[Wien] How to simulate the ionic state of a compound?

Wed Aug 23 14:14:35 CEST 2017

I am not sure where this discussion is wandering. There is one general
point of some importance: the " Charge" or "Ionic State" is not an absolute
number, it changes with the property/experiment:

1) Bader charge: a measure of the ionicity.
2) Valence charge, as normally used this is the charge measured in an
electrochemical experiment. This corresponds to a purely ionic model.
3) Spectroscopies charge, what is measured using XPS and many other
Spectroscopies, and is close to the bond valence sum.
4) Born charge, the dynamic charge, for instance the effective charge of a
moving atom that one can get from a Berry phase calculation.

1)-4) are all different, and if you use them inappropriately GIGO. For
instance, there is the infamous polar catastrophy at surfaces/interfaces
that arises when people nievely use 2).

---
Professor Laurence Marks
"Research is to see what everybody else has seen, and to think what nobody
else has thought", Albert Szent-Gyorgi
http://www.numis.northwestern.edu
Corrosion in 4D http://MURI4D.numis.northwestern.edu
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Co-Editor, Acta Cryst A

On Aug 23, 2017 06:58, "pieper" <pieper at ifp.tuwien.ac.at> wrote:

> Not knowing what your average Fe knows I still would guess it probably
> doesn't. Maybe thats why nothing else changes when you relabel atoms in
> xcrysden. However, upon adding two protons (and some neutrons) in
> Wien2k, Fe seems to know it became Ni - we learned from the initial
> question that it says so through ETOT. ;-)
>
> Best regards,
>
> Martin Pieper
>
> ---
> Dr. Martin Pieper
> Karl-Franzens University
> Institute of Physics
> Universitätsplatz 5
> A-8010 Graz
> Austria
> Tel.: +43-(0)316-380-8564
>
>
> Am 23.08.2017 11:16, schrieb Fecher, Gerhard:
> > Funny discussion,
> > I wonder wether iron (Z=26) knows that it is Nickel just if one gives
> > it a new name ?
> >
> >
> > Ciao
> > Gerhard
> >
> > DEEP THOUGHT in D. Adams; Hitchhikers Guide to the Galaxy:
> > "I think the problem, to be quite honest with you,
> > is that you have never actually known what the question is."
> >
> > ====================================
> > Dr. Gerhard H. Fecher
> > Institut of Inorganic and Analytical Chemistry
> > Johannes Gutenberg - University
> > 55099 Mainz
> > and
> > Max Planck Institute for Chemical Physics of Solids
> > 01187 Dresden
> > ________________________________________
> > Von: Wien [wien-bounces at zeus.theochem.tuwien.ac.at] im Auftrag von
> > Víctor Luaña Cabal [victor at fluor.quimica.uniovi.es]
> > Gesendet: Mittwoch, 23. August 2017 00:33
> > An: A Mailing list for WIEN2k users
> > Cc: Victor Luaña
> > Betreff: Re: [Wien] How to simulate the ionic state of a compound?
> >
> > On Tue, Aug 22, 2017 at 11:41:53PM +0200, pieper wrote:
> >> my response to Abderrahmane Reggad appears perhaps a little harsh. It
> >> was not meant that way. I wanted to emphasize that in my view the idea
> >> of telling Wien2k (or any other DFT program) its result (where the
> >> electrons are) and to simulate properties from there is completely
> >> backwards.
> >
> > Martin,
> >
> > It was not harsh at all in my perception, but I canot say what
> > Abderrahmane thought about your words.
> >
> > I perceived as a normal exchange of ideas in a subject that I love.
> >
> > I agree that using a questionable populaton technique to obtain a
> > number
> > of electrons associated to an ion from a expensive DFT calculation
> > is not neccesarily a good idea. Providing a number to say this is a
> > Ni(+1.98) O(-2.02) crystal should not be the main result from a good
> > article. However, a good physically based theory of chemical bonding in
> > molecules, solids and clusters provides far more that that.
> >
> > Let me be very particular in describing QTAIM (quantum theory of atoms
> > in molecules, aka Richard F. W. Bader et al work) as a beautiful and
> > well founded physical teory on chemical bonding.
> >
> > Bader's QTAIM can be used to partition every property into
> > contributions
> > of a particular atom or functional group.
> >
> > Then you can find an explanation why many perovskite oxides, for
> > instance, have a very similar bulk modulus, and the explanation comes
> > from the fact that the oxide is the major contribution, and that
> > contribution is common to the incumbent crystals.
> >
> >> This is, however, what I wanted to point out: DFT (or Wien2k) tells
> >> you
> >> where the electrons are. Thats its central result. It does not make
> >> any
> >> sense (to me) to use a DFT program to - as A. Reggad put it -
> >> "simulate
> >> the NiO compound in its ionic state". If NiO would be a ionic compound
> >> then DFT would (hopefully, when set up properly) calculate an electron
> >> density with a lot of weight at O and a lot less at Ni as a RESULT.
> >> The
> >> simulation of any property one wishes to study can proceed from there.
> >
> > I agree, of course. That should be the spirit of an ab initio
> > calculation.
> > However, DFT sometimes separates from the ab initio family when the xc
> > functional is chosen not because of its properties or for exploring the
> > sensibility of the predictions to the functional but because a spurious
> > predictive agreement to whatever 'experiment'.
> >
> > I believe finding explanations to peculiar facts is the real motive
> > of computational research, at least until the point is achieved at
> > which theoretical calculations become competitive with experimental
> > measurements in terms of precision, exactitude and cost.
> >
> >> And if the electron density of NiO does not really resemble the ionic
> >> picture, why use the ionic model to simulate things?
> >
> > Yes, but having a predictive measurement of the ionicity lets you
> > examine how ionicity depends on thermodynamic conditions: is the effect
> > of pressure more important than the effect of temperature or pH ... on
> > a geological scale?
> >
> > *Chemical bonding* can be at the core of geophysics, research on
> > materials, ... and many more fields. However, I'm a peculiar chemist
> > and
> > I love Physical Review since I was working on my phd thesis, long ago.
> >
> > Best regards from sunny north Spain,
> >                                     Víctor
> > --
> >     .  .    "Half of the US people use twitter to form its opinion and
> > half
> >    / `' \   also elect the US president. I only hope they are not the
> > same
> >   /(o)(o)\  half". --From a sentence by Gore Vidal
> >  /`. \/ .'\
> > /   '`'`   \ "[Technocrats, academics and journalists] are often
> > motivated
> > |  \'`'`/  | by hope while the public at large tends to be more focused
> > |  |'`'`|  | on fear." -- Russell Mead (The Washington Post, 2017)
> >  \/`'`'`'\/
> > ==(((==)))===================================+==============
> =============
> > ! Dr.Víctor Luaña, in silico chemist & prof. !
> > ! Departamento de Química Física y Analítica !
> > ! Universidad de Oviedo, 33006-Oviedo, Spain !
> > ! e-mail:  <victor at fluor.quimica.uniovi.es>  !
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