Self-interaction of electron

There are several things known about the exact behavior of Vxc(r) and it should be noted that the presently used functionals violate many, if not most, of these conditions. Two of the most dramatic failures are (a) in HF theory, the exchange terms exactly cancel the self-interaction of electrons contained in the Coulomb term. In exact DFT, this must also be so, but in approximate DFT, there is a sizeable self-repulsion error (b) the correct KS potential must decay as 1/r for long distances but in approximate DFT it does not, and it decays much too quickly. As a consequence, weak interactions are not well described by DFT and orbital energies are much too high (5-6 eV) compared to the exact values. 

While DFT has been used to successfully model activation barriers for some gas phase systems and well-defined organometallic complexes, there is a concern that DFT methods under-predict the barriers for some ft ee radical abstraction systems. The under-estimation is primarily due to the over-accounting of the self-interaction of electrons in the SCF procedure [75]. Surface-bound free radical intermediates demonstrate less localized unpaired spin due to their strong interaction with the surface. This is likely to reduce some of the problems related to self interaction effects. Below we summarize results for DFT computed barriers for the activation of simple adsorbates over different transition metals. A more thorough and systematic investigation is required to better understand what controls the accuracy in activation barrier predictions on metal surfaces. 

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