Direct summation over vibrational levels: Λ doubling in HF+

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Abstract
When different electronic states of a system are mixed by a small perturbation, expressions for molecular properties often involve summations or integrals over vibrational energy levels. A recently developed method for evaluating these expressions exactly from potential curves is tested, and is found to be numerically accurate and computationally convenient. The method does not require vibrational wave functions for the perturbing electronic state, and works for both bound and unbound perturbing states. Λ doubling in HF+ is re‐examined using the new method, and results significantly different from previous calculations are obtained. The new calculated parameters are believed to be more reliable than experimental quantities from high resolution electronic spectroscopy.