The optimal total density is the sum of individual components ρ B (r) and ρ o A (r) considered as independent variables in the total energy expression. Such a choice of independent variables makes it possible to use the information about the environment of the embedded species obtained from any physical model capable of delivering ρ B (r). Generating ρ B (r) using a wave-function description of the environment is one of several possible choices. Other choices include ρ B (r) obtained from continuum models of the environment 580 , from X-ray diffraction data 581 generated using a library of molecular electron densities, 582 or from a simplified treatment of the electronic polarization. A particular version of FDET, where a noninteracting reference system is used for both ρ A (r) and ρ B (r) and where both densities are subject to optimization, is equivalent to Cortona's formulation of DFT. 584 (4.2.5) Carter :