Negative parts of elastic tensor from VASP

[filip.formalik]

Hello
I am calculating elastic properties of cubic, organic-inorganic crystals. I use IBRION=6 and ISIF=4 (so there were 2 deformations of the crystal for each of x y z directions, and also 2 shear deformations for each of three angles). I obtained total elastic moduli:

TOTAL ELASTIC MODULI (kBar)
Direction XX YY ZZ XY YZ ZX
--------------------------------------------------------------------------------
XX 143.4104 135.0106 135.0106 0.0000 -0.0000 -0.0000
YY 135.0106 143.4104 135.0106 0.0000 0.0000 -0.0000
ZZ 135.0106 135.0106 143.4104 0.0000 -0.0000 -0.0000
XY 0.0000 0.0000 0.0000 -7.0180 0.0000 -0.0000
YZ -0.0000 0.0000 -0.0000 0.0000 -7.0180 0.0000
ZX -0.0000 -0.0000 -0.0000 -0.0000 0.0000 -7.0180
--------------------------------------------------------------------------------
As one can see, compotents responsible for shear deformations are negative indicating that the crystal is not mechanically stable. However, the energies of each deformation are higher than the relaxed strucutre:

0 E = -1723.34425 eV
1 E = -1723.21508 eV
2 E = -1723.20992 eV
3 E = -1723.21508 eV
4 E = -1723.20992 eV
5 E = -1723.21508 eV
6 E = -1723.20992 eV
7 E = -1723.28147 eV
8 E = -1723.28147 eV
9 E = -1723.28147 eV
10 E = -1723.28147 eV
11 E = -1723.28147 eV
12 E = -1723.28147 eV

where 0 is the energy of the relaxed structure, 1 is +x , 2 -x, ...,7 is +alpha and 8 is -alpha and so on.
Can someone explain this odd result?

Filip

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