[gmx-users] r_list and r_coulomb values while using PME

[OZGE ENGIN]

'The force is not the derivative of the potential', is it due to the types of calculations performed in PME, using of FFT for reciprocal sum? Because it does not matter while using reaction-field method. 
Sorry, this phenomenon is  a little complicated to me :) 

Thanks for your attention!

Oz.

-----Original Message-----
From: David van der Spoel <spoel at xray.bmc.uu.se>
To: Discussion list for GROMACS users <gmx-users at gromacs.org>
Date: Sat, 01 Sep 2007 13:37:05 +0200
Subject: Re: [gmx-users] r_list and r_coulomb values while using PME

OZGE ENGIN wrote:
> Hi all,
> 
> I can not understand why it is required to take same values for  both r_list and r_coulomb parameters when using PME for calculation of electrostatic interactions?
> 
> I read archieve, and gromacs manual. In archieve, similar question had been posted, but not replied yet.
> 
> Thanks in advance

if rlist < rcoulomb you  will have a twin range interaction, meaning the 
atom pair interactions for distance between these two will be computed 
every nstlist steps only, while the interaction beyond rcoulomb will be 
computed at each time step. this will lead to bad energy conservation, 
since the force is not the derivative of the potential.
> 
> Oz.
> 
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-- 
David van der Spoel, Ph.D.
Molec. Biophys. group, Dept. of Cell & Molec. Biol., Uppsala University.
Box 596, 75124 Uppsala, Sweden. Phone:	+46184714205. Fax: +4618511755.
spoel at xray.bmc.uu.se	spoel at gromacs.org   http://folding.bmc.uu.se
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