LAMMPS WWW Site - LAMMPS Documentation - LAMMPS Mailing List Archives
Re: [lammps-users] Dipole precessing in Electric Field

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

 From: Axel Kohlmeyer Date: Mon, 6 Nov 2017 10:54:04 -0500

On Mon, Nov 6, 2017 at 10:46 AM, carles calero wrote:
Dear axel,

Isn't it equivalent to the case of a single electric dipole in a static electric field?

​no. assume for simplicity, you represent a dipole by two point charges. in the case of the electric field​ the force on the point charges depends on the charge and the field vector. in the case of a magnetic field, however, it also depends on the velocity of the point particle.

I was making reference to a prior question that I raised on the behavior of electric dipoles on static fields:

"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 would expect Larmor precession about the applied field, but this is not what I obtain.

​Larmor precession happens when you place a *magnetic* moment into a *magnetic* field.​

​axel.​

Thank you,
C

On Mon, Nov 6, 2017 at 4:05 PM, Axel Kohlmeyer wrote:

On Mon, Nov 6, 2017 at 7:05 AM, carles calero 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 wrote:
Stan can likely look into this (CCd).

Steve

On Thu, Oct 12, 2017 at 4:34 AM, carles calero 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

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

------------------------------------------------------------------------------
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@...655....net
https://lists.sourceforge.net/lists/listinfo/lammps-users

------------------------------------------------------------------------------
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@...655....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.