I calculated the stiffness and piezoelectric tensors for LiNbO3 (R3c) and, instead of obtaining something similar to this reference [https://materialsproject.org/materials/mp-3731/#piezo-props], the OUTCAR had the following matrices without zero values.
Am I missing something? The geometry is perfectly relaxed (no imaginary frequencies) and the space group is R3c.
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TOTAL ELASTIC MODULI (kBar)
Direction XX YY ZZ XY YZ ZX
--------------------------------------------------------------------------------
XX 1533.4020 778.9732 777.0688 18.8767 64.2179 19.5538
YY 778.9732 1536.7975 777.9372 18.7576 18.6529 65.3173
ZZ 777.0688 777.9372 1531.2412 63.8484 18.9424 19.3942
XY 18.8767 18.7576 63.8484 682.6572 8.3042 8.0924
YZ 64.2179 18.6529 18.9424 8.3042 682.8536 8.1416
ZX 19.5538 65.3173 19.3942 8.0924 8.1416 682.7842
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PIEZOELECTRIC TENSOR for field in x, y, z (C/m^2)
XX YY ZZ XY YZ ZX
--------------------------------------------------------------------------------
x 0.13567 -0.07023 -0.07015 -0.22956 -0.07563 -0.22964
y -0.07022 0.13576 -0.07035 -0.22492 -0.22969 -0.07561
z -0.07022 -0.07032 0.13579 -0.07566 -0.22485 -0.22480
PIEZOELECTRIC TENSOR IONIC CONTR for field in x, y, z (C/m^2)
XX YY ZZ XY YZ ZX
--------------------------------------------------------------------------------
x -5.08505 1.77311 1.77144 -0.10799 -0.08356 -0.09122
y 1.77310 -5.08640 1.77245 -0.09438 -0.10656 -0.08319
z 1.77004 1.77045 -5.08198 -0.08278 -0.09520 -0.11030
Did you use the Rhombohedral or Hexagonal cell for the R3c structure? Could be the reason!
Thanks, Jack S. Baker
I used the rhombohedral cell.
I'm new at calculating elastic and piezoelectric properties. I suspected it could be related to the orientation of the cell but I didn't know that carrying out the calculation with the hexagonal or the rhombohedral cell would lead to such a different outcome.
The structure is completely relaxed at its equilibrium volume.
Best regards,
Cesar
P.D. I include the INCAR files I forgot to attach in my previous comment.
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INCAR for Elastic constants
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ENCUT = 650.0
EDIFF = 1.e-7
NELMDL = -10
NELM = 200
ISMEAR = -5
ADDGRID = .TRUE.
LWAVE = .FALSE.
LCHARG = .FALSE.
NSW=400
IBRION=6
ISIF=3
EDIFFG=-0.01
NELMIN = 4
NFREE = 10
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INCAR for Piezoelectric
--------------------------------------------------------------------------------------------
ENCUT = 650.0
EDIFF = 1.e-7
NELMDL = -10
NELM = 200
ISMEAR = -5
ADDGRID = .TRUE.
LWAVE = .FALSE.
LCHARG = .FALSE.
LEPSILON = .TRUE.
IBRION = 8
LORBIT = 11
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