If the chromophores are relatively far apart, the K term in equation ( ) is often also neglected, which is equivalent to the usage of Hartree products of site states instead of antisymmetrized products of site states. Another issue of the original FEM is the fact that the site states are obtained by a series of electronic structure calculations of the isolated sites, which is often referred to as the frozendensity approximation (FDA). This actually would not be an approximation, if the complete set of site states was provided on each chromophore and if a complete excitonic basis was employed. However, because the first requirement is impossible in practice (one always has a finite number of site states), and since the second requirement is not the case in the FEM (the Frenkel basis contains only GS and LE products), in practice the FDA is an approximation in the FEM. Thus, the FDA can significantly affect the performance of the FEM, especially if the fragments are close and strongly interacting. This is because the electronic wave functions of the isolated sites do not necessarily resemble the wave function of the full system in the vicinity of the respective sites, due to the perturbation one fragment causes to the other. Therefore, if the FDA is employed, one would have to provide a very large set of site states on each site in order to recover the deformations of the wave functions that happen when the chromophores are joined to the full system.