Remarks on the Use of Approximate Wavefunctions for the Second-Order Perturbation Energy

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Abstract
The use of approximate ground‐state wavefunctions in the calculation of interaction energies by second‐order perturbation theory (e.g., calculation of spin—spin coupling constants in nuclear magnetic resonance) has been investigated. It has been found that the approximate zero‐order ground‐state wavefunction must satisfy a stringent condition if reliable results are to be obtained. The condition is that the approximate ground‐state energy must lie below the true energy of the first excited state. This condition is rarely met for molecules more complicated than hydrogen, and hence reasonable results can be obtained only by a fortuitous cancellation of errors.