[lammps-users] when barostating why moving?

Dear all,

i have a simulation box with 2 things inside it. A structure that has to be still and some moving particles. When i use nve with thermostat or nvt everything is ok … I tried to control pressure either with npt or press/berendsen. My problem is that even without integrating at all the structure, it keeps moving.
Why is this happening? Or because the box with an npt is changing, the coordinate system changes (so the absolute coordinates ), so the “movement” that i see with the vmd is due to the volume change?

Regards
Jim

From the LAMMPS documention on fix npt and press/berendesen:

"Regardless of what atoms are in the fix group, a global pressure or stress tensor is computed for all atoms. Similarly, when the size of the simulation box is changed, all atoms are re-scaled to new positions, unless the keyword dilate is specified with a value of partial, in which case only the atoms in the fix group are re-scaled. The latter can be useful for leaving the coordinates of atoms in a solid substrate unchanged and controlling the pressure of a surrounding fluid. "

i have used too the keyword dilate with partial but the only thing that i achieved was to make this change smaller.

With "dilate partial "the atoms that are not in the fix group should not move at all. However, they could still move if another fix was applied to them. Also, the positions expressed in scaled coordinates will vary as the box dimensions vary. If you still think something incorrect is going on, send us a small simple example that illustrates the problem.