Subject: CCL: Re: Heme charges

From: Konrad Hinsen <hinsen*at*cnrs-orleans.fr>
Date: Tue, 5 Aug 2003 23:01:56 +0200

On Tuesday 05 August 2003 18:52, Emad Tajkhorshid wrote:

> We just optimized the geometry of a porphyrin ring using DFT and
> calculated ESP charges for the molecule. The iron carries a significant
> amoount of positive charge according to population analyses done on the
> system (it is formally 2+), but the ESP charge for it is negative! The
> iron atom is completely burried inside the molecule, and that may be one
> reason of getting this strange result. Has anybody else had similar
> problems? Any comments, references, or suggestions are
> appreciated.

The result of an ESP calculation depends a lot on how the evaluation points on 
the surface are chosen. If you have chosen them in such a way that they don't 
probe the ion at all, then the result you get for the ion is merely a number, 
a number that improves the fit of the electrostatic potential at the 
evaluation points, with no particular physical interpretation. Depending on 
your application, that may well be what you want.

Other options:

1) Try to choose other evaluation points (but don't put any of them too near 
to an atom). This may be difficult for heme.

2) Fix the ion charge in whatever way you consider reasonable, and do the ESP 
fit only for the remaining partial charges.

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