Is the Fukui function a right descriptor of hard-hard interactions?

To verify whether the maximum or the minimum Fukui function site is better for protonation reactions or an altogether different local reactivity descriptor, viz., the charge is necessary, we calculate the Fukui functions (using a finite-difference approximation as well as a frozen-core approximation) and charges (Mulliken, Hirshfeld, and natural population analysis schemes) of several hydroxylamine derivatives, their sulfur-containing variants, and amino acids using B3LYP/6-311 G(d,p) technique. While the Fukui functions provide the wrong selectivity criterion for hard-hard interactions, the charges are found to be more reliable, vindicating Klopman's idea. It is transparent from the present results that the hard-hard interactions are better explained in terms of charges, whereas the Fukui functions can properly account for soft-soft interactions known to be frontier-controlled.