The ABINIT Users Mailing List ( CLOSED )

[Chol-Jun Yu <yucj@ghi.rwth-aachen.de>]

Dear users,

thank you very much for your recommendations. In fact, our work is about very 
complicated system, that is, vicinal surface with kink (about 170 atoms). 
This situation makes the time of relaxation very crucial. I can't predict 
time of atomic relaxation (100 atoms are allowed to move). Can anyone predict 
the typical time? 

In order to save the time for relaxation, I thoght the following procedure. 
Since we don't know exact coordinates of atoms at the beginning of the 
simulation, we usually approximate the initial configuration. If this 
configuration is far away from the exact (within allowable error) one, the 
relaxation will take very much time, which is the most time-consuming of the 
work.

While so as to obtain the sufficient reasonable result, we should use enough 
large parameters such as cutoff energy, special k-points, lattice constants 
of the supercell. We should check whether our parameters can guarantee the 
reasonable results or not. But it is almost not possible in the case of very 
time-consuming work.

So I think that we can perform the relaxation of the atoms by the use of 
small 
ecut and only one k-point, so that we can get some good estimation for the 
initial configuration within short time. Then we can use this configuration 
for further relaxation with increased parameters, which maybe will finished 
within not so many Broyden steps. Finally we will check our results according 
to the paramters through the total energy calculation, with relaxed 
configuration. Does anyone criticize about this procedure?

With best regards,

Yu.

On Wednesday, 4. April 2007 17:45, Michel Côté  wrote:
> Actually, 60 steps for 50 atoms is not so bad. You have 50*3 degrees of
> freedom and you get it relaxed in much less than that number. Remember that
> when relaxing a structure, the values of the forces need to be precise
> enough for the convergence, this means that you should use toldff for
> criteria and use a value less than 5d-5.
>
> Michel
>
> Le 04/04/07 08:31, « Anglade Pierre-Matthieu » <anglade@gmail.com> a écrit :
> > Hi,
> >
> > Apparently there is no better value for ionmov at present. I don't
> > know the reasons why this algorithm is supposed to be most efficient
> > for system with few degrees of reedom either. This last statement
> > seems surprising to me since Broyden method is a very good algorithm
> > for local minimisation. If you really need it you might try to
> > implement a preconditioner to your minimisation problem. This may be
> > the only efficient improvement you can add to BFGS. It consist in
> > adding an explicit hessian guess to the implicit hessian evalutaion
> > done by BFGS.
> >
> > Regards
> >
> > PMA
> >
> > On 4/4/07, Chol-Jun Yu <yucj@ghi.rwth-aachen.de> wrote:
> >> > Dear users,
> >> >
> >> > I found that it could take much time (sometimes BFGS steps more than
> >> > 60 in order to arrive convergence, 5*10-5 Ha/Bohr) to relax the atomic
> >> > positions
> >>
> >> if
> >>
> >> > the number of atoms is much larger more than 50, for example, for
> >> > surface supercell. I usually used the ionmove=2. Is it possible to
> >> > reduce the BFGS steps by using other values of "ionmov" parameter?
> >> >
> >> > With best wishes,
> >> >
> >> > Yu Chol Jun.