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Re: [lammps-users] Difference in results for atomstyle charge and dipole
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Re: [lammps-users] Difference in results for atomstyle charge and dipole


From: Steve Plimpton <sjplimp@...24...>
Date: Fri, 7 Jul 2017 08:33:17 -0600

I would say that using PPPM slab to model a 2d system
is problematic.  That is really designed to model a 3d system
that is nonperiodic in one dimension (z), which is not
the same as an (infinitely thin) 2d system.  I'm not sure
you can trick LAMMPS into thinking it is the same.

What is really needed is a 2d PPPM (or Ewald) solver,
which LAMMPS does not currently support.

Perhaps Stan (CCd) can comment on what this
would take.

Steve

On Thu, Jul 6, 2017 at 4:37 PM, Axel Kohlmeyer <akohlmey@...24...> wrote:
there are far too many variables in your current setup. i would
recommend you take a step back and start with a small system, that has
only LJ particles (no SRD) and then you start with LJ-only, then LJ
plus cutoff dipole and finally LJ plus long-range dipole and adding
SRD only at the end. then you can see where things start to go south
and you can try to figure out why. as soon as you have the why, it
should be straightforward to decide what to do next. learning how to
do something that you haven't done before with a quite complex input
and system, is asking yourself for a lot.

axel.

On Thu, Jul 6, 2017 at 6:18 PM, John Smith <johnson21smith@...24...> wrote:
> I also need to account for the long range interactions, therefore I am
> wondering if the box size in z direction being very large affects the
> simulation in any way.
>
> On 6 July 2017 at 16:32, Axel Kohlmeyer <akohlmey@...24...> wrote:
>>
>> On Thu, Jul 6, 2017 at 5:29 PM, John Smith <johnson21smith@...24...>
>> wrote:
>> > Thank you, it works now! I am getting a warning saying that kpsace is
>> > being
>> > invoked when there are no charges present. This is true since my input
>> > consists only of dipoles. Is it still necesary for me to use
>> > kspace_style?
>>
>> if you use a "/long" style, yes. for dipole-dipole interactions only,
>> it might be acceptable to use lj/cut/dipole/cut or lj/sf/dipole/sf
>> instead and forego the long-range electrostatic handling. but that is
>> up to you to decide what is handling the interactions of your system
>> sufficiently accurate.
>>
>> axel.
>>
>> >
>> > Thanks,
>> > John
>> >
>> > On 6 July 2017 at 15:18, Axel Kohlmeyer <akohlmey@...24...> wrote:
>> >>
>> >> On Thu, Jul 6, 2017 at 4:14 PM, John Smith <johnson21smith@...24...>
>> >> wrote:
>> >> > In order to do this, I had to change the dimension to 3d and changed
>> >> > the
>> >> > lattice from sq to sc and boundary p p f. Since I cannot enforce2d in
>> >> > a
>> >> > 3d
>> >> > simulation, how do I zero out forces and velocities in z direction?
>> >>
>> >> velocity command and then fix setforce?
>> >>
>> >> axel.
>> >>
>> >> >
>> >> >
>> >> >
>> >> > On 6 July 2017 at 13:29, Axel Kohlmeyer <akohlmey@...24...> wrote:
>> >> >>
>> >> >> On Thu, Jul 6, 2017 at 2:27 PM, John Smith
>> >> >> <johnson21smith@...24...>
>> >> >> wrote:
>> >> >> > I looked into the documentation and updated the pair_style to
>> >> >> > lj/cut/dipole/long to account for dipole-dipole interactions. I
>> >> >> > also
>> >> >> > changed
>> >> >> > from 'fix nve' to 'fix nve/sphere update dipole'. The documentaion
>> >> >> > said
>> >> >> > that
>> >> >> > I need to include
>> >> >> >
>> >> >> > kspace_style ewald/disp 1e-4
>> >> >> >
>> >> >> > But I am getting an error that that command cannot be used in a 2D
>> >> >> > simulation. How do I effectively make this into a 3D simulation
>> >> >> > and
>> >> >> > get
>> >> >> > the
>> >> >> > same results? Or can is there a workaround to getting the above
>> >> >> > command
>> >> >> > working in 2d?
>> >> >>
>> >> >> please see kspace_modify slab.
>> >> >>
>> >> >> axel.
>> >> >>
>> >> >> >
>> >> >> > Thanks,
>> >> >> > John
>> >> >> >
>> >> >> > On 5 July 2017 at 18:05, Axel Kohlmeyer <akohlmey@...24...>
>> >> >> > wrote:
>> >> >> >>
>> >> >> >> On Wed, Jul 5, 2017 at 6:39 PM, John Smith
>> >> >> >> <johnson21smith@...24...>
>> >> >> >> wrote:
>> >> >> >> > Hello,
>> >> >> >> >
>> >> >> >> > I am attaching two input scripts, one with a mixture SRD of
>> >> >> >> > style
>> >> >> >> > charge
>> >> >> >> > (initial charge set to 0.0) and the other with a net dipole of
>> >> >> >> > 1.0
>> >> >> >> > (in
>> >> >> >> > reduced lj units). I expected to see an effect due to electric
>> >> >> >> > field
>> >> >> >> > on
>> >> >> >> > the
>> >> >> >> > system in the case of dipole, but I am not able to observe any
>> >> >> >> > difference
>> >> >> >> > between the results of the two runs. I am also attaching the
>> >> >> >> > graphs
>> >> >> >> > for
>> >> >> >> > vacf, msd and diffusion coeffcient calculated using msd.
>> >> >> >> >
>> >> >> >> > I have used fix efield, but I am not sure if it is being
>> >> >> >> > called?
>> >> >> >>
>> >> >> >> as the fix efield documentation states, fix efield adds a torque
>> >> >> >> to
>> >> >> >> particles with a (point) dipole.
>> >> >> >> however, there is nothing in your input, that is in any way
>> >> >> >> reacting
>> >> >> >> to this torque (or to the dipole outside of fix efield).
>> >> >> >>
>> >> >> >> axel.
>> >> >> >>
>> >> >> >> > Any advice please?
>> >> >> >> >
>> >> >> >> > Sincerely,
>> >> >> >> > John
>> >> >> >> >
>> >> >> >> >
>> >> >> >> >
>> >> >> >> >
>> >> >> >> >
>> >> >> >> > ------------------------------------------------------------------------------
>> >> >> >> > Check out the vibrant tech community on one of the world's most
>> >> >> >> > engaging tech sites, Slashdot.org! http://sdm.link/slashdot
>> >> >> >> > _______________________________________________
>> >> >> >> > lammps-users mailing list
>> >> >> >> > lammps-users@...396...sourceforge.net
>> >> >> >> > https://lists.sourceforge.net/lists/listinfo/lammps-users
>> >> >> >> >
>> >> >> >>
>> >> >> >>
>> >> >> >>
>> >> >> >> --
>> >> >> >> 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@...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.

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