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I am a student in theoretical chemistry and I am confused about the paper: Trends in adsorption of electrocatalytic water splitting intermediates on cubic $\ce{ABO3}$ oxides (Montoya, J. H.; Doyle, A. D.; Nørskov, J. K.; Vojvodic, A.; Phys. Chem. Chem. Phys. 2018, 20 (5), 3813–3818. DOI: 10.1039/C7CP06539F), where the authors report DFT calculations on the oxides:

  • $\ce{MgBaO3}$ ($\ce{Mg^3+}$ and $\ce{Ba^3+}$)
  • $\ce{NaLaO3}$ ($\ce{La^5+}$)
  • $\ce{CaBO3}$ and $\ce{SrBO3}$ ($\ce{B^4+}$)
  • $\ce{ZrNaO3}$ ($\ce{Zr^5+}$)
  • $\ce{ScSrO3}$ ($\ce{Sc^4+}$)
  • $\ce{CaCuO3}$ ($\ce{Cu^4+}$)

... and many more. Are these oxidation states even possible?

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    $\begingroup$ You can DFT pretty much everything and publish a paper about it. Some people do that once a month or so. Then again, who said the oxidation states in these compounds are what you think they are? $\endgroup$ – Ivan Neretin Jul 4 '18 at 6:10
  • $\begingroup$ If oxygen is formally 2-, in total 6-, then you can guess the oxidation states of the metals. $\endgroup$ – Carlos Martinez Jul 4 '18 at 6:46
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    $\begingroup$ Who says oxygen is 2-? $\endgroup$ – Ivan Neretin Jul 4 '18 at 7:01
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    $\begingroup$ I agree; oxygen is probably not fully -2. $\endgroup$ – orthocresol Jul 4 '18 at 7:33
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    $\begingroup$ Just to note Cu4+ while rare is known - e.g. en.wikipedia.org/wiki/Caesium_hexafluorocuprate(IV) $\endgroup$ – Ian Bush Jul 4 '18 at 7:58
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Oxidation states are just numbers, a bookkeeping tool for chemistry. These hardly ever correspond to anything observable. Point in case: hypofluorous acid $\ce{HOF}$, see for example the oxidation state of oxygen and of fluorine in the confines of the definition. Maybe also of interst in that regard is the introduction to "oxidation state"/"oxidation number" in general.

Therefore instead of asking whether the oxidation states are stable, one should ask if the bulk/ molecular structure is stable.

When it comes to computationally aided catalyst design, you can pretty much use the entire periodic table to play with. It is a matter of interpreting the results that count, and obviously if experiment can reproduce such predictions. Where you start becomes a matter of taste and starting with perovskites is as good a guess as any.

The authors of the publication make quite an effort, calculating hundreds of potential catalysts. However, they also indicate problems within their methodology. Quoting from the supporting information:

3 Oxygen evolution data
[...] \begin{array}{lcl} \text{formula} & \text{values of }\Delta G, \eta, \dots & \text{warnings}\\ \hline \ce{MgBaO3} & [\cdots] & \text{a,b}\\ \ce{NaLaO3} & [\cdots] & \text{a,b}\\ \ce{CaBO3} & [\cdots] & \text{b}\\ &\vdots&\\ \hline \end{array} a: One or more runs did not converge or failed in Quantum Espresso, missing adsorbates are reconstructed from successful runs using scaling
b: Changes in atomic positions of greater than 5.5 angstrom found in the slab or adsorbate during optimization, typically indicates structural instability
[...]

And there are a lot more unstable species.

TL;DR: Basically, in these extreme cases, where you would observe abnormal oxidation states, the calculation also predicted that the bulk/ molecular structure is not stable.

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Martin has answered well that the "abnormal oxidation state" compounds appear unstable, but what if they were to form? The sticking point in the oxidation state calculation is that oxygen in combination with most other elements shall have an oxidation state of $-2$. But at least in theory, that need not be so. Maybe some of the oxygen is in the form of the radical anion $\ce{O^-}$ rather than $\ce{O^{2-}}$, with the charges thereby balanced with the other elements in their normal oxidation states. In a proposed compound such as $\ce{CaBO_3}$ (if it were to form), for instance, the proposed $\ce{B^{4+}}$ would be so powerfully oxidizing that it would react with any $\ce{O^{2-}}$ that contacts it, forming said oxygen radical anion, until the boron gets down to the ordinary $+3$ oxidation state.

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    $\begingroup$ I agree with Oscar Lanzi, half of the proposed Catalysts will immediately oxide to their ordinary oxidation state. The proposal perovskite structures are unrealistic as the adsorption energies reported. They might have calculated hundreds of hypothetical oxides, but most will never be an actual catalyst. $\endgroup$ – Carlos Martinez Jul 5 '18 at 10:04
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    $\begingroup$ In addition, thanks to Martin I re-read the support information and about half of the structures show computational warnings. As a student, I am surprised about another fact, for Mn, Fe and Co you need to consider the strong on-site Coulomb interaction? $\endgroup$ – Carlos Martinez Jul 5 '18 at 10:16

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