[lammps-users] about the "calculating Young's modulus"

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1

Hi Rohit,

I read a message about the discussion of “calculating Young’s modulus” between you and Steve and other lammps users from Google search.

http://lammps.sandia.gov/threads/thread.3150.html

Because I joined the lammps mailing list is late (11/02/2009) I could not get this message in my mailbox.

Now I also begin to do the Young’s modulus calculation, I’m not clear about the definition of stree. I want to use the stress-strain curve to determine the Young’s modulus. If I use an external force to tensile the boundary atoms, then the stress should be defined as the external force dividing the cross section area or the calculated stress in the tensile direction in lammps (including the interatomic interaction and the kinetic energy contribution)? I think you should finish the Young’s modulus calcuation of Al using lammps, can you show your lammps input script here? Thanks so much!

In addition, can anybody tell me there is a way to receive all mails from the lammps mailing list prior to my registration time?

Dongshan

2

Hi Dongshan,

For the calculation of Young's modulus, if I remember right, I did not
write a complete script with dynamic runs discussed in the thread. I was
making runs with basic commands like 'fix NPT' or 'fix deform', and
doing post-processing using MS Excel. When doing a dynamic analysis
there are issues with pressure fluctuations that you have to deal with
through averaging over time steps or taking large simulation size.

In my view, better way to calculate elastic properties is to use static
analysis, which you can do with a very small system size. Use energy
minimization to get the system properties (energy, pressures,
dimensions, etc) for deformed and un-deformed state and calculate
elastic constants from there.

For an input script: Aidan Thompson emailed an input script on this
forum to calculate elastic constants for fcc materials some time back.
You may see that to understand how to use pressures for your
calculations.

You may also use strain energies for calculating elastic constants (see
R Zhu, E Pan, PW Chung, X Cai, K M Liew, and A Buldum, Semiconductor
Science and Techhology, 21, pp. 906, 2006).

For understanding of stress: Look at the manual carefully for how LAMMPS
defines various stresses. Some references for atomic level stresses are:
AP Thompson, SJ Plimpton, W Mattson, The Journal of Chemical Physics,
131, 154107, 2009
KS Cheung and S Yip, J. Appl. Phys, 70 (10), pp 5688, 1991
J F Lutsko, J. Appl. Phys. 64 (3), pp. 1152, 1988
DH Tsai, J Chem Phys, 70 (3), pp. 1375, 1979

Rohit

3

Hi Rohit,

Thanks for your prompt reply and these references. About the static
analysis you mentioned, how to construct the deformed state? Do you
also have some references about this method?

Thanks!

Dongshan

4

You may use displace_box command. It has various options for different types of deformations.

Just search for the input file by Aidan Thompson on the forum. It is quite easy to understand.

Rohit