total charge of the system - rounding error ?

From: Fabien Cailliez (
Date: Mon Apr 15 2013 - 11:59:33 CDT

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: 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
), 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?



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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