[gmx-users] electronic fields

[Li, Tong]

Dear all,

I am trying to apply an electric field to my system, including some polymer molecules and ions. By using '-field' in mdrun, I can see the electric field is applied:

@    title "Applied electric field"
@    xaxis  label "Time (ps)"
@    yaxis  label "E (V/nm)"
@TYPE xy
         0           1           0           0 #FIELD
     0.001           1           0           0 #FIELD
     0.002           1           0           0 #FIELD
     0.003           1           0           0 #FIELD
     0.004           1           0           0 #FIELD
     0.005           1           0           0 #FIELD

However, the ions didn't move as I have expected. Then I checked the force on the atoms by using -cf and -af in traj. The force I got is like this:

@    title "average force"
@    xaxis  label "Atom"
@    yaxis  label ""
@TYPE xy
1         981.917    -227.041      24.754
2        -544.736     560.210    2241.472
3        -554.949    -369.107   -1040.614
4        -494.053    -395.939      -2.874
5         584.973     224.759   -1199.617

It seems to be too big if the unit is in kj/mol nm (according to the manual). If the unit of electric field is V/nm, it will only generate a force at the level of '100 kj/mol nm' (assuming I didn't make mistake here). Therefore, I am guessing my simulation is incorrect. I checked the parameter of force field (54a7), but cannot find where I made mistake.

Could you guys give me some clue?

Really appreciate!
Tong