[Pw_forum] Fermi energy in 'scf' and 'nscf'(XQ Wang)

[wangxinquan]

>Dear Wang,
  Dear Prasenjit,
>
>In principle both the scf & nscf should give exactly the same value of Fermi
>energy if your calculations are converged w.r.t. the k-point sampling
>(since evaluation of Fermi energy involves numerical integration in
>k-space).The fact that that the two procedures give different energies
>suggest that the k-point mesh is not converged. Since these are numerical
>integrations, the more the no of k-points the more accurate is the value of
>the integral. Therefore for Guoying's case, the value of Fermi energy
>obtained from scf is more accurate than that from nscf as suggested by
>Hande.
>
>If the difference in Fermi energy is quite large, what I would suggest is to
>increase the no. of k-points in the nscf cal. & redo it.

As I'm a newbie in this field, the basic concept of quantum mechanics and 
condensed matter physics(DFT, the Bloch theory and plane wave) are rather
abstract for me, following contents may have some misunderstanding of your suggestions. 

>From scf and nscf results, fermi energies should be consistent with each other if they were 
evaluated (integrated and converged) with respect to the same k-points. nscf (non-scf) only 
involves integrations of the special k-points using the charge density results from scf (which
have been saved in the /tmp folder). No iteration is involved in nscf. 

Furthermore nscf obtains the mean value of the integration of wavefunctions when the k-points 
were sampled by Monkhorst-Pack method. Sometimes k-points are not so dense to integrate 
accurately (certain key points may be ignored). Compare with nscf, the fermi energy in scf 
should be mostly reasonable.

When we want to calculation DOS we need to add more k-points for nscf and confirm that the 
identical fermi energies can be achieved. Otherwise we should trust the fermi energy from scf results
when we deal with band structures and dos. 


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