[Wien] supercell structure under pressure

[Rocquefelte]

Dear Shi Kong,

We have very few data to answer properly to your question. What are the
available experimental data and more specifically:
1/ Do you have the atomic structure (cell param. and atomic positions)
for different pressures or only cell parameters variation with pressure?
2/ In the "normal" structure how many equivalent sites do you have for
the f-element? 1 or more?

If you have one equivalent position for the f-element in the "normal"
structure, then in the supercell you have many inequivalent sites for
the f-element.
In other words, during the calculation of the small cell you can easily
converged to metal state because all f-atoms are equivalently occupied.
In contrast, in the supercell you allow to have different electronic
configurations for the f-atoms in inequivalent sites. In such situation,
you should be very careful during the SCF convergency, because you could
have charge oscillations due to the alternative occupation of the
inequivalent f-atoms. In addition, you should need to help the system to
converge towards the "global" minimum electronic configuration using
such an approach:
- change the smearing method (efmod in case.in2), i.e. use GAUSS for
instance with large eval value at the beginning to 0.05
- reduce progressively eval

But it should also be due to an incorrect model for the high pressure
structure:
- because you do not have enough or accurate experimental data
- because the supercell you have generated exhibits an ordering which
lead to the opening of a band gap.

Regards

Xavier


kongshi a écrit:
> Dear WIEN2k users:
> Recently I'm calculatingthe f-electron material under pressure with
> LDA+U.There are different results between with normal structure file
> and with supercell structure file. The result of normal structure file
> with symmetry under pressure is metal. But the supercell structure
> under pressure result is insulator.The result of spuercell structure
> file in atmospheric pressure is metal. In my opinion the pressure
> drive the material to the metal state.
> Can anybody tell me what the problem is? Thank you in advance.
> shi kong
>
>
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