[gmx-users] diffusion coefficient with g_msd

XAvier Periole x.periole at rug.nl
Fri Jun 5 09:34:12 CEST 2009

On Jun 4, 2009, at 4:18 PM, Sarah Witzke wrote:

> Dear gromacs users,
> I have done several simulations with small lipophilic, molecules  
> diffusing into a DMPC bilayer.
> I would like to calculate the diffusion coefficient of the molecules  
> inside the membrane, and therefore I looked at g_msd. The manual  
> (version 4.0) states on p. 250 (manual pages) that g_msd uses the  
> Einstein relation. Reading "Molecular Modelling: principles and  
> applications" 2.ed. by Andrew Leach it is explained that the "for  
> calculating the diffusion coefficient the mean-squared distances  
> should not be limited by the edges of the periodic box. In other  
> words, we require a set of positions that have not been translated  
> back into the central simulation cell." This makes sense since I try  
> to calculate the distance a molecule diffuse.
> I haven't been able to find any mention of how gromacs handle this  
> either in the manual or on the wiki page. The search function on the  
> old webpage directs me to the new webpage, which doesn't give any  
> results for "g_msd" or "diffusion coefficient" or alike, so my  
> apologies for asking a question I could have found the answer to I  
> old emails. When I google I get some of the old emails on g_msd from  
> the gromacs website, but I can only read some of them (the others  
> direct me to an empty page in the new webpage).
> The first question is whether I can use my .xtc file made with this  
> command from the original .trr file:
> trjconv -f dmpclim32-all.trr -novel -center -fit rot+trans -pbc  
> whole -s dmpclim32-1.tpr -o dmpclim32-all.xtc  (for fitting and  
> centring the DMPC group is used)
> I would guess I could not, but should I then do the command again  
> creating a new .xtc file, but without using -center and -pbc whole?
You are right. You can not use this trajectory. The fitting procedure  
will alter the diffusion of the system overall.

If you had to do something it would be to use -pbc nojump to allow the  
molecules to freely diffuse.
g_msd takes care of the periodicity of your system. So you do not have  
to fit or anything.
> My second question is why g_msd has the option -rmcomm to remove COM  
> motion? How is g_msd created so that the diffusion coefficient can  
> be calculated if COM motion is removed?
The removal of the center of mass of the ENTIRE system is necessary  
before calculating the msd.
This is generally done during the run using options in the mdp file.
Which version you are using? I'd never seen the -rmcomm before? You  
can try to determine your
msd with and without the option and see if it affects your result.  
This would depend on the removal
or not of the COM motion in your simulation.

> Thank you very much for your time,
> Sarah
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