# [lammps-users] NVT vs NPT

Hi,

I am doing MD simulation of spherical void growth in aluminum. During deformation, i am giving NPT command at each timestep for relaxation as all experiments are at controlled temperature and pressure. Below is the command

fix 1 all npt 300 300 1 aniso NULL NULL 0 0 0 0 1 drag 1
fix 2 all deform 1 x erate .001 units box remap x
run 200000
With the above command, there is no void growth. Void only changes the shape from spherical to something like ellipsoid. So, i tried to do NVT as follow

fix 1 all nvt 300 300 1
fix 2 all deform 1 x erate .001 units box remap x
run 200000
With the above command, there is void growth seen on slip planes (about 15 - 18%), but now the volume is increasing at each time step which is thermodynamically not correct. So now i tried to preserve volume in y and z direction as follow

fix 1 all nvt 300 300 1
fix 2 all deform 1 x erate .001 y volume z volume units box remap x
run 200000
With the above command the result is similar to NPT with zero pressure on y and z (case 1). Finally i tried this

fix 1 all npt 300 300 1 aniso NULL NULL NULL NULL NULL NULL 1 drag 1
fix 2 all deform 1 x erate .001 units box remap x
run 200000
With the above command the result is similar to NVT (case 2: increasing volume at each time step). Is it so that i have periodicity in all direction(infinite system), i shouldnâ€™t worry about volume. Now i am thinking to try NPT with NULL in y and z and preserve volume in y and z, but my intuition says that results will be similar to NVT. Can anyone suggest me which is best relaxation method for void growth problem.

I don't know - what's the physical system you're trying
to model. I would set boundary conditions that correspond
to it. Your last example isn't doing any pressure control,
so that's why it is the same as NVT.

Steve