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

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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

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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