[Pw_forum] Electronic entropy

[Lorenzo Paulatto]

On 03/27/2013 09:18 AM, Nicolas FERNANDEZ wrote:
> Dear all,
>
> I try to evaluate the electronic entropy contribution to the vacancy formation.
>
> I used smearing with Fermi-Dirac function and a degauss value of 0.01 Ry
Dear Nicolas,
if you convert 0.01Ry to Kelvin/Boltzmann_constant units you'll get that 
it is equivalent to 1578.87 Ry, this is quite a high temperature!

This is because the smearing is only a computational trick, it's used to 
converge the sum over the Brillouin zone faster. It's contribution to 
the total energy is actually remove (that's why -TS) in order to get the 
benefit of faster convergence without changing the final result. More 
details: http://prb.aps.org/abstract/PRB/v40/i6/p3616_1

You could think about setting the smearing to an actual temperature 
value, and remove the parts of the code that compensate for it (i.e. 
putting the -TS back in). I don't know if this would be a meaningful 
approach, it would require some thought to find out.
> It (probably) corresponds to the temperature but the temperature of what?
the fictitious temperature you set: 0.01 Ry
> What is the temperature unit?
Rydberg atomic units, you can convert to Kelvin via the Boltzmann 
constant: k_B = 157887 Ry/K if I'm not mistaken
> How to calculate or set it?
you don't, you set it in order to converge the BZ sum without perturbing 
the system too much
> Is my approach correct to evaluate the electronic entropy?
I don't know :)
> Thank you for your responses.
>
> Best regards,

tchao

-- 
Dr. Lorenzo Paulatto
IdR @ IMPMC -- CNRS & Université Paris 6
phone: +33 (0)1 44275 084 / skype: paulatz
www:   http://www-int.impmc.upmc.fr/~paulatto/
mail:  23-24/4é16 Boîte courrier 115, 4 place Jussieu 75252 Paris Cédex 05

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