[lammps-users] CNT modeling Question

Hi Arjmand,

You can do uniaxial tensile simulation in lammps in two ways. One is
energy minimization, another is in npt MD. The former gives the
Young's modulus at 0K and the latter at any temperature. The key for
tensile simulation is that when you tensile the box along one
direction, the other direction(s) should be able to correspondingly
vary under the condition that the stress (or pressure) in this
direction equal to zero.

For the first way, I think you can try the following command:

fix 1 all box/relax aniso 0.0 0.0 NULL
displace_box all z delta 0 \{dpos\} units box minimize {etol} \{ftol\} {maxiter} ${maxeval}

In this way, you can use a loop to realize a serial of strain.

For the second way,

fix 1 all npt 300. 300. 2.0 aniso 0.0 0.0 0.0 0.0 NULL NULL 2.0 drag 2.0
fix 2 all deform 1 z scale 1.2 units box

The strain exx in tension direction is defined as (Lz/Lz0-1.0) and in
y and x directions are (Ly/Ly0-1.0) and (Lx/Lx0-1.0), respectively.
The Poisson can be calculated as -eyy/ezz or -exx/ezz. The result
should be the same for most materials. The strain energy you can use
the stress_zz or the derivate of total energy versus the strain ezz.



PS. I also paste your questions to the lammps mailing list so more
people can help you.