Virtual Winter School on Computational Chemistry
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Evert Jan Baerends
Theoretical Chemistry,Vrije Universiteit, Amsterdam, The Netherlands
We will first review many wrong statements in the literature on the nature and the (lack of) physical meaning of Kohn-Sham orbitals. Next the nature of the occupied KS orbitals, and their advantages over Hartree-Fock orbitals are highlighted.Then we address orbital energies. Exact KS orbitals have many virtues:
Unfortunately, orbital energies in the common LDA and GGA calculations are very wrong: they are typically 5 eV (more than 100 kcal/mol) higher than the exact Kohn-Sham orbital energies, an error that would be completely unacceptable in total energies. We will first analyze where this error comes from - it is not due to wrong asymptotic behavior of LDA/GGA potentials, or to a “self-interaction error” but it is caused by erroneous density dependence of the standard Exc[ρ] functionals, hence a wrong derivative (= potential). We will demonstrate that approximate potentials can be formulated that have similar good properties for ionization and excitation energies as the exact KS potential .
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