# Elastic constant

Dear all,

I noticed that in the elastic example written by Aidan, the boundary condition of the simulation box is set to be ppp. But I need to calculate the elastic constant of a system with boundary condition psp. I am wondering if this is possible since fix box/relax cannot be used in a box with non-periodic dimension. If possible, how should I modify the input? Any comment will be helpful. Thank you in advance.

Best regards,

Zhuo

You can use fix box/relax in periodic dimensions (e.g. x,z in your example).
But you can’t in y since it is non-periodic. There is nothing to “relax”

in a non-periodic dimension, since the “relax” is done by adjusting the
periodic length.

Steve

Dear Steve,

Thank you for your reply. So this means that I cannot calculate the elastic constant of a system with psp boundary condition?

Best regards

Zhuo

Dear Steve,

Thank you for your reply. So this means that I cannot calculate the elastic
constant of a system with psp boundary condition?

how should it be possible?

the parameter you compute is a material property and thus you need a
bulk system. how would you want to quantify and correct the impact of
the surfaces?

axel.

Dear Axel,

Thank you for your reply. I think when the thickness of a film is very small, its free surfaces may have effects on its elastic constant. So I am trying to build a box with psp boundary condition and change the size of the box in y dimension to study how these free surfaces affect the elastic constant. So, it is impossible to do this by using Lammps?

Best regards,

Zhuo

Dear Axel,

Thank you for your reply. I think when the thickness of a film is very
small, its free surfaces may have effects on its elastic constant. So I am

not the elastic constant, as that is a material property, but the
elastic behavior.

trying to build a box with psp boundary condition and change the size of the
box in y dimension to study how these free surfaces affect the elastic
constant. So, it is impossible to do this by using Lammps?

it doesn't make any sense. if you have a non-periodic y dimension,
changing the box size has no effect. you certainly can manually
displace atoms, and
you most certainly can compute stress/strain relations, but that would
be meaningful for the periodic directions.

your primary problem is not a problem of whether this can be done with
LAMMPS, but a problem of asking the right question and computing a
meaningful property.

Dear Axel,

Thank you for pointing out my mistake.

Best regards,

Zhuo