  We have proposed a method for correcting the Kohn-Sham eigen energies in the
density functional theory (DFT) based on the Koopmans theorem using Wannier
functions. The method provides a general approach applicable for molecules and
solids for electronic structure calculations. It does not have any adjustable
parameters and the computational cost is at the DFT level. For solids, the
calculated eigen energies agree well with the experiments for not only the band
gaps, but also the energies of other valence and conduction bands. For
molecules, the calculated eigen energies agree well with the experimental
ionization potentials and electron affinities, and show better trends than the
traditional Delta-self-consistent-field (?SCF) results.
