[Pw_forum] why don't I get an integer magnetization in a finite system

[Paolo Giannozzi]

On Jan 24, 2011, at 22:32 , Eduardo Ariel Menendez Proupin wrote:

> The reason for getting a non integer magnetization may be
> [...]
> c) other?

spin contamination? there is no guarantee in DFT to find a
solution with a well-defined spin value S(S+1), even if you
constrain S_z, in a finite system. Not sure t applies to your
case, though

P.
---
Paolo Giannozzi, Dept of Chemistry&Physics&Environment,
Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
Phone +39-0432-558216, fax +39-0432-558222

[Pw_forum] why don't I get an integer magnetization in a finite system

[lfhuang]

Dear E. A. M. Proupin:
>  total magnetization   = 2.79 Bohr mag/cell
Do you expect it to be 4.00 Bohr? If yes, it may be due to the smearing 
technique you used.  
> In this case, with an even number of electrons, it should be 0, 2, 4, or 2N.
It could also be 0, 0, ...,  or 0 (Bohr), which is dependent on the interaction 
of electrons in your system. 
> b) smearing.  occupations = 'smearing', smearing = 'cold', degauss = 0.02,
The smearing technique could influence the magnetization moment, you may get 
more reasonable results by decreasing "degauss" down to 0.005 or lower (even 
0.00). 
> And the solution is
> c) tot_magnetization = 2, ! or any even numbe
 It is not different from setting "tot_magnetization = 1.0"  PS: the smearing 
technique is very helpful in some cases, like optimization of lattice 
constants, relaxation of atomic positions, reaction-path searching et al., but 
sometimes, it may not be a good choice in precise description of electronic 
structures.   
 May these be somewhat helpful.  
Best Wishes! 
Yours Sincerely
L. F. Huang
--
==
L.F.Huang(???) DFT and phonon physics
==
Add: Research Laboratory for Computational Materials Sciences,
Instutue of Solid State Physics,the Chinese Academy of Sciences,
P.O.Box 1129, Hefei 230031, P.R.China
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[Pw_forum] why don't I get an integer magnetization in a finite system

[Eduardo Ariel Menendez Proupin]

Hi,
I am making calculations for a nanocrystal, isolated system in a supercell,
only gamma point. I have 10 angstrom separation between a nanocrystal border
and the border of the closest image.
 I am a bit surprised to obtain a non integer value of the total
magnetization.

total energy  =   -9085.20537799 Ry
 Harris-Foulkes estimate   =   -9085.20537567 Ry
 estimated scf accuracy<   0.1138 Ry

 total magnetization   = 2.79 Bohr mag/cell
 absolute magnetization= 3.11 Bohr mag/cell

There is not SCF convergence yet, but the restart is waiting in a queue. I
think that the magnetization should be integer.
Should be?
In this case, with an even number of electrons, it should be 0, 2, 4, or 2N.
The reason for getting a non integer magnetization may be
a) supercell not large enough
b) smearing.  occupations = 'smearing', smearing = 'cold', degauss = 0.02,
c) other?

And the solution is
a) Larger supercell
b) fixed occupation, but not if the system is metallic
c) tot_magnetization = 2, ! or any even numbe
d) other

Thanks for the help

Best regards




Eduardo Menendez
Departamento de Fisica
Facultad de Ciencias
Universidad de Chile
Phone: (56)(2)9787439
URL: 
http://fisica.ciencias.uchile.cl/~emenendez
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