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["Zhang, Waigen" <wg.zhang@okstate.edu>]

Title: negative elastic constant

Dear all,
I hate to ask question about Physics concept,but....
I calculated the elastic tensor with DFPT(monoclinic structure),and the outcomes are

Elastic Tensor (relaxed ion) (unit:10^2GP):
  (at fixed electric field boundary condition)

   1.1283604  -3.1643805  -2.0347373   0.0000001   1.3721603  -0.0000001
  -3.1643805 -13.2059771  -2.2664645   0.0000000   0.4939964   0.0000000
  -2.0347373  -2.2664645  -0.0102671   0.0000000   0.7978253  -0.0000001
   0.0000001   0.0000000   0.0000000  -1.1180576   0.0000000  -0.7148172
   1.3721603   0.4939964   0.7978253   0.0000000  -0.5816261   0.0000001
  -0.0000001   0.0000000  -0.0000001  -0.7148173   0.0000001  -2.3463341

Unstable eigenvalue detected in force constant matrix at Gamma point
  The system under calculation is physically unstable.

Dose that mean the structure is mechanically not stable? If so,can I say that this structure will transfer to some other structure without any external force?

Thanks a million.


Zhang,Waigen