Subject: RE: DFT:KS exact?

From: "Smith JA (Jack)" <[email-masked]>
Date: Mon, 23 Nov 1998 14:28:22 -0500

> in Kohn-Sham theory, the density is written as a sum over orbitals
> squared. The occupation numbers of these orbitals are 1 or 0. My
> question:
> can the density of _any_ molecular system be written in this way? Or,
> in
> other words, are there systems where one needs fractional occupation
> numbers to represent the exact density? If the answer to the latter
> question is 'yes' then one might question if KS-DFT yields _in
> principle_
> (if the exact Exc is known) the exact density.
> 
> _any_ comments are most welcome,
> 
There is a direct connection (equivalence) between DFT/KS with exact
exchange and Grand Canonical HF, where the density operator is formally
expressed in terms of fractional occupation numbers representing a Grand
Canonical ensemble.  See the following: 

  (1) Jorgensen and Ohrn, Phys Rev A, 8(1), p112 (1973).
  (2) Abdulnur, Linderberg, Ohrn and Thulstrup, Phys Rev A, 6(3), p889
(1972).

and compare with Slater's Hyper-HF
  (3) Slater, Mann, Wilson and Wood, Phys Rev, 184(3), p672 (1969).

- Jack

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 Jack A. Smith             ||
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