[Wien] cohesive energy

[Zsolt Rak]

Thank you Prof. Blaha,

Your explanation makes sense. Of course an alternative method to calculate
formation and cohesive energies  would be to combine experimental data with
DFT energies, such as the method described in Phys. Rev. B 85, 115104
(2012). This way one would avoid calculations of metallic/gaseous phases
and of free atoms.

Thanks,
Zsolt

On Fri, May 13, 2016 at 2:22 PM, Peter Blaha <pblaha at theochem.tuwien.ac.at>
wrote:

> I would answer it differently:
>
> No, it does not really make sense to use LDA+U for an atom;  but there are
> not many other chances to calculate formation or cohesive energies.
>
> For such quantities one MUST do all calculations with exactly the same
> calculational parameters and of course also with identical
> DFT-approximations.
>
> In all cases, where for one or the other system a certain approximation is
> not very good, the corresponding E-differences are of course very
> problematic.
>
> So if you compare a TM oxide with free TM and O atoms, all approximations
> must be the same.
>
> Of course for a TMO GGA+U is probably much better, but I would NOT say
> this for a free atom.
> You can test the sensitivity of your results be repeating the same
> E-difference using GGA (or even better a meta-GGA), i.e. you treat the atom
> more meaningful, but may even have a "metallic" TMO.
>
> The situation is similar for a formation energy of a TMO with respect to
> the metallic element and O2. GGA+U for bcc Fe with a sizable U gives
> magnetic moments up to 3 mB !
>
>
>
> Am 13.05.2016 um 19:42 schrieb Zsolt Rak:
>
>> Does it make sense to perform LDA+U/GGA+U calculation on free, isolated
>> atom?
>>
>> On Thu, May 12, 2016 at 11:47 AM, Laurence Marks
>> <L-marks at northwestern.edu <mailto:L-marks at northwestern.edu>> wrote:
>>
>>     To obtain reasonable results you have to be fully consistent with
>>     everything, e.g. RMT, RKMAX, U's etc.
>>
>>     On Thu, May 12, 2016 at 10:37 AM, Zsolt Rak <zsolt.rak at gmail.com
>>     <mailto:zsolt.rak at gmail.com>> wrote:
>>
>>         ​​
>>         Dear users,
>>
>>         I am calculating the cohesive energy of a series of transition
>>         metal (TM) oxides and I have the following questions: because
>>         the energy of the TMO is calculated within the LDA+U framework
>>         should I use the same LDA+U for the free atoms? Does it make
>>         sense to subtract LDA+U/GGA+U energies of oxides from LDA/GGA
>>         energies of free atoms? Does it make sense to use LDA+U/GGA+U
>>         for free atom calculations?
>>         Thanks,
>>         Zsolt
>>
>>
>>
>>
>>     --
>>     Professor Laurence Marks
>>     "Research is to see what everybody else has seen, and to think what
>>     nobody else has thought", Albert Szent-Gyorgi
>>     www.numis.northwestern.edu
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>>
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> --
> --------------------------------------------------------------------------
> Peter BLAHA, Inst.f. Materials Chemistry, TU Vienna, A-1060 Vienna
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