请教各位一下，在下刚刚转来用VASP做钙钛矿结构压电材料的第一性原理计算，但是现在计算中出现了一些问题，我计算P4mm的四方相结构时，压电离子贡献24与15方向不相等，导致计算出来的e24也不等于e15，请教下是什么原因。
POSCAR
KNbO3
1.00000000000000
3.9780718153912340 0.0000000000000000 0.0000000000000000
0.0000000000000000 3.9780718153912340 0.0000000000000000
0.0000000000000000 0.0000000000000000 4.0697524641330913
K Nb O
1 1 3
Direct
0.0000000000000000 0.0000000000000000 0.0208122828936224
0.5000000000000000 0.5000000000000000 0.5370745926342191
0.5000000000000000 0.5000000000000000 0.9956543127557665
0.0000000000000000 0.5000000000000000 0.4982294058581956
0.5000000000000000 0.0000000000000000 0.4982294058581956
INCAR
SYSTEM = KNbO3
ISTART = 0
ICHARG = 2
PREC = Accurate
EDIFF = 1E-06
EDIFFG = -0.005
NSW = 1
POTIM = 0.2
NFREE = 2
IBRION = 8
LEPSILON = Ture
ISIF = 3
ISPIN = 1
NELM = 60
ISMEAR = 0
SIGMA = 0.2
LREAL = Auto
LCHARGE = False
LWAVE = False
计算结果
PIEZOELECTRIC TENSOR for field in x, y, z (C/m^2)
XX YY ZZ XY YZ ZX
--------------------------------------------------------------------------------
x 0.00000 0.00000 0.00000 0.00000 0.00000 0.19952
y 0.00000 0.00000 0.00000 0.00000 0.19957 0.00000
z 0.16714 0.16714 0.06456 0.00000 0.00000 0.00000
PIEZOELECTRIC TENSOR IONIC CONTR for field in x, y, z (C/m^2)
XX YY ZZ XY YZ ZX
--------------------------------------------------------------------------------
x -0.00020 0.00055 -0.00176 0.00000 0.00000 -5.47889
y -0.00076 0.00097 -0.00008 0.00000 1.73507 0.00000
z -0.46524 -0.46524 4.83952 0.00000 0.00000 0.00000
d33值利用d=e*s计算出来只有20pC/N多点。烦请各位告知哪里有问题
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