[Wien] Query regarding Hubbard U for Ti in TiO2

[Karel Vyborny]

Dear Shamik,
it is perhaps a better idea not to try to _calculate_ the U but rather, 
use the arguments of the Hu&Metiu article you mentioned (J Phys Chem C 115, 
5841) for either hybrid functionals or GGA+U with values Hu&Metiu argue to 
give a reasonable gap, reaction energies etc. A quick GGA calculation for 
TiO2 shows that already within the sphere of Ti atoms, there's significantly 
more than 18 electrons (which would correspond to nominal Ti^4+) so it is 
wrong to think about ni=nj=0 (in terms of what you write below). If you 
wish to convince yourself (personally, I'm all in for being skeptical!) 
that some particular GGA+U calculations make sense, try comparing more 
physical quantities (optical or photoemission spectra for example) to 
anything that is available on the experimental side.

Best regards,

Karel




--- x ---
dr. Karel Vyborny
Fyzikalni ustav AV CR, v.v.i.
Cukrovarnicka 10
Praha 6, CZ-16253
tel: +420220318459


On Fri, 19 Feb 2021, shamik chakrabarti wrote:

> Dear Wien2k users,                                I am interested in
> calculating the reaction energy for the equation 
>                         2 TiO2 + H2 = Ti2O3 + H2O
> In this case, I should use U for Ti in Ti2O3. However, in earlier research
> works it has been given that the same U should also be used for Ti in TiO2.
> 
> My query is as Ti exists as +4 state in TiO2 why at all we need to use U for
> it?
> 
> We know EU = U/2 SUM (ni, nj), however as ni & nj is zero for Ti in TiO2 why
> should we also need to use U for Ti in TIO2?
> 
> Waiting forward to your response. Thanks in advance.
> 
> with regards,
> 
> --
> Dr. Shamik Chakrabarti
> Research Fellow 
> Department of Physics
> Indian Institute of Technology Patna
> Bihta-801103
> Patna
> Bihar, India
> 
>