[CPMD-list] question about CUTOFF

[Axel Kohlmeyer]

On Sat, 23 Feb 2008, liu liuxiandong wrote:

LX> Dear CPMD users,

dear xiandong,

[...]

LX> ...
LX> &SYSTEM
LX>   CUTOFF
LX>    70.0
LX> ....
LX> &END

[...]

LX> The total energy of the relaxed structure is -983.949852 at CUTOFF 
LX> 70Ry. Then I increased CUTOFF to 75 Ry and performed single-point 
LX> calculation with this optimized structure, the total energy 
LX> decreased to about -984.845447.

please note that different properties of your system converge
differently with the size of the basis set (i.e. the plane wave
cutoff). for example forces generally converge faster with
the basis set size than the total energy.

LX> Since such LSD calculation is very time demanding, is it necessary 
LX> to optimize the structure with higher CUTOFF ?

the required cutoff depends on your desired accuracy and
the "hardness" of the individual pseudopotentials in your
system. you don't have to check for the necessary cutoff
with your whole system, but can check with small subsets.
just do single points with increasing cutoff and PRINT FORCES
and see how far your have to go until your forces are
converged with respect to the accuracy you want. the general
rule of the thumb is that the forces have to be converged
roughly two orders of magnitude better than the desired
accuracy for the geometry optimization. 

with the mix of elements that you have, i would not be
surprised if you need a cutoff of 90ry or beyond.

cheers,
   axel.

LX> Thanks.
LX> 
LX> Xiandong Liu
LX> _______________________________________________
LX> CPMD-list mailing list
LX> CPMD-list at cpmd.org
LX> http://cpmd.org/mailman/listinfo/cpmd-list
LX> 

-- 
=======================================================================
Axel Kohlmeyer   akohlmey at cmm.chem.upenn.edu   http://www.cmm.upenn.edu
   Center for Molecular Modeling   --   University of Pennsylvania
Department of Chemistry, 231 S.34th Street, Philadelphia, PA 19104-6323
tel: 1-215-898-1582,  fax: 1-215-573-6233,  office-tel: 1-215-898-5425
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