Change the radius of "region sphere"

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1

Dear all,

I want to confine a droplet or membrane in a spherical domain with a harmonic spring force. And by decreasing the sphere radius at a constant velocity, compression can be accomplished.

For LAMMPS, I can use region sphere to define the spherical domain. And combined with fix wall/region, I can compress the system with a harmonic spring force. But I don’t know how to decrease the sphere radius over the simulation. I notice the defined region can be dynamics by move and rotate keywords. I am wondering if it also can be shrink or expand by changing the size of the region.

I know I can use loop in the input script to change the region every fixed time steps. But it is not what I want. The radius of the domain had better to be decreased at a constant velocity with fix commands. Is there any commands in original LAMMPS that can accomplish such kind of loading? Or am I supposed to modify the region command to be dynamics for size and radius?

Thank you for your attention.

Best,
-Wenpeng

2

That's not currently possible with the region command. Region
parameters, like the radius would have to be allowed as variable
inputs. I'll think a bit about whether that would cause other problems.

Steve

3

I forgot to mention that the fix indent command can be used
to define a sphere with time-dependent radius that pushes on
atoms inside it (see the side keyword). This is close to what
you want, except that the force it exerts on atoms is different
than with fix wall/region.

Steve

4

Hi Steve,

Thank you for your reply. It is very useful. Although the force on the atoms is a little different, I think I can modify the force equation in the fix indent command to accomplish what I want.

Thank you again.

-Wenpeng

2013/1/11 Steve Plimpton <[email protected]>

5

Just added a 21Jan patch with a variable region sphere
radius - you can try it out to see if it works with fix wall/region.

Steve