Correct dissociation limit for the exchange-correlation energy and potential



Local 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[DOUBLE BOND]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[BOND]B bond (and proper covalency at long bond distances). © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006