TIP4P , kspace_modify slab , efield !

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1

Dear LAMMPS users ,

Greetings .
I have a system with TIP4P water molecules .

…
boundry : p p f

…

kspace_style pppm/tip4p 1.0e-4
kspace_modify slab 3.0

…
I want to use electric field on this system ( with using of fix efield ) .

Is Slab configuration valid for this kspace_style ? what abou efield on TIP4P ?
I checked this " http://sourceforge.net/p/lammps/mailman/message/25013349/ " but still it is a little bit ambiguous for me .

Thanks for your attention.

Best,
Saeed.

2

Dear LAMMPS users ,

Greetings .
I have a system with TIP4P water molecules .
...
boundry : p p f
.....
kspace_style pppm/tip4p 1.0e-4
kspace_modify slab 3.0
...
I want to use electric field on this system ( with using of fix efield ) .

please note that fix efield is an extremely crude approximation to the
impact of an electric field. it merely adds a force scaled by the
charge of an atom. that would only be correct for a very diluted
system.

Is Slab configuration valid for this kspace_style ?

why do you think it may not be?

what abou efield on TIP4P ?
I checked this " http://sourceforge.net/p/lammps/mailman/message/25013349/
" but still it is a little bit ambiguous for me .

what is ambiguous?

3

Dear Axel ,

Thanks for your replay .

About slab configuration I think I understood Ahmed’s replay !

But I can not understand this section of Steve’s replay " …. you want to add force to the pseudo-charge and thus indirectly to all 3 atoms, then you’d have to add logic to fix efield to do that"
one another question :If I want to have a charged parallel plate capacitor , is it possible with Fix efield ?

Best ,
Saeed.

4

Dear Axel ,
Thanks for your replay

please note the difference between "reply" and "replay".

About slab configuration I think I understood Ahmed's replay !
But I can not understand this section of Steve's replay " ... you want to
add force to the pseudo-charge and thus indirectly to all 3 atoms, then
you'd have to add logic to fix efield to do that"

he says (and in my opinion very clearly so) that fix efield does not
know anything about the special trick that the tip4p styles in LAMMPS,
where the coordinate of the charged point M is inferred from the
positions of the other atoms. if you want the forces on M to be
computed correctly and projected back on the other atoms, then you'll
have to write a fix efield/tip4p that does those computations. or you
can just represent TIP4P as a real four site model using fix rigid and
regular lj/cut/coul/long and pppm.

one another question :If I want to have a charged parallel plate capacitor ,
is it possible with Fix efield ?

this is a question about the choice of model, not a question about
LAMMPS. so that is for your to decide whether what fix efield does is
a good model for a capacitor or not, not LAMMPS or any of us.

axel.

5

Dear Axel ,

Thanks for your reply and sorry about my mistake .

Best ,
Saeed.