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

[Víctor Luaña Cabal]

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
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