moving group in a periodic box

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Then which fix should I use for tensile simulation of Periodic box???

NVT : definitely not because the volume must change

this is nonsense. *of course* you can use fix nvt.

NVE : it cant change the dimension of periodic box

again, you are looking at things from the wrong direction. using fix
nve doesn't mean that the box cannot change. NVE only in this context
only means that it does not modify volume or kinetic energy *itself*.

the only fix that change the simulation box is NPT.


but I should zero the pressure in all dimension in order that the box dimension changes with applied velocity.
in fact, I used this NPT:

to get a stress strain curve at finite temperature, you can for
example use fix deform with fix nvt or if you *really* insist to relax
the x and y dimensions you can use fix npt where z is kept fixed.

fix NPT all npt temp temp 300 300 100 x 0 0 50 y 0 0 50 z 0 0 50 drag 2

if I remove the z component, Lz does not change.

again, incorrect.

in fact I need the force corresponding to the tensile strain not the pressure of the system.

those are not completely independent.

do you have any idea how can I modify the NPT in order that the system can behave naturally?

you should have a look through the mailing list archives. discussions
on how to compute stress strain curves happen all the time. not all
systems are the same and you can use the same approach, but that is
better than just making up stuff.