[Pw_forum] scf convergence calculation

Andrea Ferretti ferretti.andrea at unimore.it
Wed Jan 23 12:47:44 CET 2008

Hi Francesca,

>   /
>  &system
>     ibrav = 8, celldm(1) = 29.02243, celldm(2) =1.0, celldm(3)= 0.89771,
>     nat=56, ntyp=2,  ecutwfc=18.0,
>  /
> 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 


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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