[Pw_forum] cell optimization

[Axel Kohlmeyer]

On Sat, 14 Jun 2008, hania djani-ait aissa wrote:

HDA> dear users,

dear hania,

HDA> I am trying to optimize lattices parameters in tetragonal centered 
HDA> structure. putting c/a=cte and a=7.28, 7.29, 7.30 ......7.36, I've 
HDA> got the results on total energy:

HDA> a=7.28    E=-1282.635
HDA> a=7.29    E=-1282.639
HDA> a=7.30    E=-1282.658
HDA> a=7.31    E=-1282.684
HDA> a=7.32    E=-1282.654
HDA> a=7.33    E=-1282.624
HDA> a=7.34    E=-1282.679
HDA> a=7.35    E=-1282.669
HDA> a=7.36    E=-1282.652

HDA> I've done the calculations twice and in each case convergence is 
HDA> well achieved giving same results. so, why above a=7.31, total 
HDA> energy rises and drops. is it because ecutoff is too small? 

please have a look at the part of your output and check
out the number of grid points.

HDA> (ecutoff=25Ry) thanks a lot for any answer.

whether your cutoff is too low depends on the pseudopotential
files that you are using. 25ry suggests you are using ultra
soft pseudopotentials, for those you have to use a much higher
density cutoff than the default (which is 4x ecutwfc). 
please note, it is easier to give qualified advice, if you make 
your input file available...

cheers,
   axel.

HDA>  
HDA> Hania Djani-AitAissa
HDA> Center of development of advanced technologies, Algiers, Algeria
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-- 
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Axel Kohlmeyer   akohlmey at cmm.chem.upenn.edu   http://www.cmm.upenn.edu
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