[Wien] QTL-B error for Zn K-edge ELNES

[Paul Fons]

Dear Peter,

Hello from Japan.
Thank you very much for your comment.  I have always employed supercells, but the sign of the compensating charge was more subtle.  In truth, for the MgO supercell example, I tried both -1 and +1 in case.inm, but I didn't see much different in the XSPEC spectra.


Paul
On Aug 27, 2014, at 16:11, Peter Blaha <pblaha at theochem.tuwien.ac.at> wrote:

> About core-hole calculations for XSPEC or TELNES:
> 
> 1) You need to make a sizable supercell !!! Maybe start with 16 atoms/cell, but 64 or more are even better. It is WRONG to make a core-hole calculation with just 2 Zn atoms/cell. Why: Consider reality: You excite only VERY few atoms in such a measurements (actually VERY few even means 1 out of 10^10 or less!!!), the majority of atoms remain in the ground state and can contribute to the screening. This is, what we want to simulate.
> 
> 2) We need to have charge neutrality, i.e. the sum of nuclear charges and electronic charges must cancel. By removing 1 e- from one atom in the cell, you need to add this charge somewhere. 2 possibilities:
> a) add it as valence electron (increase NE in case.in2; but don't forget to remove this e- AFTER the scf cycle and before calculation the spectra)
> b) add a background charge in case.inm.
> MSR1  -1.0  YES  (BROYD/PRATT, extra charge (+1 for additional e), norm)
> As the comment says, you should set this value to +1, when you have added an e-. For core holes, we have REMOVED an electron, thus we must set this to   -1.0   !!!
> You SHOULD ALWAYS CHECK your charge neutrality using
> grep :NEC01 case.scf.
> 
> Now which method is better, a) or b) ???
> In many cases there is NOT much difference, though in some cases there could be a difference.
> Again, remember we want to simulate experiment as close as possible:
> the Zn 1s electron will be excited (dipole rule) into Zn-4p electrons. The electronic structure of ZnO has mainly Zn-s and p character at the conduction band minimum, thus when we use method b), we actually add this e- into Zn-s+p states, which is "approximately" correct. However, since the Zn-4sp states are very delocalized states with little weight inside the Zn-sphere, I don't think it matters too much when you put the charge into the background.
> An opposite example would be eg. CeO2, which has very localized Ce-4f states just above EF. When simulating O-K edges, it is NOT a good idea to add the e- into the valence, because it will go into LOCALIZED Ce-4f states and not contribute significantly to screen the core-hole on oxygen. The method b) would be better in this case. Of course, for Ce-M edges obviously method a) should be much better.
> 
> Hope this clarifies the unclear discussion on the mailing list.
> 
> Peter Blaha
> 
> -------------------------------
> Subject: Re: [Wien] QTL-B error for Zn K-edge ELNES
> From: Paul Fons <paul-fons at aist.go.jp>
> Date: 08/27/2014 06:37 AM
> To: A Mailing list for WIEN2k users <wien at zeus.theochem.tuwien.ac.at>
> Hi Kevin,
>    Are you sure you are correct for using "-1" in case.inm.  I can reproduce the example from the Wien2K course (MgO supercell) just fine when I use a "+1" in case.inm.  The documentation states that "1" adds another electron.  From what I understand this is to compensate for the electron that is removed when a core hole is generated by changing the occupancy in case.inc.  For the case of MgO.inc
> 
> 1 0.00  0  NUMBER OF ORBITALS (EXCLUDING SPIN), SHIFT, IPRINT
> 1,-1,2               ( N,KAPPA,OCCUP)
> 1 0.00  0  NUMBER OF ORBITALS (EXCLUDING SPIN), SHIFT, IPRINT
> 1,-1,2               ( N,KAPPA,OCCUP)
> 1 0.00  0  NUMBER OF ORBITALS (EXCLUDING SPIN), SHIFT, IPRINT
> 1,-1,2               ( N,KAPPA,OCCUP)
> 1 0.00  0  NUMBER OF ORBITALS (EXCLUDING SPIN), SHIFT, IPRINT
> 1,-1,2               ( N,KAPPA,OCCUP)
> 1 0.00  0  NUMBER OF ORBITALS (EXCLUDING SPIN), SHIFT, IPRINT
> 1,-1,2               ( N,KAPPA,OCCUP)
> 0
> 
> 
> I changed the second line to 1,-1,1 to make a core hole.  I then compensated for the missing charge with case.inm to add a +1 background charge via
> 
> MSR1  1.0   YES  (BROYD/PRATT, extra charge (+1 for additional e), norm)
> 0.20            mixing FACTOR for BROYD/PRATT scheme
> 1.00  1.00      PW and CLM-scaling factors
> 9999  8         idum, HISTORY
> 
> After this, I run a SCF calculation.  When it is done, I then remove the charge from case.inm and carry out a XSPEC calculation.  This is what I learned in the Wien2K course a couple of years ago when the developers had a seminar here in Japan.  As I mentioned, the above procedure reproduces the MgO core hole present plot in the exercise notes.  I would be very much interested in hearing if I am doing something incorrect.
> 
> Cheers,
> Paul
> 
> 
> 
> 
> -- 
> 
>                                      P.Blaha
> --------------------------------------------------------------------------
> Peter BLAHA, Inst.f. Materials Chemistry, TU Vienna, A-1060 Vienna
> Phone: +43-1-58801-165300             FAX: +43-1-58801-165982
> Email: blaha at theochem.tuwien.ac.at    WWW: http://info.tuwien.ac.at/theochem/
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Dr. Paul Fons
Senior Research Scientist
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