Correct dissociation limit for the exchange‐correlation energy and potential

Abstract

AbstractLocal and semilocal functionals like the local density (LDA) and generalized gradient approximations (GGA) fail to provide correct behavior during the dissociation of electron pair bonds: the asymptotic potential energy curve is wrong, the dissociation in the heteronuclear case leads to partially ionic instead of neutral atoms, and the time‐dependent density functional theory (TD‐DFT) excitation energies behave incorrectly upon bond stretching. We demonstrate that such errors can be avoided by the use of an orbital‐dependent exchange‐correlation (xc) functional. Such a functional needs to incorporate, in contrast to the popular exact exchange functional, the electron correlation effects. The demonstration is given for the electron pair bond of a diatomic molecule AB with just one orbital on A and one on B. This simple two‐orbital model affords an analytical inversion of the density response function, which is used to show that the orbital‐dependent xc functional described previously, possesses the correct dissociation limit for the xc energy and potential. The correct potential (xc hole potential in the symmetric case AB, for example, for the dissociating H2 molecule) properly describes strong nondynamical (left–right) electron correlation, while in the heteroatomic case (A ≠ B) it develops an additional “counter‐ionic” step behavior in its response part, which is needed to ensure proper covalent dissociation of the AB bond (and proper covalency at long bond distances). © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006