[Wien] simple question about scf convergence of hematite supercell

[swati chaudhury]

Hi Laurence, Pavel and all Wien users,
  Thanks for your suggestion. I have done ground state calculation of hematite as non-magnetic, spin-polarised and antiferromagnetic way.No scf convergence problem. The supercell contains 120 number of atoms.But on the basis of your suggestion, now I will treat the supercell in antiferromagnetic way becoz hematite is antiferromagnetic at 0K.
  Regards.
  Swati
  
Laurence Marks <L-marks at northwestern.edu> wrote:
  The answer to this is that you have to think about the physics of your
problem, and setup a calculation which is representative of this. If
you pose the physics very badly in general you will get nowhere; the
iterations may not coverge, the forces will be inconsistent and the
results rubbish. Key things:
1) Read the UG carefully, as well as the other notes on the web pages
so you understand the underlying physics.
2) Don't just use an RKMAX, think about the plane wave sampling
(RKMAX/min(RMT)) as well. For instance, this should be at least 3.5
for a "good" calculation.
3) Think about what the k-point sampling means. 1 k-pt for a small
supercell is not reasonable.
4) Think about whether it is reasonable to run Fe2O3 without spin. Fe
is magnetic, so you are trying to run a spin-polarized system without
spin which is incorrect physics.
5) Think about whether it is reasonable to run Fe2O3 using simple GGA,
or if this will give incorrect physics.

On Mon, Jun 16, 2008 at 6:19 AM, swati chaudhury wrote:
> Dear all Wien users,
> I am having problems with convergence of supercell of
> hematite (Fe2O3, non-magnetic).
> I have tried by changing so many parameters like mixing factor
> (PRATT/BROYD/MSEC with 0.05,0.01,0.10,0.40), global E-parameter (0.35,0.30),
> K-points (1,8) etc. So I am now little bit confused that is there any other
> parameters, which can help to convergence scf cycle.
> Basic inputs:
> Supercell : 2x2x1 with 10 bohr slab
> Space group=147 (P –3)
> 13 GGA, -9.0 cut-off value, Rkmax =5.40, Emin = -9.0, Emax = 2.50, 14 - Gmax
> Please help.
> Thanks in advance.
> Regards.
> Swati.
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>



-- 
Laurence Marks
Department of Materials Science and Engineering
MSE Rm 2036 Cook Hall
2220 N Campus Drive
Northwestern University
Evanston, IL 60208, USA
Tel: (847) 491-3996 Fax: (847) 491-7820
email: L-marks at northwestern dot edu
Web: www.numis.northwestern.edu
Commission on Electron Diffraction of IUCR
www.numis.northwestern.edu/IUCR_CED
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