Re: total charge of the system - rounding error ?

From: JC Gumbart (gumbart_at_ks.uiuc.edu)
Date: Mon Apr 15 2013 - 12:20:09 CDT

You have 75x as many atoms as that post. Your error is commensurate with this fact. Don't over-think things.

On Apr 15, 2013, at 12:59 PM, Fabien Cailliez wrote:

> Dear all,
>
> I noticed recently that the total net charge given at the beginning of the
> output of namd for my systems is not exactly 0, even for a neutralized system.
> For example, for one such system (around 110000 atoms) I get :
>
> Info: 4 ATOMS IN LARGEST MIGRATION GROUP
> Info: TOTAL MASS = 670097 amu
> Info: TOTAL CHARGE = 8.46386e-06 e
> Info: MASS DENSITY = 0.961233 g/cm^3
> Info: ATOM DENSITY = 0.0948485 atoms/A^3
>
> I found in the archive a message about such kind of problem
> (http://www.ks.uiuc.edu/Research/namd/mailing_list/namd-l.2006-2007/3027.html
> ), that said this was due to rounding error. However, in the post, the charge
> was only around 1e-8, whereas in my systems, it is around 1e-5. It seems to me
> that this is quite a big rounding error. Am I wrong? Is there an accepted
> value for rounding error in that sort of case?
>
> Thanks,
>
> Fabien
>
> --
> -------------------------------------
> Fabien Cailliez
> Laboratoire de Chimie Physique, CNRS UMR8000
> Bat 349 - Université Paris-Sud 11
> 91405 Orsay Cedex
> tel : +33 (0)1 69 15 32 49
> -------------------------------------
>

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