[Pw_forum] scf convergence calculation

[Andrea Ferretti]

Hi Francesca,

>   /
>  &system
>     ibrav = 8, celldm(1) = 29.02243, celldm(2) =1.0, celldm(3)= 0.89771,
>     nat=56, ntyp=2,  ecutwfc=18.0,
>  /
>...
> ATOMIC_SPECIES
> Si 28.086 Si.pw91-n-van.UPF
> H   1     H.pw91-van_ak.UPF


since you are using ultrasoft pseudopotentials (USPP), besides ecutwfc,
you should also specify ecutrho...
as a default ecutrho = 4 * ecutwfc, and this is what it is analytically 
obtained for norm-conserving pseudopotentials...
in the case of USPPs, typical values for ecutrho range from 8 to 12 times 
ecutwfc (as a rule of thumb to be tested)

of course, this can explain convergence problems

look also in the forum archive where this topic has been already 
discussed...

cheers
andrea

-- 
Andrea Ferretti
National Research Center S3, CNR-INFM  ( http://s3.infm.it )
Dip. Fisica, Univ. di Modena e Reggio E. (Italy)
Tel: +39 059 2055301;  Fax: +39 059 374794;  Skype: andrea_ferretti
URL: http://www.nanoscience.unimo.it

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