  A method is presented, which employs the density matrix renormalization group
technique in order to construct exact ground state exchange correlation
functionals for models of correlated electron systems coupled to external
reservoirs. The technique is applied to the M-site resonant level model. We
calculate its exact Kubo-conductance, which is available within DMRG, and
compare to the single-particle conductance obtained from Kohn-Sham energies and
orbitals of the exact ground state density functional theory (DFT). It is found
that the position of transport resonances is reproduced essentially exactly,
while deviations in the level broadening can be less than 1% and do not exceed
10%. Our findings lend strong support to a recently held point of view, namely
that approximations in the ground state functionals used in DFT based transport
calculations can lead to drastic errors in transport calculations while
exchange correlation contributions to the induced effective potential tend to
be less significant.
