  As density functional theory conventionally assumes that the density of a
chosen model system (e.g., the Kohn-Sham system) is the same as the exact one,
one might expect that approximations to the exact density introduce
supplementary errors by falsifying the density. In fact, this is not true: by
modelling the exchange-correlation holes for all densities, density functional
approximations avoid this problem. The technique used to show it is a
potential-driven adiabatic connection which hopefully will also permit
constructing new approximations in the spirit of DFT.
