[Pw_forum] How to figure out machine accuracy

[Manoj Srivastava]

Dear Lorenzo, 
 Thank you for your reply. I figured out my problem. You are right, it is
not coming from lapack routine. 

Regards, 
Manoj

On Fri, 4 Sep 2009, Lorenzo Paulatto wrote:

> In data 04 settembre 2009 alle ore 15:54:38, Manoj Srivastava  
> <manoj at phys.ufl.edu> ha scritto:
> >  The input is CMACH, I can specify various input variables for 'CMACH' to
> > get output, but what variable is returned as output, as there is only one
> > in this subroutine?  Would you mind having a look at this- its in
> > espresso4.0.4/flib/dlamch.f
> 
> 
> Dear Manoj,
> QE does not use simple LAPACK routines for most of its algebra.  
> Hamiltonian diagonalization is computed using a custom implementation of  
> the iterative Davidson (or conjugate-gradient) algorithm that only uses  
> LAPACK to diagonalize a reduced hamiltonian during each iteration.
> 
> Said that, you can control the accuracy to which eigenvalues are computed  
> cannot be controlled directly by the user (without modifying the code) and  
> is decreased automatically as the self-consistency approaches convergence.
> 
> The formula used is the following:
>   ethr = MIN( ethr, 0.1D0*dr2 / MAX( 1.D0, nelec ) )
> where dr2 is the estimated scf accuracy.
> As you can see you can increase the accuracy (at convergence) of  
> eigenvalues and eigenvectors by increasing the convergence threshold.
> 
> 
> In a more general frame, I think you could disclose some more information  
> on your "symmetry problem"; personally I doubt it is caused by  
> insufficient diagonalization accuracy.
> 
> best regards
> 
> -- 
> Lorenzo Paulatto
> SISSA  &  DEMOCRITOS (Trieste)
> phone: +39 040 3787 511
> skype: paulatz
> www:   http://people.sissa.it/~paulatto/
> 
>      *** save italian brains ***
>   http://saveitalianbrains.wordpress.com/
> _______________________________________________
> Pw_forum mailing list
> Pw_forum at pwscf.org
> http://www.democritos.it/mailman/listinfo/pw_forum
>