[Pw_forum] Interpretation of vc-relax / pi-pi-stack

[Guntram Schmidt]

Dear solid-state-theorists ;-),

I'm wondering what the vc-relax-result of an optimizing of a measured 
crystal structure can tell me.
I have a sodium salt of a benzene-sulfonic acid (Na+ -O3S-Ph type) and 
the optimization at vdW-rpb-rrkjus-level grows the unit cell 
significantly along the pi-pi-stacking!
The measured crystal structure has an approx. intercentroid distance of 
3.5 A which grows to app. 5 A after "optimization".

This happens with two different salts of this type.

I thought, vdW was capable of treating such interactions - isn't it?
Or do I have to modify my input-parameters (attached)?

Thanks,
Guntram


----------------------------------------
&control
    calculation = 'vc-relax'
    restart_mode='from_scratch',
    prefix='zacdatenzschmidti2t0537i2t0537',
    tstress = .true.
    tprnfor = .true.
    pseudo_dir = '/gpfs/home/aasfu/espresso-4.3.1/pseudo/',
    outdir='/gpfs/home/aasfu/tmp/'
    verbosity = 'high'
/
&system
  ibrav = 14,
  A = 6.6986,
  B = 22.181,
  C = 6.1146,
  cosAB = 0,
  cosAC = -.1452369383,
  cosBC = 0,
  nat = 88,
  ntyp = 5,
  ecutwfc = 70,
  ecutrho = 700,
  input_dft = 'vdW-DF',
  occupations = 'smearing' ,
  degauss = 0.05D0 ,
  smearing = 'gaussian',

/
&electrons
     conv_thr        = 1.0d-5
     electron_maxstep= 1000
/
&ions
/
&CELL
  press = 0.0,
/

ATOMIC_SPECIES
  S 32.066 S.rpb-rrkjus.UPF
  Na 22.98977 Na.rpb-rrkjus.UPF
  C 12.011 C.rpb-rrkjus.UPF
  H 1.00794 H.rpb-rrkjus.UPF
  O 15.9994 O.rpb-rrkjus.UPF


ATOMIC_POSITIONS angstrom
  O       5.189644    1.223060    1.773489
...
K_POINTS automatic
1 1 1 0 0 0
------------------------------------------