# Nanotube stretch fix deform NPT/NVT

Hello all:
I am doing simulations of CNT stretch to calculate Poisson’s ratio，and i am pretty puzzled about the command “fix deform”.
1）"fix deform"needs ensure that the boundary condition is P in the direction of pressure control.
If Y is the tensile direction, X and Z use NPT，then the nanotube will be extruded into like a cuboid by adjacent nanotubes. I tried to add vacuum into the box，but the vacuum would disappear after simulation. Is it possible to use NPT and this method？

2）I also try to use NVT, the results show the strainX/Z are vary small and i am not sure if it is appropriate to choose NVT to compute Poisson ratio.

Thank you in advance for checking this topic.

There usually is no need to use fix npt under these conditions. Please note that using fix deform in one dimension and fix npt in the other two is a procedure for bulk systems. If you stretch in one direction, you need to let the system relax in the other two or else would impose additional, unphysical strain.

When applying this approach to a CNT, you must make certain that the CNT is perfectly periodic in the deform direction, since fix deform will evenly stretch all atom as the box is stretched. You don’t really want periodic boundaries in the other two directions unless you want to determine the properties for a bulk system, i.e. a perfectly periodic layer of bundled CNTs. For a single CNT you can just use “f” boundaries.

If you have an entirely non-periodic system with a CNT in the center, then you cannot use fix deform. Instead you need to define regions with atoms at each end and either immobilize one and and use fix move to pull the atoms at the other end and run fix nvt on the remainder, or pull from both sides.

That is what should happen when using fix npt. if you want space between the CNT(s) then you must not use a periodic boundary or must not use fix npt.

What is appropriate depends on the specifics of the system. There is no general “do this, not that” kind of rule.