[Pw_forum] deguss

[Stefano de Gironcoli]

Nicola Marzari wrote:
> yaldaa kh wrote:
>   
>> dear all
>> 1) why does the total energy (Escf ) decrease with increase of the 
>> deguss value in smearing technique?
>> 2) How can I choose a good value (the best choise) for deguss?
>>  
>> thanks a lot
>> yalda
>> student (chemistry)
>>     
>
>
> 1) It shouldn't for standard smearings (e.g. Fermi-Dirac, gaussian).
> E-TS decreases. For m-p, m-v, we cannot say.
>   
In PWscf in case of smearing the total energy is the variational 
quantity and contains an entropy-like term (-TS contibution).
In case one is using  Fermi-Dirac smearing this variational quantity is 
the electronic free-energy, for other smearing it is an auxiliary 
quantity analogue to that but with no relationship with electronic free 
energy.
As Nicola says, for FD and Gaussian smearing "E-TS" (that is total  
energy in PWscf)  decreases  with increasing T (smearing).
for other smearing type we cannot say.

Stefano de Gironcoli - SISSA and DEMOCRITOS

> 2) See Chap 4 of http://quasiamore.mit.edu/phd/ for plenty details.
>
> 			nicola
>
>
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