A semi-empirical method for the calculation of spin—orbit splitting in degenerate electronic states of linear polyatomic molecules

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A semiempirical method for the calculation of spin—orbit coupling constants in linear polyatomic molecules is discussed. The method uses the readily available output from a CNDO/2 calculation and a set of effective atomic coupling constants obtained from observed spectroscopic data on atoms. Two procedures for obtaining the spin—orbit splitting are discussed: one based on a closed shell CNDO/2 calculation and one based on an open shell calculation. The average error for the molecules studied was 6.8% and 7.7% for the two procedures used in the calculations, the easier closed shell procedure giving the lower percentage error. It should be noted that the largest errors occurred in the case of diatomic molecules where the error would be expected to be the greatest. Thus, the average error involved in the values of the spin—orbit splitting for polyatomic molecules is much less than 3.7% for the closed shell procedure. The method used appears to be extendable to non-linear molecules in which the only atoms off the principle symmetry axis of the molecules are hydrogen. The method should be useful in the interpretation of photoelectron spectra.