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Re: [lammps-users] Long range ewald force output
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Re: [lammps-users] Long range ewald force output

From: "Moore, Stan" <stamoor@...3...>
Date: Fri, 8 Dec 2017 22:27:26 +0000

Why not just use “pair_modify compute no” on a snapshot and then dump out the forces on the atoms? Others wanted electric field, not force, so they needed to hack the code.




From: teddy baker [mailto:edwardbaker86@...24...]
Sent: Friday, December 08, 2017 11:29 AM
To: LAMMPS Users Mailing List <>
Subject: [EXTERNAL] Re: [lammps-users] Long range ewald force output


I made a mistake previously, the box has side length three nanometers (not angstroms of course), sorry about that.  Also, the gewald parameter I am using is .25, so that the length scale of interaction is 4 angstroms (units real).  


On Fri, Dec 8, 2017 at 1:24 PM, teddy baker <edwardbaker86@...24...> wrote:


   In my group, we are interested in long range Coulomb forces and a modified Ewald summation technique, so I am studying the long range Coulomb forces.  I have seen this post for outputting the long range Coulomb forces, and I have written my own code to do it.  I also have my own algorithm that should compute the same forces, but I am getting different results when I look at the autocorrelation functions (same order of magnitude, but different decay behavior).  It is certainly possible that my algorithm is faulty, but I want to ask to make sure I am doing things correctly on the Lammps side. 

   In order to output the long range force, I modified the fieldforce_ik function in the pppm.cpp file, and outputted the qfactor*ekx, y and z on a tagged oxygen particle, which as far as I know should be the long range force on the particle.  I also used the gewald command to set the gewald parameter to a value that would be appropriate to compare to my results.  I have attached the autocorrelation functions that I computed in this way, which you will see are very different (Ewald split are the forces outputted from LAMMPS).  This is a simulation of bulk water in a box of sidelength 3 angstroms, and the tagged particle is an oxygen.  Any help in understanding this issue would be greatly appreciated.  Thank you!