RELATIONSHIP OF KOHN-SHAM EIGENVALUES TO EXCITATION-ENERGIES

In Kohn-Sham density functional theory, only the highest occupied eige nvalue has a rigorous physical meaning, viz., it is the negative of th e lowest ionization energy. Here, we demonstrate that for finite syste ms, the unoccupied true Kohn-Sham eigenvalues las opposed to the those obtained from the commonly used approximate density functionals) are also meaningful in that good approximations to excitation energies can be obtained from them. We argue that the explanation for this observe d behavior is that, at large distances, the Kohn-Sham orbitals and the quasiparticle amplitudes satisfy the same equation to order 1/r(4). ( C) 1998 Elsevier Science B.V. All rights reserved.