Inconsistency between Elastic Moduli Tensor Obtained from Theory and from VASP (IBRION=6)
Posted: Fri Jul 21, 2023 7:18 am
Dear users and developers,
I obtained the elastic tensor for a material using IBRION=6. The obtained total elastic moduli (kBar) is as follows:
TOTAL ELASTIC MODULI (kBar)
Direction XX YY ZZ XY YZ ZX
--------------------------------------------------------------------------------
XX 228.6477 83.8015 0.1557 0.0060 1.6150 -0.0357
YY 83.8015 228.5368 0.1111 -0.0005 -1.5993 0.0633
ZZ 0.1557 0.1111 -0.2190 0.0280 -0.0101 -0.0598
XY 0.0060 -0.0005 0.0280 72.1719 -0.0181 1.7111
YZ 1.6150 -1.5993 -0.0101 -0.0181 -4.7055 0.1320
ZX -0.0357 0.0633 -0.0598 1.7111 0.1320 -4.6553
My material is a hexagonal crystal and hence, C66 should be equal to ((C11-C12)/2) but that does not seem to be the case. Any suggestions as to why this inconsistency exists? I have calculated the elastic moduli for other 2D materials as well and the C66 obtained from (C11-C12)/2 seems to make more sense to me than the one obtained via IBRION=6. I would like to mention that my material is 2D and hence there is a vacuum length of around 20 Å along the z-axis.
I obtained the elastic tensor for a material using IBRION=6. The obtained total elastic moduli (kBar) is as follows:
TOTAL ELASTIC MODULI (kBar)
Direction XX YY ZZ XY YZ ZX
--------------------------------------------------------------------------------
XX 228.6477 83.8015 0.1557 0.0060 1.6150 -0.0357
YY 83.8015 228.5368 0.1111 -0.0005 -1.5993 0.0633
ZZ 0.1557 0.1111 -0.2190 0.0280 -0.0101 -0.0598
XY 0.0060 -0.0005 0.0280 72.1719 -0.0181 1.7111
YZ 1.6150 -1.5993 -0.0101 -0.0181 -4.7055 0.1320
ZX -0.0357 0.0633 -0.0598 1.7111 0.1320 -4.6553
My material is a hexagonal crystal and hence, C66 should be equal to ((C11-C12)/2) but that does not seem to be the case. Any suggestions as to why this inconsistency exists? I have calculated the elastic moduli for other 2D materials as well and the C66 obtained from (C11-C12)/2 seems to make more sense to me than the one obtained via IBRION=6. I would like to mention that my material is 2D and hence there is a vacuum length of around 20 Å along the z-axis.