Elastic constant

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#1

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

#2

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

#3

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

#4

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.

#5

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

#6

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.

#7

Dear Axel,

Thank you for pointing out my mistake.

Best regards,

Zhuo