[Pw_forum] Convergence test

[Axel Kohlmeyer]

On Tue, 13 Nov 2007, Huiqun Zhou wrote:

HZ> List users,

dear huiqun zhou,

HZ> I'm testing convergence on ecutwfc for chromite using USPP. Below are
HZ> results of different ecutrho with different ecutwfc. I may be wrong, I think
HZ> I should watch the difference of total energy between successive runs till
HZ> it's less than 1-2 mRy. But the trend is not apparent in the results below
HZ> and it seems that the larger the ecutwfc, the lower the total energy.

you are using an ultra-soft pseudopotential and for those
actually the density cutoff is _much_ more important than
the wavefunction cutoff. depending on the choice of the 
pseudopotential parameters, 35-40ry for the wavefunction 
cutoff should be a conservative choice, however a density
cutoff of 250 is not. if you want to converge the energy
very tightly, you may need to go up to 10x the wavefunction
cutoff. you should also keep in mind that the total energy
converges rather slowly, and that for most calculations
you need energy differences or derivatives, which may converge
faster, i.e. you may want to look at forces (use a non-symmetric
input for that!!) or stress instead. what is sufficiently
converged depends on your the kind of calculation you want
to do. so for simple geometry optimization or MD 35/210
or less may already be sufficient.

HZ> 
HZ> I calculated E-V relations using two sets of ecutwfc and ecutrho:
HZ> for ecutwfc = 35 ry and ecutrho = 240 ry, a = 8.405, B0 = 172.8, B0' = 4.3
HZ> for ecutwfc = 55 ry and ecutrho = 250 ry, a = 8.407, B0 = 178.6, B0' = 3.6
HZ> 
HZ> There are no big differences in unit cell parameter and elastic properties.
HZ> So, my questions are
HZ> (1) usually, the ecutwfc of USPP should be around 35 ry or below, is my
HZ>   case an exception?

no. it does not look like an exception.

HZ> (2) how small the difference of total energy of successive runs should be
HZ>   in judging convergence against ecutwfc?

total energy is not a good parameter. see above.

cheers,
  axel.

HZ> 
HZ> Thank you in advance.
HZ> 
HZ> Dr. Huiqun Zhou
HZ> @Earth Sciences, Nanjing University, China
HZ> 
HZ> 
HZ> 
HZ> 
HZ> ecutrho = 240 ry
HZ> ecutwfc                     total energy
HZ> 35                     -1461.54573299 Ry
HZ> 40                     -1461.64206824 Ry
HZ> 45                     -1461.71978835 Ry
HZ> 50                     -1461.76587314 Ry
HZ> 55                     -1461.76316597 Ry
HZ> 60                     -1461.76692396 Ry
HZ> 
HZ> ecutrho = 250 ry
HZ> 40                     -1461.64351876 Ry
HZ> 45                     -1461.70130510 Ry
HZ> 50                     -1461.74724764 Ry
HZ> 55                     -1461.76462943 Ry
HZ> 60                     -1461.76838812 Ry
HZ> 
HZ> ecutrho = 280 ry
HZ> 50                     -1461.74778158 Ry
HZ> 55                     -1461.76516515 Ry
HZ> 60                     -1461.76892327 Ry 
HZ> 
HZ> _______________________________________________
HZ> Pw_forum mailing list
HZ> Pw_forum at pwscf.org
HZ> http://www.democritos.it/mailman/listinfo/pw_forum
HZ> 

-- 
=======================================================================
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
=======================================================================
If you make something idiot-proof, the universe creates a better idiot.