Comentario de grupos funcionales

Jonatan encontró el siguiente comentario acerca de grupos funcionales y la densidad electrónica. Lo coloco a continuación:
"It has been shown that despite the non-analytical character of ρ (because of the spikes) the function ρ has the following remarkable property: if we know ρ even in the smallest volume, this determines ρ in the whole space. A by-product of this theorem is of interest for chemists. Namely, this means that a functional group in two different molecules or in two conformations of the same molecule cannot have an identical ρ characteristic for it. If it had, from ρ in its neighbourhood we would be able to reproduce the whole density distribution ρ(r), but for which of the molecules or conformers?
Therefore, by reductio ad absurdum we have the result: it is impossible to define (with all details) the notion of a functional group in chemistry."
P.D. Walker, P.G. Mezey, J. Am. Chem. Soc. 116 (1994) 12022