The values of <L2> in diatomic molecules: implications for adiabatic and molecular fine structure calculations

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Abstract
The possibility of using calculated ab initio values of <L 2> to assess convergence of second order sums in molecular fine structure parameters is investigated for several diatomic molecules with 3Σ- ground states. Values of <L 2> for a number of molecules in 1Σ+ states have also been obtained. The calculated values of <L 2> are in general very large, especially for non-hydride molecules, and they show a marked dependence on the internuclear separation R. The implications of these large values of <L 2> are discussed both for the calculation of molecular fine structure parameters (where the convergence criterion is seen to be of little use) and for the adiabatic corrections often needed in discussing isotopic effects. In this latter context, ab initio values of have also been calculated and found to be an important term in adiabatic corrections, contrary to some assertions in the literature. The implications of the strong dependence of <L 2> on the internuclear separation are also discussed.