Hi Boaz,
For the most reasonable values, make sure that when you color by electrostatic potential, you use the “Surface offset” (default 1.4 Angstroms) to color the solvent-excluded surface by values measured some distance outside the surface.  The default distance is a “water” probe radius, approximating the location of the solvent-accessible surface. 

The (displayed) solvent-excluded surface is about that much closer to the atomic centers, the locations of the point charges used to calculate the ESP, than the solvent-accessible surface (not displayed).

Another potential foible is using some all-atom charge model on a structure that doesn’t have any hydrogens, so the whole thing is overwhelmingly negative.  However, if you are also seeing large magnitudes on positive regions, that probably isn’t an issue for you.
I hope this helps,
Elaine
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Elaine C. Meng, Ph.D.                       
UCSF Chimera(X) team
Department of Pharmaceutical Chemistry
University of California, San Francisco

> On Feb 3, 2018, at 3:26 PM, Boaz Shaanan  wrote:
> 
> Hi,
> I'm seeing some weird values when I use this option, for example high +ve value when touching a very red area or -ve on a very blue area ( I use 3 colours scheme). I'm running linux alpha version v. 1.13 (build 41653) from 5 Jan 2018. The electrostatic potential was calculated in delphi. I have a vague recollection that this feature behaved more reasonably in the past when I used it (I could of course be wrong).
> Thanks for your help.
>         Boaz