Dear all
I have read some of the discussions in the mailing list
(https://www.mail-archive.com/wien@zeus.theochem.tuwien.ac.at/msg11037.html)
and somewhere else but I couldn’t find some useful information to solve my
doubts; I actually got more confused. I am doing core hole calculations for
alpha-Fe2O3 and I want to compute just the DOS for this system for holes from
Fe 1s, 2p, and 3s (and possibly 3p). I have some doubts regarding the correct
procedure. I would appreciate any feedback on the following points.
1) I am not sure if the crystal structure I made is correct. As a starting
point I used the .cif file downloaded from AMCSD web site with space group R-3c
(167), then converted it to .struct file with RMT Fe=1.91, RMT O=1.64. This
structure is corundum and all Fe atoms are equivalent. However, to reproduce
experimental observations, spin-polarized calculations with LDA+U are required.
So I need to make Fe atoms inequivalent and assign the spin orientation along
the [0001] direction, with Fe atoms alternating spin in planes {0001}. For this
I created a 1X1X1 supercell from the original file and later execute x nn, x
sgroup and x symmetry. Sgroup suggested a new space group P3c1 (158) with 6
inquivalent Fe atoms and 3 O equivalent atoms which I accepted and then
labelled the Fe atoms as Fe1,... Fe6. Then I called instgen and assigned the
spin orientation to each Fe and O atom (oxygen is non-magnetic), and run the
rest of the initialization programs with 100 k points, ecut= -6 Ry, RKmax=7,
and for the functional PBE. Thus, since now I have six inequivalent Fe atoms, I
would have, after the scf cycle, 6 partial DOS (actually 12, 6 up and 6 down),
one for each Fe atom. Is this correct?
2) For core hole calculations there exist at least two options to send the core
electron. After removing a core hole (from a core level: 1s, 2s, 2p, or 3s) in
the file case.inc we can send this electron to:
a) the valence electrons in case.in2c; or,
b) to the background charge in case.inm
I’m not sure where to add the electron, what physical criterion should I apply?
In any case, I decided to try both cases. For a), I removed the electron from
the file case.inc for the levels 1s, 2s and 3s (of course, in three different
computing sessions). For b), I removed one electron from the same three levels
in case.inc. The core electron (for each calculation) was removed from only one
Fe atom, say Fe1 (the remaining atoms kept the same configuration), as follows:
Core hole from Fe1 1s
1,-1,1 ( N,KAPPA,OCCUP) 1s electron removed
2,-1,2 ( N,KAPPA,OCCUP)
2, 1,2 ( N,KAPPA,OCCUP)
2,-2,4 ( N,KAPPA,OCCUP)
3,-1,2 ( N,KAPPA,OCCUP)
Core hoe from Fe1 2p
1,-1,1 ( N,KAPPA,OCCUP)
2,-1,2 ( N,KAPPA,OCCUP)
2, 1,1 ( N,KAPPA,OCCUP) 2p electron removed
2,-2,4 ( N,KAPPA,OCCUP)
3,-1,2 ( N,KAPPA,OCCUP)
Core hole from Fe1 3s
1,-1,1 ( N,KAPPA,OCCUP)
2,-1,2 ( N,KAPPA,OCCUP)
2, 1,1 ( N,KAPPA,OCCUP)
2,-2,4 ( N,KAPPA,OCCUP)
3,-1,1 ( N,KAPPA,OCCUP) 3s electron removed
Electron added to the valence electrons
The electron was added to the valence electrons in case.in2. Before we had
-12.0 276.0 0.50 0.05 1 EMIN, NE, ESEPERMIN, ESEPER0, iqtlsave
And after adding the electron the file look like
-12.0 277.0 0.50 0.05 1 EMIN, NE, ESEPERMIN, ESEPER0, iqtlsav
3) Here I have a question, where exactly is the electron added? The 3d band,
the 4s band? In the user’s guide, it reads that the -12 stands for the lower
energy cut-off for defining the range of occupied states and NE is the number
of electrons (per unit cell) in that energy range. Does it mean that those 277
electrons are in states with energies higher than -12 Ry. In other words, is it
possible to decide where exactly in the valence to put the excited electron?
Electron added to the background charge
The electron was added to the background charge in case.inm. Before we had
MSR1 0.0 YES (BROYD/PRATT, BG charge (-1 for core hole), norm)
According to Prof. Blaha for core hole we need to type -1, so the file after
the change looks like this
MSR1 -1 YES (BROYD/PRATT, BG charge (-1 for core hole), norm)
4) Here I have another question. The user’s guide reads that the second number
in the line, bgch, means “background charge to apply to the cell”. Normally,
this is set to 0.0, meaning that the cell is not charged??? I do not have clear
what is called background charge. Could you please explain what is the nature
of this background charge. As I understand, the cell/material is electrically
neutral, and we only have charges due to nuclei and electrons, where does this
background charge come from? This question is connected with the following.
Since I want LDA+U, I also included the files case.orb and case.indmc. Then I
executed:
runsp_lapw -orb -ec 0.001
After the scf cycle, Prof. Blaha suggested to check charge neutrality. I did in
all cases using grep :NEC01 in case.scf and obtained:
NEC01: NUCLEAR AND ELECTRONIC CHARGE 456.00000 454.90177
so, I have no charge neutrality. I added the electron to the valence or
background before running the scf cycle, as suggested anywhere, why there is no
charge neutrality?
5) After the scf-cycle, for the case in which the electron was added to the
valence electrons, I removed the electron from the file case.in2c and computed
the DOS. But, what should I do for the case in which the electron was added to
the background? Should I reset to 0.0 in case.inm before calculating the DOS?
6) My final question. When we run lstart we define the cut-off energy, say – 6
Ry, that separates core from valence states. In my case, with this energy, I
have, in case.outputst, as core states 1s, 2s, 2p, and 3s; and as valence
states, 3p, 3d, 4s. However, in the file case.inst core states are considered
those of Ar and 3d and 4s as valence states. So, during the computation of the
scf, how is the 3p level taken, as a core or a valence state? If I changed
cut-off energy=-4 Ry to consider the 3p as valence state (and so do core hole
calculations for this level) then I get a charge leakage warning! I am confused
about this apparent contradiction in these two files.
I would be happy if you could help me.
Best Regards
Israel Perez
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