How to use Fix field command ?


First of all , I have to say thanks for fast response.

As you said I will explain more specific.

please always reply to the mailing list, not individual people.

I attached a picture so please see this.


As you ca see, this is visualization by using VMD.

On the top and the bottom, there are graffine electrodes and between
electrodes sea water flow but you can see only Na+ and Cl- ions


The point is how many Na and Cl ions are attached to the electrode
depending on the intensity of eletric field.


So we use fix efield command to add a force F = qE to Graffin elctrode so
that we could see the movement of ions

this doesn't make any sense.


Na+ and Cl- ions are supposed to move Z-direction so we use

fix gra efield 0.0 0.0 10.0 command, we only specify Z-direction electric
field unit.

But the problem is ions did not move to Z-direction. it is more likely to
move disorderedly. we couldn't see any tendency toward distribution of ions
according to eletric field.

why should Na+ and Cl- be affected, if you apply the efield fix to the

So we tried many time by changing the fie efield command such as

fix gra efield 100.0 100.0 100.0 / fix gra efield 0.0 0.0 200.0 / 0.0
0.0 100.0

but there was no difference.

we conclude that there were some errors within the input script attached

the script is working correctly. the error is in your overall model. please
read the documentation of fix efield more carefully.

on top of that, fix efield is a very crude approximation (there is no
screening), for your particular setup it is probably more realistic, if you
model the field through explicitly charging the two electrodes.