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
Stan can likely look into this (CCd).
Steve
Hi,
Have you tried to simulate the case of a single dipole in a static magnetic field? Do you obtain the Larmor precession?
Thank you,
Carles
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.
Dear axel,
Isn’t it equivalent to the case of a single electric dipole in a static electric field?
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.
Thank you,
C
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.