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

[Pietro Bonfà]

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