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[Xifan Wu <wuxifan@physics.rutgers.edu>]

Hi Waigen,

The physical elastic constant (relaxed-ion one) will be coupled with the
pseudo-inverse of the force constant matrix through the internal-strain
matrix. The output shows that there is probably a negative eigenvalue of
force-constant matrix in your system, which means that the energy is not 
at the global minium point with respect to the ionic displacements and
means that the claculated structure maybe not physical stable, or maybe 
wrong, or the calculations is not converged yet, etc.

You could check the input structure, the convergences. And you could also
take a reference at the paper, PHYSICAL REVIEW B 72, 035105(2005) and 
maybea good way to make sure the calculation is correct is try to
reproduce some numbers already been calculated.

good luck,

Xifan Wu

On Wed, 2 May 2007, Zhang, Waigen wrote:

> 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