[gmx-users] Wrong Diffusion/Compressibility for Coarse-grained Water

[X.Periole]

Dear,

All time related observables have to scaled by a factor
of 4 in the CG. This was originaly derived from the
diffusion porperties of Water an lipids. You've found it 
by
yourself on the water diffusion.

XAvier

-----------------
XAvier Periole
MD-Group
University of Groningen
The Netherlands.
-----------------

> I am trying to get my feet wet with the Coarse-grained 
>Force Field 
> developed by Marrink et al.
> I've downloaded the examples of top-File, gro-file and 
>mdp-file for the 
> Water model on Marrink's website.
> 
> Unfortunately, I didn't succeed to reproduce the 
>diffusion constant and 
> isothermal compressibility data found in Marrink's 
>paper:
> "Coarse-grained Model for Semi-Quantitative Lipid 
>Simulations", J. Phys. 
> Chem. B, 2004, 108, 750-760.
> 
> The gro-file contains 1728 W beads.
> I didn't changed the parameters in the mdp-file: 100000 
>time Steps of 50 
> fs.
>>From g_energy, I obtained an isothermal compressibility 
>>of 1.25x10-5 bar-1 
>  instead of 6x10-5 bar-1 found by Marrink.
>>From g_msd, I obtained a diffusion constant of 1.5x10-5 
>>cm2/s instead of 
> 5x10-6 cm2/s found by Marrink.
> The Coarse-grained diffusion constant has to be multiply 
>by 4 (a CG Water 
> bead representing 4 Water molecule) in order to obtained 
>a real diffusion 
> constant of 2x10-5 cm2/s...
> 
> Do you have any idea why I am not able to reproduce 
>Marrink's data?
> 
> Thank you for your help
> 
> Best Regards
> 
> Pierre Verstraete