[CPMD-list] slow convergence with ODIIS

[Veronique Van Speybroeck]

It is true that convergence problem occur for initializing the 
wavefunctions with VDB pseudopotentials.  A lot helps when you request 
not to reset the ODIIS as often as is standard done.  

This can be obtained by putting

>  OPTIMIZE WAVEFUNCTION
>  ODIIS NO_RESET=50
>   10
>  TIMESTEP
>   20


furthermore it is essential to put the GC cutoff large enough

f.e.
&DFT 
 LDA
 GC-CUTOFF
 5.D-5
&END
 but this is well documented in the manual

hope this helps

veronique



Christian Tuma wrote:

>Hello,
>
>these wavefunction convergence problems with Vanderbilt-PPs seem to be
>quite common. Chapter 11.1 of the manual mentions them, there have been
>postings in the mailing-list, and Axel Kohlmeyer just gave some hints
>again. However, my own experience is that even when the geometry of the
>system is well optimised, one still should not believe that the wavefunction
>always converges properly when using these keywords:
>
>INITIALIZE WAVEFUNCTION RANDOM
>PCG MINIMIZE
>TIMESTEP
> 20
>
>I was running calculations on zeolite systems (using Vanderbilt-PPs for
>hydrogen, oxygen, and carbon) and encountered situations where the
>final result of a wavefunction optimisation using these keywords depends
>on the chosen TIMESTEP value, e.g. (using the BP86 functional):
>
> TIMESTEP     result
> 3            -2613.5011 Hartree
> 5            -2613.4979
> 8            -2613.5011
> 9            -2613.4979
> 10           -2613.5011
> 11           -2613.4951
> 12           -2613.4979
> 15           -2613.4979
> 20           (no convergence, oscillation)
>
>This corresponds to relative energy differences of several kJ/mol, and
>e.g. for some questions of chemical interest this cannot be neglected.
>
>However, testing several values for TIMESTEP is no guarantee to find the
>"correct" wavefunction and so the convergence problem still persists.
>Axel, you suggested to put some initial ATOMIC CHARGES. I would like
>to try this and to see whether this improves the situation, but, I
>have no clear idea which values to choose. When doing a properties
>calculation for atomic charges I get a set of charges derived from
>real space integration and another one from the electrostatic potential,
>the absolute values from the latter being about one order of magnitude bigger
>than the ones from real space integration. Which of these two sets should I
>use as an orientation for the ATOMIC CHARGES to follow your suggestion?
>
>Does anybody have another new idea how to make life easier when trying
>to converge the wavefunction using Vanderbilt-PPs?
>
>cheers,
>Christian.
>
>PS: I agree to Atte, PBE calculations seem to converge easier than e.g.
>BP86 calculations, at least when using normconserving PPs.
>
>  
>

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Dr. ir. Van Speybroeck Veronique
Laboratorium voor Theoretische Fysica
Universiteit Gent
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email : veronique.vanspeybroeck at ugent.be
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