Calculated 1 J CC values in HC-CH fragments were shown to be very sensitive to the lengths of the C-H bonds, with longer bonds correlating with larger 1 J CC values. This behavior bears directly on the choice of computational methods to obtain geometrically optimized structures for use in 1 J CC calculations. The use of functionals and/or basis sets that provide accurate C-H and C-C bond lengths, both of which strongly determine calculated 1 J CC values, is an important consideration when calculating 1 J CC values. C-H bond lengths extracted directly from conventional X-ray crystal structures are inappropriate for use in these calculations, since these lengths are often held constant at 0.998-1.000 Å during refinement (i.e., are not determined experimentally) to simplify analyses of electron densities.