Difference between 'fix deform' and 'fix move'?

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1

Dear All,

I have a simple question on the difference between ‘fix deform’ and ‘fix move.’

I am doing uni-axial tension on a polymer system (period boundary conditions on 3 dimensions). I have tried ‘fix deform’ and ‘fix move’ to stretch the system along x direction, and use our post-processing to calculate the stress tensor.

Even I used the same deformation rates for these two cases, but I got very different stress responses. The stress from ‘fix deform’ looks like already after equilibrated; but the stress from ‘fix move’ looks more instaneous.

My confusion here is: does the ‘fix deform’ try to equilibrate the system during I deform the box? I combined it with ‘fix npt’ for y and z directions.

What is the difference between ‘fix deform’ and ‘fix move’?

Thank you for your help,

Lili

2

Dear All,

I have a simple question on the difference between 'fix deform' and 'fix
move.'

I am doing uni-axial tension on a polymer system (period boundary conditions
on 3 dimensions). I have tried 'fix deform' and 'fix move' to stretch the
system along x direction, and use our post-processing to calculate the
stress tensor.

Even I used the same deformation rates for these two cases, but I got very
different stress responses. The stress from 'fix deform' looks like already
after equilibrated; but the stress from 'fix move' looks more instaneous.

what does that mean?

My confusion here is: does the 'fix deform' try to equilibrate the system
during I deform the box? I combined it with 'fix npt' for y and z
directions.

i don't understand that question, either. there is a fundamental
difference between what fix deform and fix move do. please also, note
that both commands have many options that enable different behavior,
so you have to be very careful when comparing results. for the most
part you will be comparing the proverbial "apples" with "oranges".

What is the difference between 'fix deform' and 'fix move'?

fix deform changes the box. fix move moves atoms.

axel.

3

Thanks Axel, for pointing out the comparison may not be fair.

My doubt here is : when I deform the box using ‘fix deform,’ is it possible that the molecules are getting equilibrated at the same time, so the molecules don’t carry as much load as I direct move the atoms by using ‘fix move’?

You gave a comment on an old post below, but I am not very clear on what you said ‘due to equilibration being faster than deformation,’ why this can happen? Thanks, Lili
http://lammps.sandia.gov/threads/msg24540.html

4

Thanks Axel, for pointing out the comparison may not be fair.

My doubt here is : when I deform the box using 'fix deform,' is it possible
that the molecules are getting equilibrated at the same time, so the
molecules don't carry as much load as I direct move the atoms by using 'fix
move'?

that depends on the exact commands used. as i said, there are many
options for both commands and the answer depends on how you use the
commands. please look very closely at the manual and see what it is
exactly that they do if you use them the way you do.

axel.