STRESS_TENSOR Convergence Issue

[Sebastian Hütter]

Hi all,

I am using cp2k to calculate elastic properties of metals (example: 
Molybdenum, bcc). I believe my solution is well converged, as I can 
calculate E(V) for different volumes around the relaxed state, fit 
Birch-Murnaghan (i.e. using gibbs2) and can find the correct bulk modulus 
(~300GPa) from this. 
However, when I try to use analytical STRESS_TENSOR output to compute the 
full stiffness tensor from 6 deformations (x,y,z,xy,xz,yz), there appears 
to be a strong sensitivity on simulation volume. I have then tried 
different MULTIPLE_UNIT_CELL values, and so far have not seen anything 
looking like convergence. I have tried:
2x2x2 (16 Atoms), plausible result for c11&c22, but c44 is wrong,
3x3x3 (54 Atoms), plausible result,
4x4x4 (128 Atoms), plausible c11&c22, c44 wrong,
5x5x5 (250 Atoms), wrong result,
6x6x6 (432 Atoms), plausible result,
7x7x7 (686 Atoms), wrong result
All converge to the same energy/cell with high precision, so I wouldn't 
expect any problem there. It should be noted that the 'plausible' results 
are very close to each other and the 'wrong' results are very close to each 
other as well. The 'plausible' values are also very close to what I have 
found going back from known E, B and G and also from published experimental 
values, that is C11 ~550GPa, C12~150GPa, C44~100GPa.

Please find attached the base input file. Deformation in X with e = 0.05 
replaces the cell parameters with the following, and so on:
A      3.27239      0.00000      0.00000
B      0.00000      3.11656      0.00000
C      0.00000      0.00000      3.11656

Do you see something wrong with my input file? I am a bit confused by the 
fact that the energies seem to be converged well but forces not so much...


Thanks in advance,

Sebastian Hütter
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