# Deformation simulation: center of mass is moving

Dear Lammps users,

I am using lammps to simulate mechanical response of a bulk polymer.
I apply different cyclic loading to the bulk (uniaxial/multiaxial deformation) with the commands below
(ex. for uniaxial deformation):

fix 1 all nvt/sllod temp 300.0 300.0 100.0
fix 2 all deform 1 x wiggle 7.98 100000 remap v units box

My initial structure is well equilibrate and the interaction between atoms is describe by the DREIDING FF.
I can fallow the motion of the center of mass by the command:

variable Xpoly equal xcm(all,x)
variable Ypoly equal xcm(all,y)
variable Zpoly equal xcm(all,z)

It appears that during several simulations the center of mass is moving.
I don't know the origin of this motion and how to fix it.

Thank you very much,

Nicolas

The applied deformation is asymmetric (top is tilted,
bottom is held constant), so the COM will move,
Simply subtract it out of your analysis if you need to.

Steve

I am stretching the bulk in the x direction and for low deformation (<5%)
everything works great (no motion of the COM).
In the case below (10% of deformation: my bulk is 79.8A) the COM start moving...
It seems that there is limit deformation load?

Nicolas

Hi,

It may be possible that for low deformation the system configuration doesnt change the COM . For large deformation I belive the COM should change… QUESTION : Why the COM should remain fixed? , If your deforming the body the COM should change position. I’m not able to grasp the idea of having a fixed COM (Please explain to me)

limit deformation load? (more like a Limit deformation of Strain) .

Oscar G.

If the system responds to the deformation, then the COM will
move, as I said, b/c the applied deformation is asymmetric.
See the fix deform doc page. Whether the atoms respond,
depends on many things: how fast you apply the deformation,
is the system a liquid or solid, what "remap" options you
use with fix deform, etc.

Steve