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

[H.S.Domingos]

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.
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


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|  Dr. Helder S. Domingos                                             |
|                                                                     |
| INESC Microsyst & Nanotechnol, Lisbon, P-1000 Portugal              |
| and                                                                 |
| R&D unit for  Molecular Chemical Physics                            |
| Chemistry Department, University of Coimbra                         |
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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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