# Effect of quenched disorder study using LAMMPS

Dear all,

Can we use LAMMPS to study the effect of quenched disorder in 2-spatial dimensions ?

Any help !!

Thank you,

Quenching in LAMMPS is simply the minimize command.
Re: disorder, you have to be more specific. How do
you propose to measure “disorder”?

Steve

Dear Steve,

The idea is to study the time evolution of a system in the presence of disorder (or random) potential, not to measure the disorder.

We discretize the simulation box into small boxes and put in the disorder potential values (gaussian distributed) that are fixed during the whole simulation.

Can we use addforce region to carry out such a simulation ?

Sincerely,

Shankaraih

Fix addforce region will let you define one geometric region and only add
forces to atoms currently in that region. That doesn’t sound like
an atom-style variable to specify the force on each atom. So if
you an define a formula (e.g. using a random value) that represents
your potential (the force you want to apply to each atom), then
you may be able to do what you want.

Worst case, you may need to write your own fix (or pair style if these
are pairwise interactions) that applies the “disorder” potential you
have in mind.

Steve

Also, you could try the new fix python command to encode your
per-atom force in Python and try it out quickly w/out having
to write any new LAMMPS (C++) code.

Steve

I will update you when I succeed to use one of your suggestions.

Sincerely,

Shankaraiah

Dear Steve,

Thank you for your kind help.

All I have to do is at every time step, I need to check the atom positions x_i and y_i and then add force f_i, as my random potential (is not just a random number and can’t use a formula) is site (x,y) dependent but not time dependent.

It seems I can use fix addforce region with region styel as EDGE so as to consider the full simulation box as my region. If so, then the next step is to find the positions of each atom x_i and y_i and add force f_i at every time step.

Sincerely,

Shankaraiah

Dear Steve,
Thank you for your kind help.

All I have to do is at every time step, I need to check the atom positions
x_i and y_i and then add force f_i, as my random potential (is not just a
random number and can't use a formula) is site (x,y) dependent but not time
dependent.

It seems I can use fix addforce region with region styel as EDGE so as to
consider the full simulation box as my region. If so, then the next step is
to find the positions of each atom x_i and y_i and add force f_i at every
time step.

​you are not making much sense here and you didn't pay attention to both,
what steve wrote and what is written in the fix addforce documentation.

1) if you don't want to restrict your addforce command to a specific
region, you do not need to use the region keyword
2) if you have a force term f_i for atom i, that depends on the x and y
position, you can define it as an atom style variable and then use that
variable as force component in the fix addforce command.

axel.