Most of previous perturbational calculations of nuclear spinmdash;spin coupling constants either made the mean excitation energy approximation or took account of only a few lower excited states. We have examined these approximations by including higher excited states, taking as an example the hydrogen fluoride molecule, for which several molecularhyphen;orbital wavefunctions are available. Contributions from the orbital and spin dipolar terms besides the Fermi contact term are also calculated with due regard to twohyphen;center integrals. The present results indicate that the calculated values of the coupling constant are extremely sensitive to the wavefunctions employed, and the estimates by these approximate wavefunctions available at present cannot be fully trusted as yet.
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