[Wien] [SPAM?] Should the valence electrons configuration of charge transfer insulators be changed ?

Mon Nov 18 09:02:12 CET 2019

>2) Slaters transistion state is a well known concept to calculate the
>XPS binding energy of a core state, where you would remove the excited
>electron from the system (it comes out and goes to the detector). It has
>NOTHING to do in EELS , where the excited electron stays in the system
>(except if you would attempt to calculate the absolute energy of an edge).

Hmmm. If you put the excited state into the system it goes into the LUMO as
you say in 3), in general the wrong place. Therefore I will argue that
treating it as "gone" via a Slater method in EELS makes physical sense. I
don't agree with the physics of adding it back as a real electron, sorry,
as background seems the only appropriate way.

N.B., of course, if due to the core hole (fractional or full) previously
unoccupied states at the target atom drop below the Fermi energy they will
now get filled which can be unphysical. I can't see how one avoids this for
a metal or a small gap insulator within standard DFT; neither the mixer nor
lapw2 currently have atom occupancy constraints. Using runfsm can avoid
this in some cases.

_____
Professor Laurence Marks
"Research is to see what everybody else has seen, and to think what nobody
else has thought", Albert Szent-Gyorgi
www.numis.northwestern.edu

On Mon, Nov 18, 2019, 01:12 Peter Blaha <pblaha at theochem.tuwien.ac.at>
wrote:

> I'll add a few statements about core-EELS:
>
> 1) Core hole: In principle we want to simulate the excitation of ONE
> core electron into the conduction band. Thus one should create a big
> supercell (as big as possible, at least 64 atoms) and put a full core
> hole (I guess this was NOT yet mentioned, but is the most important
> point of the discussion !!!!!). This hole will be partially screened,
> and with our limited supercell size and the static DFT approximation,
> this screening could be incomplete and thus one sometimes uses
> "empirically" 1/2  or no core hole (in particular for metals) at all.
> This is an often used method, but of course it is no longer "ab initio".
>
> 2) Slaters transistion state is a well known concept to calculate the
> XPS binding energy of a core state, where you would remove the excited
> electron from the system (it comes out and goes to the detector). It has
> NOTHING to do in EELS , where the excited electron stays in the system
> (except if you would attempt to calculate the absolute energy of an edge).
>
> 3) excited electron: In principle it is clear that the excited electron
> should go into a dipole allowed conduction band state. However, we have
> NO MEANS to select such a state and the electron will go into the first
> empty states in the system in a scf procedure.If we feel that this state
> is not the state where it would go in experiment, it is better to put
> the electron into the "background" charge (mixer). E.g in NiO the O-1s
> electron should go into a O-2p state. However, the first conduction
> bands are Ni-d states in the supercell calculation and thus adding an
> electron to the valence electrons is not appropriate. In the case of
> cuprates, I'd probably add it to the valence, since the "hole" state is
> a mixture of Cu-d-x2-y2 - O-2p and thus at least partly it is ok to put
> the electron into it. In any case, I'd do the calculation with both,
> adding the electron to valence or to background.
>
> 4) spin state: It is of course clear, that the photon does not change
> the spin state of the excited electron.In a spin-polarized calculation
> when you put the electron into the valence, it is usually obeyed anyway,
> because the missing core electron of "spin-up" will lower the potential
> of spin-up and the electron will go into the spin-up conduction bands,
> preserving the total spin of the system.
> However, correlations within the conduction bands could change this
> anyway, because the "other electrons" could react on the presence of an
> additional spin-up electron.This is in particular true for correlated TM
> oxides. And if you use the background-option, the spin.state is not
> defined anyway, since the background option cannot be done spin-selective.
>
> In non-spinpolarized calculations it should not really matter.
>
>
>
> Am 17.11.2019 um 14:58 schrieb 丁一凡:
> > As we all know, DFT deals with the system in the ground state. When
> > dealing with the charge transfer insulator system, can I modify the
> > valence electronic configuration after initialization and before SCF and
> > EELS (Electron Energy Loss Spectroscopy) calculations ?
> >
> > The Cu-based high temperature superconducting (HTSC) oxides are known to
> > be insulators of a charge-transfer type, with the charge-transfer (CT)
> > gap originating from the energy difference between the O(2p) and the
> > Cu(3dx2-y2) orbitals. Before calculating EELS of Cu-based HTSC oxides,
> > will it make the result reasonable if their valence electron
> > configuration is changed ? For example, we remove one oxygen 2p electron
> > and add one electron in Cu 3d orbit. Just like the treatment of core
> > hole effect. For a “core-hole” calculation we will edit super.inc and
> > remove one core electron from the desired atom and state (1s or 2p,
> > ...). Then we add the missing electron either in super.inm (background
> > charge) or super.in2 (add it to the valence electrons).
> >
> > This problem haunts me for several weeks, and my question is still
> > unsolved after consulting the previous mailing list. Any comment(s)
> > would be highly appreciated. Thanks in advance!
> >
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> --
> --------------------------------------------------------------------------
> Peter BLAHA, Inst.f. Materials Chemistry, TU Vienna, A-1060 Vienna
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