[Pw_forum] XDM stress dependent on k-point parallelization?

[Malte Sachs]

Dear all,

I have tried to perform vc-relax calculations with Quantum Espresso v6.0 
using the XDM Van-der-Waals correction. I get some strange results, 
because it seems that the calculated XDM stress depends on the k-point 
parallelization. As a test I performed three scf calculations using the 
same input (see attached) just variing the number of k-point pools:

2 pools with 3 k-points: XDM stress (-33.37 -33.37 -40.79)

3 pools with 2 k-points: XDM stress (-58.75 -58.75 -61.19)

6 pools with 1 k-point: XDM stress (-117.51 -117.51 -122.37)

The stress ratio 1: 1/2 : 1/3 is the inverse of the k-point ratio per 
pool... Has somebody seen something like this before?

Best regards,

Malte

-- 
Malte Sachs
Anorganische Chemie, Fluorchemie
Philipps-Universität Marburg
Hans-Meerwein-Straße 4
35032 Marburg (Paketpost: 35043 Marburg)
Tel.: +49 (0)6421 28 - 25 39 5
http://www.uni-marburg.de/fb15/ag-kraus/

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&CONTROL
    calculation = 'scf',
    title = 'UCl6_P-3m1'
    verbosity = 'high'
    prefix = 'UCl6_P3'
    outdir = './outdir/'
    pseudo_dir = '/home/Sachsm/Promotion/Pseudos/'
    tstress = .true.,
    wf_collect = .true.
    etot_conv_thr = 1.0D-8,
    forc_conv_thr = 1.0D-6
 /
&SYSTEM
    ibrav = 4,
    celldm(1) = 20.55532424,
    celldm(3) = 0.461663392,
    nat =  21,
    ntyp = 2,
    nbnd = 90,
    occupations = 'fixed',
    ecutwfc = 100,
    ecutrho = 800,
    vdw_corr = 'xdm',
    xdm_a1 = 0.6512,
    xdm_a2 = 1.4633,
 /
 &ELECTRONS
    mixing_beta = 0.4,
    mixing_mode = 'plain',
    conv_thr = 1.D-10,
 /
&IONS
 /
 &CELL
 /

K_POINTS (automatic)
  2 2 4 0 0 0

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