[Wien] cohesive energy

[Peter Blaha]

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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Peter BLAHA, Inst.f. Materials Chemistry, TU Vienna, A-1060 Vienna
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