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

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
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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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