Uniform radial pressure

Hi,
It seems bizzare but can we perform an uniform radial pressure for a nanoparticle (amorphous silica for example) in a molecular dynamics simulation?
Thanks.

You can define a circular region with all your
atoms inside and use fix wall/region to “push”
on atoms that are near the wall. You can even
use a dynamic region (with time-varying radius)

to change the sphere radius over time. That will
impart an effective “pressure” to the enclosed system.

Steve

Thanks for your response.
So I need to have first a spherical region to use fix wall/region.
In my simulation, I had a simulation box after thermostate and barostate, at equilibrium. I used the fix indent command to push all atoms in a sphere (with keyword side in). But I have a problem now with fix indent command. I juste send a message to mailing list, with a visualization after used fix indent.
Thanks.

Also take a look at fix spring/rg,

http://lammps.sandia.gov/doc/fix_spring_rg.html
(I usually set the RG0 parameter to 0 and vary K.)

This fix exerts an inward harmonic force on each atom proportional to
it's distance from the center. You don't have to define a region or a
wall to use this fix.

Problems with this approach:

1) Generally, you will end up with a density which is Gaussian, not
uniform (for dilute systems, at least). If you don't want this, you
can try Steve's suggestion of using a hard (spherical) wall.

2) Unfortunately the energy associated with this force is not
currently tallied or included in the thermodynamic output.

3) You have to make sure that your simulation box is large enough to
gaurantee no atom will cross the boundaries.

Good luck!
Andrew

or ‘fix indent’ (sphere option) with ‘side in’