# [Pw_forum] regarding stress in pw.x output file

bramha pandey pandey.bramha at gmail.com
Sat Feb 25 18:01:48 CET 2012

```Sir,  Stefano de Gironcoli Thank you For your kind reply.
Yes i have tested my convergence issue but unable to figure out exactly
which energy cut off and k point should i select.
First i was tried with kpoint=gamma (0 0 0 0 0 0), for fast convergence and
i obtained stress(Kbar)=0.24 at 95 Ry of ecut. The number of iteration is
8-9 in every case.

Convergence test at kpoint= 0 0 0 0 0 0 (gamma)
ecut(Ry)   stress(kbar)      etot(Ry)
23            -856.12         -22.45678789
28            -816.21         -22.74277097
33            -717.31         -22.99913807
38            -640.04         -23.13068593
43            -538.05         -23.24446298
48            -388.44         -23.36265311
53            -298.19         -23.41743393
58            -228.34         -23.44939498
63            -147.46         -23.48057851
68            -107.24         -23.49317545
73            -81.19          -23.50039187
78            -48.03          -23.50793794
83            -22.26          -23.51292707
88            -10.40          -23.51479400
93            -3.80           -23.51569322
94            -2.96           -23.51578788
95             0.24           -23.51615658
96             0.24           -23.51615658
97             1.44           -23.51628362
98             1.44           -23.51628362

Second with taking kpoint in consideration again the stress was increased
at fix ecut=95Ry. And if i decreased or increased  the ecut from 95 Ry to
63Ry and 115Ry , the stress value increases and decreased very small
respectively .
Please see the below which i was mention above in wording.
when cosider k point...fix ecut=95
k mess           stress(kbar)        etot(Ry)
1*1*1              0.24             -23.51615658
2*2*2             -107.01          -24.24731157
3*3*3             -146.30          -24.33808965
4*4*4             -149.74          -24.34988328
5*5*5             -149.91          -24.35200695
6*6*6             -149.97          -24.35245199
8*8*8             -149.85          -24.35256921
10*10*10        -150.01          -24.35257467

when we chnage the ecut 95 to 63:::
8*8*8             -279.21          -24.31814193

when we chnage the ecut= 115Ry:::
8*8*8             -142.60          -24.35372918

Please any suggestion will be appreciable.

On Fri, Feb 24, 2012 at 1:45 AM, Stefano de Gironcoli <degironc at sissa.it>wrote:

> **
> A norm conserving Nitrogen pseudo-potential is likely to require much more
> that 28 Ry.
> have you studied the convergence of your calculated energy and stress with
> respect to cutoff and k-point sampling ?
>
> Stefano de Gironcoli
>
> On 02/23/2012 07:41 PM, bramha pandey wrote:
>
> Dear all QE users and developers..
>
> I want to calculate the property of system at T=0 K and at P=0 kbar at
> starting of my pw.x run.
> But at the end of run, i obtained force=0 but stress p =-832.77 kbar.
> I am afraid of this stress value. my input file is given below.
>
> &CONTROL
>                  calculation = 'scf' ,
>                 restart_mode = 'from_scratch' ,
>                       outdir = '.' ,
>                   pseudo_dir = './' ,
>                       prefix = 'aln' ,
>                      tstress = .true. ,
>                      tprnfor = .true. ,
>  /
>  &SYSTEM
>                        ibrav = 2,
>                    celldm(1) = 8.285,
>                          nat = 2,
>                         ntyp = 2,
>                      ecutwfc = 28 ,
>  /
>  &ELECTRONS
>                     conv_thr = 1.0d-8 ,
>                  mixing_beta = 0.7 ,
>  /
> ATOMIC_SPECIES
>    Al   26.98000  Al.pz-vbc.UPF
>     N   14.01000  N.pz-vbc.UPF
> ATOMIC_POSITIONS alat
>    Al      0.000000000    0.000000000    0.000000000
>     N      0.250000000    0.250000000    0.250000000
> K_POINTS automatic
>   4 4 4   0 0 0
>
> Any type of help is highly appreciated.
>
>
>
>
>
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--
Thanks and Regards