[Pw_forum] about ecutwfc convergence for total energy

[Carlo Nervi]

Hi,
as far as i know it seems a problem related with bands, so low number of 
k-points (for insulators).
I would suggest in any cases to try to use a couple of k-points.
Another important point: are you sure your system is not metallic?
In this case you could try smearing.
HTH,
	Carlo

Il 08/10/2012 17.05, James Mao ha scritto:
> Hi all,
>
> I am a little bit confused about ecutwfc convergence. I am working on
> calculating
> NMR of a crystal structure. Before that I need test SCF energy convergence.
> However I noticed the total energy is not convergent with respect to
> ecutwfc.
>
> It reached minimum at ecutwfc=90 then began oscillating up and down.
>
> Ecutwfc  Total Energy
>
> 70           -3249.733804
>
> 80           -3250.674250
>
> 90           -3250.827326
>
> 100         -3250.661301
>
> 120         -3250.346091
>
> 160         -3250.582989
>
> 200         -3250.395401
>
> I think higher ecutwfc means more basis functions, therefore should
> always result
> in a lower total energy. If I am wrong, should I keep increasing ecutwfc?
>
> The PP I used is (pbe-tm-new-gipaw-dc.UPF), the atoms include C, O, N,
> F, S, H, P.
>
> Since PP is NC, I left ecutrho default. And since the cell is pretty big
> (a~10, b~16, C~15),
> I only calculated at gamma point.
>
> Any hint is helpful! Thanks.
>
> Best,
>
> James Mao
>
> ----------------------------------
>
> Postdoc, Department of Chemistry, University of Pittsburgh
>
>
>
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