[Wien] 'NN' - overlapping spheres during the volume optimization

[Volodymyr Svitlyk]

Dear Stefaan,

Thank you for the answer.

I am using the PBE-GGA functional. According to the 
http://prb.aps.org/abstract/PRB/v79/i8/e085104 paper, it is indeed the 
best choice for Fe. However, LDA seems to be the best for Ge. Well, it 
looks like that the Fe-Ge system is not very convenient for this kind of 
optimizations.

Best regards,
Volodymyr

Le 15/07/2011 15:19, Stefaan Cottenier a écrit :
>
>> I am working on a Fe-Ge compound with about 17% vacancies on one site. I
>> have set this site as a fully occupied and I am currently trying to
>> optimize the volume and the atomic coordinates.
>>
>> For the initial volume optimization run, I have reduced RMTs by 5%, and
>> as a result I got an the "optimized" volume smaller than the original. I
>> believe this is expected due to the reduced RMTs.
>
> No, this is not related to the choice of RMT. RMT is a mathematical 
> construct only. Whatever RMT you choose (within reasonable limits), 
> you should get the same observable properties (e.g. the volume, in 
> your case) if you perform well-converged calculations.
>
> First estimate what is the minimal volume that you will ever want to 
> calculate for this compound, and then take the RMTs such that they'll 
> not overlap for this minimal volume.
>
>> However, in reality,
>> it would be expected to get a slightly larger volume for this new
>> "stochiometric" phase.
>
> Which functional are you using, and how large is 'slightly'? If you 
> use e.g. LDA, it will usually underestimate the volume by several 
> percent, making the volume for the stochiometric compound perhaps 
> smaller than the experimental volume of the off-stochiometric compound.
>
> You might have a look at 
> http://prb.aps.org/abstract/PRB/v79/i8/e085104 to get an idea about 
> which functional is likely to provide the optimal volume for the 
> elements you are using.
>
> Stefaan
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