Elimination of the fractional dissociation problem in Kohn-Sham DFT using the ensemble-generalization approach

Many approximations within density-functional theory (DFT) spuriously predict that a many-electron system can dissociate into fractionally charged fragments. Here, we revisit the case of infinitely separated diatomic molecules, known to exhibit this problem when studied within standard approximations, including the local spin-density approximation (LSDA). We apply the recently suggested ensemble-generalization to LSDA (eLSDA) [1,2] and find that fractional dissociation is eliminated in all systems examined. The eLSDA Kohn-Sham potential develops a spatial step, associated with the emergence of the derivative discontinuity in the exchange-correlation energy functional. This step, predicted in the past for the exact Kohn-Sham potential and observed in some of its more advanced approximate forms, is a desired feature that prevents any fractional charge transfer between the system's fragments. Our findings show that, if appropriately generalized for fractional electron densities, even the most simple approximate functionals correctly predict integer dissociation [3]. [1] E. Kraisler, L. Kronik, Phys. Rev. Lett. 110, 126403 (2013) [2] E. Kraisler, L. Kronik, J. Chem. Phys. 140, 18A540 (2014) [3] E. Kraisler, L. Kronik, submitted.