[Pw_forum] fermi level (XQ Wang)

wangxinquan wangxinquan at tju.edu.cn
Fri Aug 15 11:47:19 CEST 2008

Dear Eyvaz,

>1. Scf calculations do not give the Fermi level, because you use fixed occupations used by default for semiconductors (insulators). I.e. if nothing specified as occupations, >your system is considered as a semiconductor
>(insulator). In this case the number of bands treated is exactly half of the number of valence electrons, i.e. 4 in your case.
Do you mean that the occupations of electrons were constrained during iterations? To my limited experiences, 
the occupations should be changed by setting the "starting_ns_eigenvalue" parameters (lda+u calculation) and
the occupation matrix will be tuned through the iterations process. 

I'm sorry for that I confused "electron density" with "occupation". In my opinion, both
concepts describe the probabilty of electrons locating at lattic site. The "electron density" 
which is derived from wavefunctions belongs to quantum mechanics field, while "occupation" 
belongs to condensed matter physics field with respect to band structure.The electron density 
should be changed but the occupation of bands should be fixed in scf calculation. right?

The electron states(eigenvalue of the density matrix) which construct the band structures will not
be occupated entirely. For insulator, the valence band is full while the conduct band is empty. 
In the output file of scf I have no idea what the "fermi energy" results are. Does it not mean the 
"real" fermi energy? E.J.Yoffa and D.Adler (Phy. Rev. B, 12, 2260) have talked about the calculation 
of fermi energy for Mott insulator. I'm afraid the definition of fermi energy were misunderstood.

My brain doesn't work. Any help to remove my understanding to fermi energy will be deeply appreciated.

Best regards,
XQ Wang
X.Q. Wang
wangxinquan at tju.edu.cn
School of Chemical Engineeringand and Technology
Tianjin University
92 Weijin Road, Tianjin, P. R. China
tel:86-22-27890268, fax: 86-22-27892301

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