request for information

Dear all,
I am having a simulation box with two regions joining along a common boundary. the boundary is normal to z axis. i am maintaining periodic boundary conditions along x and y. whereas along z it is “s”. I want to apply force along the z direction(tensile force). which means i want to apply force in +ve direction on the upper region and in -ve direction in the lower region. I tried fix aveforce, fix setforce commands but both of the them can apply only on periodic boundary conditons not in “s”. I also tried fix box/relax which is also the same case. So can any body tell me how to apply force on regions with non periodic boundary condition (“s”).
your suggestion will be very helpful to me.
with regards

Just a general note to everyone. When you post
a message to the mail list, you want to be sure
your "subject" is clearly LAMMPS or MD related.
The subject of this email was "request for info".
As the gatekeeper of the maillist, when I accept/reject
postings on SForge, all I see are the subjects. We get a lot
of spam, so if the subject isn't obviously relevant, I typically
delete the message without even looking at it.

Now to answer your Q. There is no restriction
on fix aveforce or fix addforce with respect to
periodic boundaries. They act on groups of
atoms. They know nothing about the boundaries.