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Re: [lammps-users] Dipole precessing in Electric Field
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Re: [lammps-users] Dipole precessing in Electric Field


From: Axel Kohlmeyer <akohlmey@...24...>
Date: Mon, 6 Nov 2017 10:05:10 -0500



On Mon, Nov 6, 2017 at 7:05 AM, carles calero <carles.calero@...24...> wrote:
Hi, 
Have you tried to simulate the case of a single dipole in a static magnetic field? Do you obtain the Larmor precession? 

​that would be somewhat difficult, since there currently is no feature in the LAMMPS distribution that can model a static magnetic field. i know some people are working on it (and ​many more have promised), but this is far from trivial to implement properly.

axel.

 
Thank you,
Carles



On Thu, Oct 12, 2017 at 3:58 PM, Steve Plimpton <sjplimp@...24...> wrote:
Stan can likely look into this (CCd).

Steve

On Thu, Oct 12, 2017 at 4:34 AM, carles calero <carles.calero@...24...> wrote:
Hi,

I have performed a simple test to check the behavior of point dipoles in electric fields and the outcome seems to indicate that there is something wrong.

Basically, I run a simulation with one point dipole initially directed along the Z-direction in a static field directed along the Y-direction. The particle starts with zero velocity and I run the simulation in the nve ensemble (see below). I would expect a precession about the Y-axis, but instead I get a partial precession about the X-axis. By "partial precession" I mean that the moment rotates 180 degrees and then goes backwards. Both, the direction of precession and the trajectory seem to be unphysical.

I didn't find any reference to this problem in the mailing list.

I define the atom using:
 
atom_style    hybrid sphere dipole bond

The field is given by:
fix 0 dip1 efield 0.0 10.0 0.0

I use the integrator:
fix 2 all nve/sphere update dipole


The whole script is:
units        lj
atom_style    hybrid sphere dipole bond
bond_style      harmonic
dimension    3

read_data    input


pair_style    lj/cut/dipole/cut 10.0

pair_coeff 1 1 1.0 1.0 5.0 10.0
bond_coeff 1 1.0 4.0

group        dip1 type 1

velocity     all zero linear

fix 0 dip1 efield 0.0 10.0 0.0
fix 2 all nve/sphere update dipole

timestep    0.05
compute         t1 all temp/sphere
thermo_modify   temp t1

thermo        500

dump        0 dip1 custom 100 dump.dipole id type x y z mux muy muz
dump        1 all atom 1000 dump.lammpstrj


timestep     0.0001
thermo        50
run        1000000



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--
Dr. Axel Kohlmeyer  akohlmey@...12...24...  http://goo.gl/1wk0
College of Science & Technology, Temple University, Philadelphia PA, USA
International Centre for Theoretical Physics, Trieste. Italy.