[gmx-developers] calculating dielectric constant

[Berk Hess]

Qiao Baofu wrote:

> Hi all developers,
>
> I have posted this problem on the gmx-user list. David gave me some 
> suggestions, but the problem has not been solved completely.
>
> I simulated an ionic liquid [mmimCl], 981 molecules for 1.5 ns, every 
> 5 time steps saved.  In the .mdp file, I used PME:{ coulombtype  = 
> pme; epsilon-r = 1}, the simulation temperatuer is 425K.
>
> Then I used  "g_dipoles -temp 425 -f -s -enx" to calculate the the 
> dielectric constant, the result is: Epsilon = 826.496. I checked the 
> epsilon.xvg, in which the result is converged to some extent with some 
> fluctuations at the end.  I think the result is too big. Even though I 
> have no exerimental data for [mmimCl] at hand, I have some reported 
> data of some other ionic liquids, 10-20.
>
> Is there anyone who can give some suggestions? Great thx!
>
I recently found out that the dipole calculation is incorrect for periodic
system with charged groups with net charge.
It is not trivial to determine the dipole for a periodic system when ions
are present.
Does anyone know a reference for this?

Berk.