[Pw_forum] problem to converge using constrained_magnetization

Gabriele Sclauzero sclauzer at sissa.it
Fri Aug 13 10:45:30 CEST 2010

Il giorno 13/ago/2010, alle ore 01.23, Alejandro Rébola ha scritto:

> Dear all,
> I'm performing a scf calculation for an AFM structure. I fix the magnetic
> moments of the Fe ions using, for instance:
> starting_magnetization(1)=0.7
> starting_magnetization(2)=-0.7
> constrained_magnetization='atomic'
> I use the experimental structure of the system and perform a scf
> calculation for different magnetic moments (I want to estimate the GS
> magnetic moment).
> My problem is that the system only converges for one value of the starting
> magnetization (0.5) and doesn't converge in all the other cases I've
> tried.
> According to the pwscf documentation, LAMBDA, the Lagrange multiplier for
> the magnetic penalty term, ".. is reduced in the first iterations and is
> increased slowly up to the input value." (I've tried LAMBDA=1,10,20,30,50)

Which version of the code are you using? In v4.2 the documentation about lambda reads:

   Variable:       lambda
   Type:           REAL
   See:            constrained_magnetization
   Default:        1.d0
   Description:    parameter used for constrained_magnetization calculations
                   N.B.: if the scf calculation does not converge, try to reduce lambda
                         to obtain convergence, then restart the run with a larger lambda

which I think is consistent with what the code does (i.e., lambda in NOT reduced and then increased automatically).
Please try what suggested in the Nota Bene. You may also need to play a little bit with the mixing_beta value. 

The constraints on the magnetic moments are applied to the system in subroutine PW/add_bfield.f90.

> Then, could it be that I should use much bigger values for LAMBDA? Is
> there a default value for it? I couldn't find it.

The default is 1.d0, as stated in the documentation.



> Thank you in advance for you help.
> Best regards,
> Alejandro
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> Pw_forum at pwscf.org
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§ Gabriele Sclauzero, EPFL SB ITP CSEA
   PH H2 462, Station 3, CH-1015 Lausanne

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