[Pw_forum] difference in energy calculated by "gaussian" and "m-v" smearing

Nicola Marzari marzari at MIT.EDU
Wed Nov 25 13:31:49 CET 2009

Dear Mohnish,

roughly speaking you should use a similar degauss for m-v and gaussian. 
.05 Ry is fairly large, I'd stick with
.02 Ry or so.

Try this - study a bulk system (i.e. as small as possible) and plot for 
3 or 4 k-point meshes the free energy (E-TS)
as a function of smearing. At very small smearings the free energy will 
not be converged with respect to k-points;
at high smearings it will, but will also be very dependent on smearing.

You want the sweet spot in between.


mohnish pandey wrote:
> Dear users,
>                       I have calculated the energy of zinc oxide thin 
> film having nine layers. But the energy calculated by "m-v" and 
> "gaussian" smearing is coming out to be very different. The difference 
> is around 0.1 Rydberg. I have used used "degauss" to be .05 and 10^-3  
> respectively for "m-v" and "gaussain smearing". Can anybody please 
> tell me why the energy is coming out to be very different. I have gone 
> through the sections of book by Martin and Sholl also but I couldnt 
> find that.
> Sincere thanks in advance.
> Mohnish Pandey
> -- 
> Mohnish Pandey
> Y6262,4th Year Undergraduate,
> Department of Chemical Engineering,
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Prof Nicola Marzari   Department of Materials Science and Engineering
13-5066   MIT   77 Massachusetts Avenue   Cambridge MA 02139-4307 USA
tel 617.4522758 fax 2586534 marzari at mit.edu http://quasiamore.mit.edu 

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