[gmx-users] PME accuracy

[Bert de Groot]

Vincent Ballenegger wrote:

> In conclusion:  SETTING pme_order TO 4 LEADS TO VERY LARGE ERRORS IN THE
> FORCES.
> 

Somewhat worried by this statement (and the rest of the email), I performed 
two reruns (with pme_order 4 and 6) to calculate the electrostatic part
of the force on an ion in 200 MD snapshots (system: protein/lipids/water, 
approximately cubic box with ~80.000 particles). The differences for each component 
(and hence the sum) were very small: maximally 2 kJ/mol.nm (whereas the forces 
themselves have typical values of +/- 1000 kJ/mol.nm. 

So I don't know if the problems described by Vincent are particular to
his system (unshielded charges as far as I could see), but at least I was 
not able to reproduce the problem for a typical simulation system. 
(at least not to the extent described). 

--

Bert

____________________________________________________________________________
Dr. Bert de Groot

Max Planck Institute for Biophysical Chemistry
Theoretical molecular biophysics group
Am Fassberg 11 
37077 Goettingen, Germany

tel: +49-551-2011306, fax: +49-551-2011089

email: bgroot at gwdg.de
http://www.mpibpc.gwdg.de/abteilungen/071/bgroot
____________________________________________________________________________