# Question about external electric field

Hello everyone,

I am trying to study behavior of a polymer confined between two plates (carbon ) under external electric field.
The top field has a field, for example, -1.0 v bottom one is grounded.

So I tried the fix :

fix kick external-field efield 0.0 0.0 -1.0

My question is

1. do I put only the polymer in the group ‘external-field’ , or I include both the plates also, or only the polymer and the top plate ?

2. I want the direction of the field toward positive Z direction.
So if my bottom plate has z coordinaye say ‘0’ and the top plate has z coordinate say ‘40’
Does the fix allow field direction from bottom plate to top plate or I need to do something else ?

Thanks for the replies.

Mani

Hello everyone,

I am trying to study behavior of a polymer confined between two plates
(carbon ) under external electric field.
The top field has a field, for example, -1.0 v bottom one is grounded.

So I tried the fix :

fix kick external-field efield 0.0 0.0 -1.0

My question is
1) do I put only the polymer in the group 'external-field' , or I include
both the plates also, or only the polymer and the top plate ?

neither. you use only the polymer in the fix group.

2) I want the direction of the field toward positive Z direction.
So if my bottom plate has z coordinaye say '0' and the top plate has z
coordinate say '40'

the absolute values have no meaning. what fix efield
does is to apply a force that is proportional to the charge
of the atom. so it isn't much different than fix addforce,
only that the added force is multiplied by the charge
and thus the opposite sign of a charge would reverse
the direction of the force. thus you can easily estimate
the sign of the field that you need to use.

Does the fix allow field direction from bottom plate to top plate or I need
to do something else ?

as mentioned above. there is no real field,
you simply simulate the effect of the field.
after all, you are doing a simulation.

cheers,
axel.

p.s.: of course, this approach requires your polymer
to have partial charges on the atoms and it doesn't
model any polarization effects in the polymer.