[Wien] Questions about the difference charge density in the Ti core

[Ding Peng]

Dear Prof Peter Blaha,

Many thanks for your feedback. I'm very grateful to this. 

I'm sorry for my bad description of the "core" region. The "core" region that mentioned in my last email is the "round region" at the centre of Ti atomic site , which is most likely to be contributed by the core states (1s, 2s, 2p, etc.) and the semicore states (3s, 3p) of Ti. If the difference charge density is defined as the charge density of the solid minus the superposed charge densities, the centre region would be about zero since the core states haven't participate in the bonding (please correct me if I'm wrong). But what I can see from the graph is very deep purple-coloured region at the centre of the Ti site, which suggests a dense accumulation of electrons at the Ti site. So could I ask why the difference charge densities in this "round region" is very positive?

Many thanks,
Ding

 

> On 2 May 2020, at 3:48 pm, Peter Blaha <pblaha at theochem.tuwien.ac.at> wrote:
> 
> Hi,
> 
> Please read the rest of the article. There is NO indication of a Ti4+, because this is not a very ionic compound. What is evident, is a strong anisotropy of the occupied 3d electrons, both on Co and Ti.
> 
> What you seem to call "core" region, comes from the 3d electrons, whose maximum is really quite localized.
> 
> Remember, this is a "chemical valence" (in a purely ionic picture), a "formal" quantitiy.
> 
> In addition: If Ti is 4+, where would these electrons go ? Sb is in group V, so can at most take 3 electrons, thus in your ionic picture you get:  Ti4+ Co-1 Sb-3 .   This can't be correct and while it is impossible to give quantitative numbers of a charge transfer, there is some indication (Table II) that Co takes some electrons from Ti (the Co 3d states are lower in energy than Ti3d/4s), while Sb is slightly negative (Bader) or even positive (QTL). As is evident from the difference density, there is a huge region of nearly zero difference-density. These interstitial charge, however, cannot be easily and uniquely attributed to specific atoms and can stem from all delocalized wave functions (Ti-4s, Co-4s, Sb-5p - see Table I abould "localized 3d" and delocalized sp electrons).
> 
> Am 02.05.2020 um 07:45 schrieb Ding Peng:
>> Dear Wien2k experts,
>> I'm reading the latest publication for Wien2k (P. Blaha, K.Schwarz, F. Tran, R. Laskowski, G.K.H. Madsen and L.D. Marks, J. Chem. Phys. 152, 074101 (2020)) and confused about the figure showing the difference charge density of TiCoSb (Figure 9 in the paper). I noticed that the core charge densities for Ti and Co are very positive and the ones for Sb are very negative. To my best knowledge, Ti in TiCoSb is thought to have the chemical valence of +4, which means it tends to lose the electrons. However, this looks to contradict to the this graph, in which the difference charge densities in Ti are positive. So can someone explain why the difference charge densities in Ti core are very positive?
>> Sorry for asking this question that may look stupid.
>> Many thanks,
>> Ding
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> -- 
> Peter Blaha
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> TU Vienna
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