AlN mp-661 piezoelectric calculation?

#1

Hello,
I’ve noticed that the MP piezoelectric calculation for wurtzite AlN, mp-661, doesn’t appear to match the values listed in de Jong et al.'s Scientific Data paper. The paper says " As an example, for the wurtzite-compound AlN, we calculate e 33=1.46 C/ m 2 and e 31=−0.58 C/ m 2 which is within 10% of the values reported elsewhere, obtained using GGA as well, but with a different implementation of DFPT and slightly different convergence parameters". However, here’s the MP piezoelectric tensor:

0.00000 0.00000 0.00000 0.00000 0.00000 0.00000
0.00000 0.00000 0.00000 0.00000 0.00000 0.00000
-0.66500 -0.66500 1.67637 0.00000 0.00000 0.00000

I tried calculating the piezoelectric properties in VASP the old-fashioned way with LCALCEPS = .TRUE., and got (electronic + ionic)
0.0 0.0 0.0 0.0 0.0 -0.30
0.0 0.0 0.0 0.0 -0.28 0.0
-0.58 -0.58 1.46 0.0 0.0 0.0
which at least agrees with de Jong et al. for e31 and e33. (According to the VASP wiki the sign of this piezoelectric tensor is wrong for VASP 5.4.4 and older, but this seems to affect everyone equally) Are the MP values transformed to a different set of axes, or something?

#2

I’ve actually been scratching my head at this problem for nearly 5 months now, thinking that it’s been the symmetry, but your LCALCEPS results helped a TON in diagnosing the issue. It turns out there are some bad DFPT calculations that were performed using LDA that were improperly tagged as GGA, but because of the difference in functionals, had a lower energy, and were being preferred, over the correct GGA calculations that are also reported in the de Jong paper.

We’re working on fixing on another problem as well right now which we hope to release as a minor update on Monday/Tuesday. I’ll work this fix into that release as well.

Thank You!

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