On Wed, Sep 09, 2015 at 10:49:57AM +0200, pieper wrote:
> I took the freedomn to use your data to test a little fitting procedure  
> I just wrote. I tried to fit parameters a, b, c of the function
> y=a(x-b)**2+c
> to your data.
>
> The fit crashed - probably like the one you may have used - when  
> presented the data and asked to fit the parabola from scratch. The  
> reason for such a behaviour of a fit procedure frequently is bad user  
> behaviour: The program is quite understandably unable to guess  
> reasonable starting values for the fit parameters from such data.
>
> Presented with starting values
> a=1.000000e-06,   b=9.500000e+03,  c=-8.857000e+04
>
> the fit finds without complaining a very good fit with
> a=5.687893e-07,  b=9.612186e+03,  c=-8.856770e+04
>
> So you should do your DFT volume optimization around a volume of 9612 -  
> and probably give a little thought to the problem of why you started so  
> far off the optimum at larger volumes.

Marzieh,

Martin description of your problem shows you perfectly what is happening.

May I indicate my practice in studying elastic properties and phase
transition problems?

If you know the equilibrium volume of your material (previous
calculations, experiments ...) try to use a +/- 10 or 15% for your
calculations. The contraction part is essential to determine bulk
modulus and high pressure properties. The expansion part approaches
linear behavior if the range is small enough.

Best regards,
             Víctor Luaña

--
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