[Pw_forum] a superficial question about the same parameters in relax and scf

[Axel Kohlmeyer]

On Wed, 5 Dec 2007, xu yuehua wrote:

hi!

please respect the policy of this forum to add an
affiliation and then pick up a text book about quantum
chemistry and read about the variational principle.

the basis set size (in plane waves) is determined by
ecutwfc. the higher the cutoff, the larger the basis set
the lower the energy. we are essentially all doing calculations
with an incomplete basis set, but as long as the error
is small enough, it does not matter. it is you job to
find out what is enough and that can be different for
each calculation.

ecutrho is usually coupled to ecutwfc and determines the
real space representation of the density (which enters
the evaluation of the density functional(s)). for norm-
conserving pseudopotentials, this has a ration of 1:4
and there is little reason to change it. for ultra-soft
pseudopotentials, additional contribution to the density
(e.g. augmentation charges) have to be added to the 
real space density and for that you may need a higher
density cutoff. as ratio of 1:10 is frequently used when
high accuracy is wanted, but in some cases it may need
to be higher.

you didn't mention k-point sampling. depending on the
size and nature of your system, this is another dimension
of parameters that need to be converged to get accurate
results. please take the time and search through the
mailing list archives on these topics. there were many
discussions and each system and problem has its own 
quirks and needs adjustments (or not). 

hope this helps,

   axel.

XY> hi everyone :
XY> i have a question about the main  parameters used in relax and scf ,for
XY> example :ecutwfc   ,
XY>   ecutrho  ,if  i have test them in relax ,and whether i should retest
XY> them when i do scf ,or enlarge them ,because ,in other software ,if you want
XY> to get more accurate energy ,you are asked to enlarge some
XY> parameters similar to  ecutwfc  , ecutrho    etc.
XY> maybe ,it is a foolish or misunderstanding  questions .but i hope you could
XY> help me ,to understanding how and why .
XY> tahnk you
XY> 
XY> 
XY> 
XY> 

-- 
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Axel Kohlmeyer   akohlmey at cmm.chem.upenn.edu   http://www.cmm.upenn.edu
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