Subject: DFT polarisation question

From: Steve Greatbanks <[email-masked]>
Date: Thu, 20 Aug 1998 15:43:10 +1000 (EST)

Hi folks,
	my questions relate to the applicability of DFT methods to the
study of biological anion systems. I'll set the scene... 
	I am interested in a biological molecule (folate) which has a
formally doubly anionic group (a glutamate) at one end. Both HF and MP2
(using a range of basis sets, including diffuse functions) predict this to
be the case, giving a charge very close to -2 (so far so good) for the
GLU.  My question relates to my DFT results (both LDA and GGA), which all
give charges of ~-1.4 (rather less than I'd expect). Before I get picked
up on this point, that is for Mulliken charges summed over the residue.
The important point is that I get the same trend (i.e. DFT predicts the
molecule to be far less polar than the HF or MP2) regardless of the charge 
partitioning scheme or basis set used. 

Does other people's experience agree with this?

Are there any theoretical reasons why DFT should struggle to model the
anionic system correctly? 

	Interestingly, if I take the same molecule and embed it in a
protein environment (using point charges), the molecule polarises
substantially at the DFT level and I get the expected double anion, though
the HF and MP2 results change very little (which is more what I'd expect).

	I'd be interested in hearing any opinions people have on this
issue and pointers to any relevant literature I may have missed on the
relative merits of HF and MP2 vs. DFT for anions, particularly their 
relative polarizability. I will, of course summarise any replies I get.

Thanks,

Steve Greatbanks
-- 
+--------------------------------------+--+------------------------------+
|Dr Stephen P. Greatbanks              |  |                              |
|Biochemistry & Molecular Biology      |w:|http://biocomp.anu.edu.au/~spg|
|John Curtin School of Medical Research|e:[email-masked]   |
|Australian National University        |p:|(+61) (02) 6279 8305 (JCSMR)  |
|ACT 0200, Australia                   |  |                              |
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