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

[pieper]

High Victor,

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.

Of course I agree with you that the electron density of a solid is a 
real observable - even in the XXI century. Quantum mechanics was 
invented to calculate such properties and the density is, after all, the 
D in DFT. I also agree that there are any number of solids where a ionic 
description makes perfect sense. I never tried but I am very confident 
that the ionic nature of alkali halides will be evident from the result 
of a DFT calculation.

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.

And if the electron density of NiO does not really resemble the ionic 
picture, why use the ionic model to simulate things?

Best regards,

Martin


---
Dr. Martin Pieper
Karl-Franzens University
Institute of Physics
Universitätsplatz 5
A-8010 Graz
Austria
Tel.: +43-(0)316-380-8564


Am 22.08.2017 20:45, schrieb Víctor Luaña Cabal:
> On Tue, Aug 22, 2017 at 07:00:07PM +0200, pieper wrote:
>> DFT in general and Wien2k especially are there to tell you with
>> remarkably high precision what the charge distribution in a given
>> structure actually looks like. Thats FAR better than any hand-waving 
>> Ni
>> is 2+, O 2-. If you don't like what DFT tells you and want to draw 
>> some
>> fictitious ionic representation of NiO, take xcrysden, plot the
>> structure, and add labels saying that the (maybe) red circles are Ni2+
>> and the (maybe) blue circles are O2-.
>> 
>> It should be obvious to you from your lectures and readings on solid
>> state physics that the total energy depends on Z. If you change the
>> nuclear charge Z you change the element at that position. One can 
>> adjust
>> the number of electrons and their starting distribution in Wien2k, but 
>> I
>> plainly won't tell you how. You will learn much more if you find out
>> yourself - start with the User Guide and a solid state physics text
>> book.
>> 
> 
> martin,
> 
> Nothing against your description and I agree basically with
> everything. However, the electron density (\rho) of a molecule or
> solid is a real and mensurable property that exists in 3D real position
> space, so the topology of \rho can be examined properly: that's Bader
> topological analysis. A mostly physical and matematical description of
> molecules and solids. Similarly, there are other scalar properties that
> are physically well defined.
> 
> By Bader topological analysis we know that some systems are truly
> ionic in nature, like alkali halides or simple oxides, but that is not
> any surprise and crystallographers have used that since Laue and the
> Braggs. Other materials are more interesting, however, and pressure
> (thermodynamics, in general) modifies behavior of BP from being mostly
> boron phosphide to phosporus boride, including a pressure range in 
> which
> electrons behave as independent chemical items and we have an 
> electride.
> [Phys. Rev. B  63 (2001) 125103, Polarity inversion in the electron
> density of BP crystal, Paula Mori-Sánchez et al.]
> 
> In molecules, for instance, we have the Electrostatic potential as a 
> well
> defined scalar property. In some ways it brings us from XX century
> (the century of quantum mechanics and electron density, quite complex
> due to the effect of correlation and basis sets) to XIX century with
> Maxwell and Faraday (positive and negative charge distributions are
> what we need to know).
> 
> Unfortunately, in solids there is a real indetermination with the zero 
> of
> the EP, that it is basically arbitrary, as much as I know.
> 
> 
> Nice subject, if you let me tell, by the way. Best regards,
>                                                Víctor Luaña
> --
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> ==(((==)))===================================+===========================
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