Developments at the front of wave function and density functional theories: The screening charge density

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• Nikitas Gidopoulos (STFC,RAL,ISIS)
• Thursday 12 May 2011, 13:45-15:00
• Theory Library.

If you have a question about this talk, please contact Dr Dimitri M Gangardt.

A frequently encountered statement is that Density Functional Theory has revolutionized the way we perform electronic structure calculations. A fact is that DFT is major player in the field electronic structure theory for longer than 4 decades in physics and in the last 2-3 decades in chemistry.

Nowadays the physics and chemistry communities believe that after exhaustive investigations for so long time in the foundations of DFT , we understand the theory deeply and for example it is undisputed that the Kohn-Sham (KS) Slater determinant cannot be expected to represent in any way the interacting ground state (compared with other noninteracting states, such as the Hartree-Fock (HF) Slater determinant), since the purpose of the KS state is to yield the exact electronic density and no more. Such views hold even stronger in the traditional communities employing wave-function methods, where senior scientists have little affinity to the language and methods of DFT .

In the talk, I shall give a brief review of DFT allowing some time for discussion over physical meaning, if any and if there is interest. Then, I shall present a simple proof of a basic theorem that bridges DFT with wave function methods [1]: the KS potential turns out to be the optimal potential in a straightforward and unconstrained Rayleigh Ritz energy minimization. In this way the exact density property of the KS system arises from the energy optimization rather than from Walter Kohn’s intuitive but ad hoc construction. It turns out that the exact KS state is as variationally “energy-optimal” as the HF state.

In the second half of the talk, the related Optimised Effective Potential (OEP) theory will be reviewed briefly. OEP leads naturally the exact treatment of exchange in KS theory and about a couple of decades ago, it raised high expectations that it would also lead to accurate correlation energy expressions and accurate correlation energy potentials. So far this has not happened, but I shall use OEP techniques to show how common approximations in DFT , such as the local density approximation (LDA), can be improved by removing self-interaction errors in the approximate potential. For example calculations on simple atoms will show that the approximate LDA KS potential felt by an electron decays correctly as 1/r. Crucial in this development is the introduction of the notion of the screening charge density. Depending on time and interest, the interpretive properties of the screening charge density and a theorem on it will be presented and discussed.

NIG, Phys Rev A 83, 040502(R) (2011)

This talk is part of the Theoretical Physics Seminars series.

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