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["D. R. Hamann" <drhamann@mat-simresearch.com>]

Dear Zhang,

In general, if you have negative diagonal elements in your elastic 
tensor, it indicates that you have an unstable structure which would 
also  be indicated by negative gamma-point phonon frequencies in your 
RF-run output.  The relatively small negative acoustic-mode eigenvalue 
flagged in the anaddb warning shouldn't cause major problems like this.  
Getting very different results from two runs of the same data is very 
strange, and indicates that some quantities are poorly converged, with 
the errors perhaps magnified when anaddb takes the pseudo-inverse of a 
nearly-singular dynamical matrix to calculate the relaxed-atom elastic 
tensor.

Beyond this, I'm not sure I understand your input data or if this 
relates to your recent posts on Al2O3.  This looks like an N4O8 unit 
cell, in which case nband is wrong.  If this is supposed to be an (NO2)4 
molecular crystal, these are notoriously hard to structurally optimize 
because you have very stiff covalent bonds and very soft van der Waals 
bonds (which are poorly described by LDA anyway).

Don Hamann

waigen zhang wrote:
> Hi,
> Thanks for the reply for my last question.
> Another problem is that I am getting some results(elastic constant) 
> case sensitive for my material.I used the same structural geometry and 
> same ecut,ngkpt,psp...,but each time the outcomes of elastic constants 
> are quite different.BTW,I did  the structural optimization and the 
> optimized structure seems no problem before any calculation.
> I notice that there is a warning in the output file
>
> elast9 : WARNING - :
>   Acoustic sum rule violation met : the eigenvalues of accoustic mode
>   are too large at Gamma point
>   increase cutoff energy or k-points sampling.
>   The three eigenvalues are:    2.354541E-04    3.769631E-06   
> -9.936443E-05
>
> and the elastic tensor from two different runs with same input 
> parameters are
>
> Elastic Tensor (relaxed ion) (unit:10^2GP):
>   (at fixed electric field boundary condition)
> 0.5479039  -0.2078001   0.4736602   0.0000000  -0.4454188   0.0000000
>   -0.2078002  -1.6226535  -0.1456602   0.0000000   -0.0503831   0.0000000
>    0.4736602  -0.1456602  -1.7632272   0.0000000  -0.8610705   0.0000000
>    0.0000000   0.0000000   0.0000000  -0.1100148   0.0000000  -0.0896016
>   -0.4454189  -0.0503831  -0.8610705   0.0000000    0.4057265   0.0000000
>    0.0000000   0.0000000   0.0000000  -0.0896016   0.0000000  -0.2808213
>
>  Elastic Tensor (relaxed ion) (unit:10^2GP):
>   (at fixed electric field boundary condition)
>  -2.4936683  -1.4720537   -0.7323263   0.0000000   0.4624234   0.0000000
>   -1.4720537  -3.5190605  -1.2099844   0.0000000  -0.3453437   0.0000000
>   -0.7323263  -1.2099843  -0.5868132   0.0000000   0.0348794   0.0000000
>    0.0000000   0.0000000    0.0000000  -1.1759601   0.0000000  -0.3877170
>    0.4624235  -0.3453437   0.0348793   0.0000000  -0.7692686   0.0000000
>    0.0000000   0.0000000   0.0000000  -0.3877171   0.0000000  -1.5326429
>
> Attachment is the input file.Thanks for you suggestion.

--
D. R. Hamann
Mat-Sim Research LLC    | Deptartment of Physics
P.O. Box 742            |  and Astronomy
Murray Hill, NJ 07974   | Rutgers University
phone: 908-370-8079     | 732-445-4381

email: drhamann@mat-simresearch.com