Can someone please answer Lili’s question. I also can not figure it out.



Hi All,

May I ask a question following this thread regarding the meaning of the
"args" of keywords "*pos*" and "*vel*" in "fix wall/piston command?"  The
doc says that

*"pos* args = z
    z = z coordinate at which the piston begins (distance units)
  *vel* args = vz
    vz = final velocity of the piston (velocity units)"

which seems: pos needs only one argument z, and vel needs only one
argument vz (the z component of the piston).

But in the Example part of the doc:  fix walls all wall/piston zlo pos
1.0 0.0 0.0 vel 0.0 0.0 10.0 units box

There are three args for each, what is the meaning of them?

Thank you very much,

That is an error with the doc page. This command requires x, y, and z coordinates for the pos keyword and x, y, and z velocities for the vel keyword. Note the wall can only move in the +z direction, so x and y velocities have no effect at all. The same for x and y for pos keyword - they have no effect. There may be a need to change the source code to match the doc page, since the doc page makes more sense to me.


I changed the code to just use the z and vz values, not x,y.

The doc page is thus correct, except the examples were also
changed to just take a single value.