Setting a time-dependent dipole moment

Hello,

I’m trying to set a time dependent dipole moment for a pair of particles by calculating a variable with:

  • variable Field atom time*1e-9

and then I write:

  • set group Atoms dipole 0 0 v_Field

The problem is that instead of assigning the value of the calculated variable to the z component of the dipole moment, it assigns zero. Even if I set the initial value to something other than zero, the set command will put it back to zero.

However if I dump the value of v_Field, it is being updated every loop. Also, if I substitute time for xlo, the calculated value is correctly set to the dipole moment. The same problem arises if I try to set the position to a time dependent variable.

I’m sending a working example.

Thanks in advance,
Antonio Ortiz.

----- LAMMPS Input File -----

units micro
atom_style hybrid sphere dipole bond
boundary s s p
dimension 3
neighbor 4.0 nsq
pair_style lj/cut/dipole/cut 20

read_data TimeDependentInput.data

group Atoms type 1
mass * 1

timestep 10000

variable thermoname equal time/1e9
variable Field atom time*1e-9

set group Atoms dipole 0 0 v_Field

dump 3 all custom 10 TimeDependentField.lammpstrj id type x y z v_Field muz
thermo_style custom step atoms v_thermoname
thermo 1000

run 6000

----- TimeDependentInput.data File -----

4 atoms

2 atom types
-30.00 30.00 xlo xhi
-30.00 30.00 ylo yhi
-1.50 1.50 zlo zhi

Atoms

1 2 15.00 0.00 0.000000 10.30 0.0017 0.0 0 0 0 1
2 2 -15.00 0.00 0.000000 10.30 0.0017 0.0 0 0 0 2
3 1 15.01 0.00 0.014681 10.30 0.001678 0.0 0.0 0.0 0.002 1
4 1 -15.01 0.00 0.016334 10.30 0.001678 0.0 0.0 0.0 0.002 2

PairIJ Coeffs

1 1 0.0 0.0 0.0 60.000000
1 2 0.0 0.0 0.0 0.0
2 2 0.0 0.0 0.0 0.0

Hello,

I'm trying to set a time dependent dipole moment for a pair of particles by
calculating a variable with:

variable Field atom time*1e-9

and then I write:

set group Atoms dipole 0 0 v_Field

The problem is that instead of assigning the value of the calculated
variable to the z component of the dipole moment, it assigns zero. Even if I
set the initial value to something other than zero, the set command will put
it back to zero.

you are misunderstanding how the set command works. set is
*instantaneous*, i.e. it will take the current value and assign it.
it will *not* associate an atom property with an expression during the
simulation.
if you want something like that, you would have to program a new fix
style, that overwrites the dipole components during the simulation in
a similar fashion as fix setforce overwrites the forces. in that case
the variable expression is evaluated at every step *during* the run
and the resulting value assigned.

the other examples you describe all the same issue. the problem is not
really a LAMMPS problem but a matter of PEBCAC.

axel.