[QE-users] Problem with vc-relax simulation of Pb (cell dimensions are much smaller than they should be)

[Marzari Nicola]

Indeed, the stress tensor converges poorly with cutoff, and an equation of state with ev.x would be better than vc relax (ie. 7-10 scf calculations at different volumes).

For all - please, *before* asking a question like this, try the parameters and pseudopotentials suggested by:
https://www.materialscloud.org/work/tools/qeinputgenerator

Nicola

Sent from a tiny keyboard... Contact info:
http://theossrv1.epfl.ch/Main/Contact

On 9 Dec 2019, at 09:39, Pietro Bonfà <pietro.bonfa at unipr.it> wrote:

Dear S. Arash Sheikholeslam,

your settings for the basis set seem quite low, you should consider increasing these two values:

ecutwfc = 25.0, ecutrho = 300.0

You can check this web page https://www.materialscloud.org/discover/sssp/plot/efficiency/Pb to identify the appropriate basis set parameters for the pseudopotential that you are using. The actual values to be used should be converged according to the specific scientific case that you intend to investigate.

Reciprocal space sampling should probably also be improved.

Best regards,
Pietro


On 12/8/19 9:54 PM, sarashs wrote:
Hi quantum espresso users,
I am performing a vc-relax simulation on pb. I have included my input file for your convenience. The problem that I have is that the minimum energy cell is more than 1 Angstrom smaller on each dimension (3.25 Angstrom) while it should be around 4.95. I am wondering if you can help me figure out why? I have tried increasing the number of K points, ecutwfc and I tried with different degauss values but I was not successful in converging to the right size.
Is this an xc functional issue? or something else?
&control
  calculation =   "vc-relax"   ,
   restart_mode = 'from_scratch' ,
   outdir='$TMP_DIR/' ,
   pseudo_dir = '$PSEUDO_DIR' ,
   disk_io = 'default' ,
   verbosity = 'default' ,
   tstress = .true. ,
   tprnfor = .true. ,
 /
 &system
    ibrav = 2, celldm(1) =4.9505, nat= 1, ntyp= 1,
    ecutwfc = 25.0, ecutrho = 300.0
    occupations='smearing', smearing='gaussian', degauss=0.02
 /
 &electrons
    diagonalization='david'
    conv_thr = 1.0e-8
    mixing_beta = 0.7
 /
&IONS
 /
 &CELL
   cell_dynamics = 'bfgs',
   press = 1.00 ,
 /
ATOMIC_SPECIES
 Pb 207.2 Pb.pbe-dn-kjpaw_psl.1.0.0.UPF
ATOMIC_POSITIONS alat
 Pb 0.0 0.0 0.0
K_POINTS (automatic)
 5 5 5 0 0 0
Best regards,
S. Arash Sheikholeslam
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--
Pietro Bonfà
Department of Mathematical, Physical and Computer Sciences,
University of Parma,
Italy
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