Subject: DFT and dispersion forces

From: [email-masked]
Date: Sun, 17 Oct 1999 17:40:45 +0200 (MSZ)

Just to get the discussion on formal grounds again:

imagine two argon atoms at both sides of a "wall" (i.e., an infinite potential
barrier). Since both densities do not overlap, the DFT exchange-correlation
hole at a reference point at one side of the barrier cannot distinguish if
at the other side there are MOVING electrons or a static charge distribution.
Hence, while induction forces etc. are present in the DFT calculation, all
dispersion interactions are necessarily absent (yes there is dispersion
force between the two atoms).

You will find the example esoteric, but two argon atoms at a distance where
their densities practically do not overlap are just the same.

Who can come up with a functional that gives dispersion in these cases:
it is not impossible but necessarily non-local: if the reference point is on
one side, the density at the other side must be taken into account as well.
A gradient expansion ist not sufficient because of the convergence radius!

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