QuantumATK Forum

QuantumATK => General Questions and Answers => Topic started by: Cyrille on August 17, 2015, 16:14

Title: Broadening technique
Post by: Cyrille on August 17, 2015, 16:14

Hello

I have a question regarding the broadening technique in a standard scf calculation within ATK.
From the vnl interface it seems that I have only access to Fermi-Dirac broadening.
Is it possible to use Marzari-Vanderbilt or Methfessel-Paxton broadening as well?

thanks in advance

Cyrille


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Cyrille Barreteau
CEA Saclay, IRAMIS, SPEC Bat. 771
91191 Gif sur Yvette Cedex, FRANCE
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Title: Re: Broadening technique
Post by: Jess Wellendorff on August 17, 2015, 22:11

Hi Cyrille,
FD broadening is the only option with ATK. Is there a particular reason you would prefer MV or MP broadening instead?

Title: Re: Broadening technique
Post by: Anders Blom on August 17, 2015, 23:29

We'll probably add the other options to ATK 2016, for bulk materials. But the question is interesting - are they really much better? What is your experience, Cyrille?

Title: Re: Broadening technique
Post by: Cyrille on August 18, 2015, 10:52

Hi Anders

Thanks for your answer.
I would say it is mainly for convergence (and total energies) issues. With MV and MP it is possible to use larger broadening (temperature T) than Fermi Dirac (FD) while keeping a good estimate of the total energy. Indeed if you plot E(T) it is very flat over a very large range for MV and MP while it increases rapidly with FD. This usually facilitates the convergence keeping a behaviour similar to T=0..

Cyrille

Title: Re: Broadening technique
Post by: Jess Wellendorff on August 18, 2015, 11:18

Great, thanks for the input! We will keep this in mind for a future release of ATK.

Title: Re: Broadening technique
Post by: asanchez on August 18, 2015, 22:35

Nicola Marzari's PhD thesis has some interesting information regarding this whole issue if anyone's interested;

http://theossrv1.epfl.ch/Main/Theses

Title: Re: Broadening technique
Post by: Jess Wellendorff on August 19, 2015, 08:40

Thanks very much for the reference. Yes, I am aware that the PWscf code offers several different options for electron occupation smearing, and that there may be good reasons to prefer the cold smearing scheme. Very interesting.

Title: Re: Broadening technique
Post by: Cyrille on August 19, 2015, 09:20

MP and MV have rather similar behaviours. The only  problem with MP is that you can sometimes get negative charges which is not the case of MV.
Another very interesting discussion (with many additional references) on the same website:
http://theossrv1.epfl.ch/Main/ElectronicTemperature

Cyrille