[gmx-users] how to simulate at a specific dielectric constant

David van der Spoel spoel at xray.bmc.uu.se
Wed Nov 15 11:55:20 CET 2006

Qiao Baofu wrote:
> Hi David,
>     There is not a simulation parameter that you can change that will
>     influence the dielectric constant of your system. 
> I don't grasp this. If the system has two fluid, one as solute, another 
> as solvent. I had thought that I should change the value epsilon_r in 
> the .mdp file from the default 1 to some other value. Is it wrong?
Yes. If you change epsilon_r the Coulomb interaction will be scaled but 
not the Van der Waals. Hence the balance of forces is disturbed and you 
system will implode (epsilon_r < 1) or explode (epsilon_r > 1). You are 
probably thinking of implicit solvent methds where this might apply, but 
these are not implemented in GROMACS.

David van der Spoel, PhD, Assoc. Prof., Molecular Biophysics group,
Dept. of Cell and Molecular Biology, Uppsala University.
Husargatan 3, Box 596,  	75124 Uppsala, Sweden
phone:	46 18 471 4205		fax: 46 18 511 755
spoel at xray.bmc.uu.se	spoel at gromacs.org   http://folding.bmc.uu.se

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