[Pw_forum] convergence not achieved

[Giovanni Cantele]

On Feb 16, 2010, at 12:27 PM, Shyam Khambholja wrote:

> Dear all,
>             I am doing total energy calculation for a system having two atoms per unit cell. When doing it I am getting converged total energies upto lattice constant a=7 bohr. When doing calculation for higher values of a=8 bohr, I am getting following message.
>  
> convergence not achieved.
> end of calculation. 
>  
> and as a result, i am not getting value of total energy.
> so, help me
>  
> thanks in advance
> 
> 
> -- 
> Shyam G Khambholja
> Reseach student,
> Depratment of Physics,
> Sardar Patel University
> 


Convergence related issues might be found here:
http://www.quantum-espresso.org/wiki/index.php/Frequently_Asked_Questions#Self-consistency_is_slow_or_does_not_converge_at_all

Also, please explore the user forum, I think there should be lots of convergence-related posts!

What kind of system/crystal/whatever are you calculating? Usually scf is not expected to converge for ANY input file. Your input structure should be physically meaningful.

Why do you change the "lattice constant" by a large amount? This might be useful if you are considering an "isolated" molecule and want to check convergence with
respect to the vacuum space separating the periodic replica (in this case I would say 7 bohr is too small), but if you are trying to optimize the unit cell of some crystal,
usually you explore the total energy vs lattice constant by varying the latter by small amounts (your Delta a = 1 bohr means 14%!!!!).

Giovanni

--

Dr. Giovanni Cantele
Coherentia CNR-INFM and Dipartimento di Scienze Fisiche
Universita' di Napoli "Federico II"
Complesso Universitario di Monte S. Angelo - Ed. 6
Via Cintia, I-80126, Napoli, Italy
Phone: +39 081 676910
Fax:   +39 081 676346
E-mail: giovanni.cantele at cnr.it
              giovanni.cantele at na.infn.it
Web: http://people.na.infn.it/~cantele
Research Group: http://www.nanomat.unina.it
Skype contact: giocan74

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