[Pw_forum] point charge-spin

[Giovanni La Penna]

> I'm a bit confused...
> due to Gauss theorem, doesn't all the [A] procedure boil down to be
> the integral of the
> ionic+electronic charge inside the vdW radius ...

Using for the fitting ALL the points of the
electrostatic potential, one would end with
the reconstruction of the potential due to
the ionic charges + the diffuse valence electrons.
And the Gauss theorem would work (consistently
with the numerical accuracy), whatever
the surface is.

I understood that using in the fitting a sub-set of points
(like those laying on a surface)
of electrostatic potential means using a different
estimate of the electric field on the spanned surface.
This latter estimate satisfies the Gauss
theorem with different charges.

The ESP idea (see the Kollman & Co. citations
in Bayly et al 1993) was to use ONLY the
points on (or around) the molecular solvent accessible surface.
Empirically, the more points you collect away from the surface and
the closer to the real (original) charge distribution you get.
Apparently, the worse is the definition of the
electric field on the given surface (due to the
inaccuracy of the potential in providing a good
field by finite differences) and the better are the point charges
(at least for chemists!, but I would say for
describing intermolecular interactions, the main
goal of the ESP/RESP procedure).

Maybe I increased the confusion.
As I said, I gave a recipe for providing
results consistent with another recipe.

                Giovanni

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Giovanni La Penna - National research council (Cnr)
Institute for chemistry of organo-metallic compounds (Iccom)
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