[Pw_forum] why total energy vs. lattice parameter is not a parabola?

[Stefano Baroni]

On Oct 20, 2006, at 2:53 PM, H.S.Domingos wrote:

> I would suggest that you test all the potentials that you are using  
> individually. By, for example, making small bulks with just one  
> element.
> After you have made certain that the convergence of the cutoffs is  
> addequate and that the k-point density also converges every  
> calculation, then you consider a strontium titanate cell with all  
> well tested.

Hi Helder: I think your suggestion is a sensible one, particularly in  
what concerns the choice of the cutoff (which is an atomic property,  
hence independent of the structure) vs. potential. As for k-point  
sampling, instead, it depends on the specific band structure, and so  
a k-point set that is adequate for a material may prove to be  
inadequate (or even meaningless, if the structure is totally  
different) for another ...

S.

> I have had a similar problem with ZnO and it is essential to get  
> potentials, cutoffs, k-points and geometries right.
>
> I would also warn you that strontium titanate has displacive  
> transitions and  that for very large bulks your results should be  
> different since there are coordination polyhedra that will be rotated.
>
>
> Helder
>
>
> ====================================================================== 
> =
> |  Dr. Helder S.  
> Domingos                                             |
> |                                                                      
> |
> | INESC Microsyst & Nanotechnol, Lisbon, P-1000  
> Portugal              |
> |  
> and                                                                 |
> | R&D unit for  Molecular Chemical  
> Physics                            |
> | Chemistry Department, University of  
> Coimbra                         |
> ====================================================================== 
> =
>
>
> On Thu, 19 Oct 2006, vu ongphuong wrote:
>
>> Dear all users,
>>
>>  I calculate the lattice canstant of SrTiO3. When I plot the   
>> total energy vs. lattice parameter, the shape is not a parabola  
>> even the  lattice parameter is taken in very near-minimum-region.  
>> The the lattice canstant obtained is 3.851Angstrong (the   
>> experimental value 3.905)
>>
>>  When I calculate with VASP package the result is 3.86 and the  
>> total energy vs. lattice parameter is really a parabola.
>>
>>  Anybody can explain me?
>>
>>
>> ---------------------------------
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---
Stefano Baroni - SISSA  &  DEMOCRITOS National Simulation Center -  
Trieste
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