Every property of a molecule is given by the sum of the contributions from
each of its constituent atoms or groups, the groups being defined as proper
open systems. The observation of "experimental group additivty" requires t
hat in addition to the propel-ties of the groups being additive, the group
and its properties be transferable from one molecule to another, different
molecule. It is shown that such transferability of a group and its properti
es is in general, only apparent, being the result of compensatory transfera
bility wherein the changes in the properties of one group are compensated f
or by equal but opposite changes in the properties of the adjoining group.
These compensating changes are in some cases vanishingly small, but even wh
en the energy changes are in excess of 20 kcal/mol, the experimental heat o
f formation is still predicted to be additive to within 0.1 kcal/mol, The o
peration of compensatory transferability is illustrated for the linear homo
logous series of hydrocarbons and polysilanes and for the formation of pyri
dine from fragments of benzene and pyrazine. The properties considered are
the energy and the delocalization of the electrons, the former determined b
y the one-electron density matrix and the latter by the pair density. It is
shown that the transferability of the degree of localization of the electr
ons to a given group, a property of the pair density, is a result of the co
nservation of the delocalization of its electrons over the remaining groups
in the molecule. The results presented here emphasize the important observ
ation that all the properties of a proper open system-a functional group-wh
ether determined by the first-order density matrix, the pair density, or fi
eld-induced charge and current densities, are all functionally related to i
ts form in real space, that is, to its distribution of charge.