaining the exact denominator correction to the Van Vleck transformation whether or not the zero-order energy is known in closed form. The denominator correction can be expressed in terms of a commutator involving the perturbation and the zero-order Hamiltonian. The denominator correction is given for a general case, and the correct denominator correction is given for the separation of internal and over-all rotation. The effect of the denominator correction on the structure of acetaldehyde and its implications with regard to asymmetric methyl groups is discussed.