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Re: [lammps-users] slab correction in PPPM/disp and Ewald/disp
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Re: [lammps-users] slab correction in PPPM/disp and Ewald/disp


From: "reza N.Z" <mrn.nz91@...24...>
Date: Thu, 26 Oct 2017 12:17:08 +0330

I asked about non-neutral systems because I have not seen the sentence, which is added to the documentation, before and this questions comes to my mind that, might further changes is added to the slab correction. 
The system I want to perform is a slab-like system(neutral-system) in order to calculate surface tension. The impact of electric field is going to be studied on the system. As far as I know, it is essential to use slab-correction in z-direction. So if I want to use PPPM/disp method I should cancel both coulombic and Lenard-Jones long-range interactions in z-direction while the slab-correction implemented in lammps just cancels the Coulombic one. 

On Tue, Oct 24, 2017 at 2:49 PM, Axel Kohlmeyer <akohlmey@...24...> wrote:
On Tue, Oct 24, 2017 at 4:07 AM, reza N.Z <mrn.nz91@...24...> wrote:
>>>that is not what i am saying. the slab correction applies to the
>>>*residual* dipole of the entire system.
>
> So, the system should be entirely neutral.

no, but the larger the net charge the larger the risk of deviation
from the behavior of a neutral system.

>>>i don't understand what you are asking here. please elaborate in more
>>>detail and quote the particular sections to the manual you are
>>>referring to.
>
> In the lammps documentation on the following page:
> http://lammps.sandia.gov/doc/kspace_modify.html
> on the last line of the paragraph which the slab keyword is introduced, this
> sentence was added;" The slab option is also extended to non-neutral systems
> (Ballenegger)."

if you have a system with a net charge, Sum(r_i * q_i) is not
invariant to translation, so for the slab correction to be meaningful
you need to remove the translational bias, e.g. by subtracting
Sum(q_i)*(geometric center)

>
> Actually I am looking for a way to use PPPM/disp or Ewald/disp method for a
> 2 dimensional system and it is important to cancel both Lenard-Jones and
> Coulombic long-range interactions across the non-periodic direction. Could
> you give a suggestion?

you are not making much sense here. the attractive LJ term decays with
r**6 (the r**12 term is not at all considered in the /disp styles,
since it is so extremely short ranged).
if you have a thin slab of, say, 10 \AA width, then particles will be
30 \AA apart from their images in z-direction, so that the magnitude
of their interactions will have decayed by almost 9 orders of
magnitude.
that is far smaller than the typical PPPM/Ewald accuracy threshold
used. i don't see a convincing point for having a slab correction. if
you feel this strongly about accuracy, you should rather go about and
implement a proper 2d-ewald summation for coulomb (only) with no PBC
in z, and then use an LJ cutoff long enough to span the entire width
of the slab. of course, you will also have to re-parameterize your
model, since i'd doubt that any existing force field parameters will
be tuned for such a setup at the high accuracy you are looking for.

i don't even want to argue systematic force field accuracy issues or
the fact that you are - after all - simulating an essentially
unphysical system when you have a system with a net charge.

rather than arguing about how to do what cannot be done or makes no
sense, you should explain what in your planned simulations makes this
all necessary and provide some convincing proof that the existing
features in LAMMPS are not sufficient.

axel.


>
>
>
> On Thu, Oct 19, 2017 at 5:22 PM, Axel Kohlmeyer <akohlmey@...24...> wrote:
>>
>> On Thu, Oct 19, 2017 at 5:42 AM, reza N.Z <mrn.nz91@...24...> wrote:
>> > Thank you so much Axel
>> >
>> >>the slab correction is for coulomb only and only cancels inter-slab
>> >>interactions of the residual dipole moment across the slab.
>> >
>> > You mean that Coulombic interaction of Ions wont be canceled across the
>> > slab?
>>
>> that is not what i am saying. the slab correction applies to the
>> *residual* dipole of the entire system.
>>
>> > Actually, I am confused by your _expression_ and the Lammps
>> > documentation where the kspace_modify is introduced, it is stated that
>> > slab
>> > correction is extended to non-neutral systems.
>> > Could you explain how the modification changes the Yeh slab-correction?
>>
>> i don't understand what you are asking here. please elaborate in more
>> detail and quote the particular sections to the manual you are
>> referring to.
>>
>> any periodic system with a non-neutral cell has essentially a
>> diverging energy. all reciprocal space based long-range solvers work
>> around this by ignoring the divergent (k=0) term. this applies to slab
>> systems as well. if you had a residual non-zero charge, the slabs
>> would repel each other, but since you have the same repulsive force in
>> positive and negative directions, it will cancel.
>>
>> axel.
>>
>> >
>> > Thank you so much for your precious guides.
>> > Best regards
>> > Reza
>> >
>> > On Tue, Oct 17, 2017 at 11:38 PM, Axel Kohlmeyer <akohlmey@...24...>
>> > wrote:
>> >>
>> >> On Tue, Oct 17, 2017 at 1:16 PM, reza N.Z <mrn.nz91@...24...> wrote:
>> >> > Dear lammps Users
>> >> >
>> >> > As I concerned by reading the documentation, the slab correction is
>> >> > performed to constrain the Coulombic long-range interaction in one
>> >> > direction
>> >> > (as performed in Yeh and Berkowitz work).
>> >> > I wonder If the slab-correction removes both Coulombic and
>> >> > Lenard-Jones
>> >> > dispersion interactions in PPPM/disp or Ewald/disp method?
>> >>
>> >> the slab correction is for coulomb only and only cancels inter-slab
>> >> interactions of the residual dipole moment across the slab.
>> >>
>> >> axel.
>> >>
>> >> > I will appreciate any comments.
>> >> >
>> >> > Best regards,
>> >> > Reza
>> >> >
>> >> >
>> >> >
>> >> >
>> >> >
>> >> > ------------------------------------------------------------------------------
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>> >> > _______________________________________________
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>> >> >
>> >>
>> >>
>> >>
>> >> --
>> >> Dr. Axel Kohlmeyer  akohlmey@...24...  http://goo.gl/1wk0
>> >> College of Science & Technology, Temple University, Philadelphia PA,
>> >> USA
>> >> International Centre for Theoretical Physics, Trieste. Italy.
>> >
>> >
>>
>>
>>
>> --
>> Dr. Axel Kohlmeyer  akohlmey@...24...  http://goo.gl/1wk0
>> College of Science & Technology, Temple University, Philadelphia PA, USA
>> International Centre for Theoretical Physics, Trieste. Italy.
>
>



--
Dr. Axel Kohlmeyer  akohlmey@...43...4...  http://goo.gl/1wk0
College of Science & Technology, Temple University, Philadelphia PA, USA
International Centre for Theoretical Physics, Trieste. Italy.