calculating elastic constant

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1

Hi all,

I am using script developed by Aidan Thompson from example sub-directory in lammps to calculate elastic constant of amorphous polymer, however, the results produced are two magnitudes higher than what I’ve expected. At the same time, elastic constants are different from each other with various deformation magnitude variable while as far as I know they should not vary so much. I’ve only modified the init.mod and potential.mod to read data from a structure file and leaves displace.mod unchanged. The rdf, energy fluctuation of the system seems OK to me. Could someone please tell me what causes could it be to such a huge elastic constant? Since I am relatively new to lammps, it’s hard to locate the problem.

Thank you all!

Best,
Jing

tri_equ_cfg1.structure (771 KB)

2

Maybe Aidan can comment.

Steve

3

There are three issues:

-The script you are using measures the elastic constants in the limit of
zero temperature, which may not be a good approximation for your amorphous
polymer, which presumably is supposed to be at 300 K. See
http://lammps.sandia.gov/doc/Section_howto.html#4_18.

-The zero temperature elastic constants require that you start the
calculation at a local energy minumum, with positive curvature of the
enrgy surface is all 3N-3 degrees of freedom. This can be hard to achieve
for amorphous polymer.

-Finally, there is the usual problem of making sure that the atom
positions are converged to the new energy minimum after the deformation.

Aidan