Dear professor,

I calculated the stiffness coefficients C11, C12 and C44 of single crystal with a default orientation of x[100], y[010], and z[001] using the examples “ELASTIC” in LAMMPS. The calculated value of pxx, pyy and pzz are approximate to 0, and finally I can obtain the right result of stiffness coefficients.

But if I change the crystal orientation or change the model, the calculation results are totally wrong. I checked the log file, after using minimize command, the pxx, pyy and pzz I obtained are around 84741.059, 43951.288 and -66543.871 bar, respectively. I think that’s why I got the wrong result.

I want to know how to calculate the stiffness coeff C11, C12 and C44 of single crystal with different orientations, bicrystal and also polycrystals.

Could you help me with my problem?

Thanks a lot!

Hi Jiaqi.

Dear professor,

I calculated the stiffness coefficients C11, C12 and C44 of single crystal with a default orientation of x[100], y[010], and z[001] using the examples “ELASTIC” in LAMMPS. The calculated value of pxx, pyy and pzz are approximate to 0, and finally I can obtain the right result of stiffness coefficients.

But if I change the crystal orientation or change the model, the calculation results are totally wrong.

If you changed the orientation of the model in “ELASTIC”, do pay attention to rotate both the atoms and the simulation cell’s vectors consistently. Try adding a dump of the atomic coordinates before and after your modification.

As for “changing the model”, no one but you can judge what is the “right” result: if you e.g. used a differently sized crystal of the same material, you should get similar results (but probably not identical due to finite-size effects). If you switch material, all bets are off.

I checked the log file, after using minimize command, the pxx, pyy and pzz I obtained are around 84741.059, 43951.288 and -66543.871 bar, respectively. I think that’s why I got the wrong result.

I want to know how to calculate the stiffness coeff C11, C12 and C44 of single crystal with different orientations, bicrystal and also polycrystals.

The results here are uniquely determined by how well your model is constructed. Take care of properly setting up your poly-crystalline system, and equilibrating it at the target temperature and pressure before attempting the computation.