different behavior with fix rigid and fix npt (iso vs. yz)

Please post to the list, not to me.

I don’t know why your wall atoms are moving with fix npt,

if you have defined your groups correctly, nor why

they would only move in x and not in y,z. There must

be something else in your input script going on.

I suggest you remove things from your script to produce the

simplest possible problem and see if the issue persists.

And indicate how you determine that the walls are moving.

Are you time integrating the wall atoms? Do they have an

initial velocity?

If the problem still persists, post the simple script to the mail list.