[Pw_forum] Fwd: pp convergence once more

[Guntram Schmidt]

Hello together!

I started using QE to calculate some total energies of molecular
crystals. It was said, that using rpb-pseudopotentials together with
input_dft = 'vdW-DF'
should lead to good results.
So I created the potentials using the PSlibrary-code. Some elements
didn't converge, but the ones, which generated a UPF-file should be fine
- aren't they?

To find the ECUTWFC it was said to be sufficient, to just calculate one
atom of the type you are interested in, so I took the following input:

&control
    calculation = 'scf'
    restart_mode='from_scratch',
    prefix='',
    tstress = .false.
    tprnfor = .false.
    pseudo_dir = '/gpfs/home/aasfu/espresso-4.3.1/pseudo/',
    outdir='/gpfs/home/aasfu/tmp/'
    verbosity = 'high'
/
&system
  ibrav=0,
  celldm(1)=2,
  nat = 1,
  ntyp = 1,
  ecutwfc = 30,
  ecutrho = 120,
  input_dft = 'vdW-DF',
  occupations='smearing',
  smearing='gaussian',
  degauss=0.01,
  force_pairing = .true.

/
&electrons
     conv_thr        = 1.0d-8
     electron_maxstep= 1000
/

CELL_PARAMETERS cubic
10.0 	0.0 	0.0
0		10.0	0
0.0	 	0.0		10.0

ATOMIC_SPECIES
  O 1.0 O.rpb-rrkjus.UPF

ATOMIC_POSITIONS angstrom
  O      0 0 0

K_POINTS automatic
1 1 1 0 0 0



for the elements of interest (H, C, N, O, P, Sn) with increasing
cutoff-value and different multiplicators to get ecutrho (4 times
ecutwfc and 10 times ecutwfc (ultrasoft?) ).

Some of them converge (e.g, C at 36 / 140 and 37 / 370 for rpb-rrkjus),
but others do oscillate quite large (O, Sn).

What is a good convergence criterium?
I found numbers like 1mRy but also 5mRy per atom
(http://users.aims.ac.za/~daniel/hands_on/Brandon_workshop_handout.pdf)!
Does this mean, the more atoms I have in my structure (will be ~200),
the bigger the allowed deviation (1Ry in my case)?
I always took the difference between n Ry and n+1 Ry in ecutwfc as a
criteria - is this right?

Or is it better to test convergence with small molecules, which contain
the atoms in the configuration of my molecule (sp2-carbons, carboxylate
functions etc.)?

Thanks a lot for clearification,
Guntram