[Pw_forum] Questions about vc-relax

[Giuseppe Mattioli]

Dear Shaofeng
vc-relax calculations are VERY sensitive to unconverged wfcs and densities. The corresponding ecutwfc and ecutrho variables are generally kept on the 
low side in the examples, because fast execution is more important than accuracy in this case. So be sure that your results are converged before 
comparing them, and keep in mind that different kinds of pseudopotential (NC, US, PAW) require different wfc and density cutoffs.

> (3) how can I get
> correct reuslts?

Define "correct results" in the context of ab initio calculations, please...:-)

HTH

Giuseppe

On Tuesday, November 25, 2014 09:21:49 PM Shaofeng Wang wrote:
> Dear all,
> 
> I am  new user of QE and trying to do some structure optimization of
> minerals. When I was running examples of As structure (VCSexample) in PW
> folder, I found the unit volume increased dramatically from 245 to
> 274.7a.u.^3 at 0 kbar like follow:
>       unit-cell volume          =     245.3705 (a.u.)^3
>       new unit-cell volume =    273.57167 a.u.^3 (    40.53914 Ang^3 )
>       new unit-cell volume =    282.73649 a.u.^3 (    41.89722 Ang^3 )
>       new unit-cell volume =    283.67515 a.u.^3 (    42.03632 Ang^3 )
>       new unit-cell volume =    280.14013 a.u.^3 (    41.51248 Ang^3 )
>       new unit-cell volume =    274.78389 a.u.^3 (    40.71877 Ang^3 )
>       new unit-cell volume =    273.45636 a.u.^3 (    40.52205 Ang^3 )
>       new unit-cell volume =    273.55922 a.u.^3 (    40.53729 Ang^3 )
>       new unit-cell volume =    273.85007 a.u.^3 (    40.58039 Ang^3 )
>       new unit-cell volume =    274.29816 a.u.^3 (    40.64679 Ang^3 )
>       new unit-cell volume =    274.73620 a.u.^3 (    40.71170 Ang^3 )
>       new unit-cell volume =    274.77140 a.u.^3 (    40.71692 Ang^3 )
>       new unit-cell volume =    274.77140 a.u.^3 (    40.71692 Ang^3 )
> Then, I tried to use other pseudopotential to calculate. When I used
> As.pbe-n-kjpaw_psl.0.2.UPF, the volume increased greatly to 302 a.u.^3 .
>       unit-cell volume          =     245.3705 (a.u.)^3
>       new unit-cell volume =    284.84576 a.u.^3 (    42.20979 Ang^3 )
>       new unit-cell volume =    297.17056 a.u.^3 (    44.03613 Ang^3 )
>       new unit-cell volume =    302.05275 a.u.^3 (    44.75960 Ang^3 )
>       new unit-cell volume =    301.67350 a.u.^3 (    44.70340 Ang^3 )
>       new unit-cell volume =    298.41441 a.u.^3 (    44.22045 Ang^3 )
>       new unit-cell volume =    296.79550 a.u.^3 (    43.98056 Ang^3 )
>       new unit-cell volume =    296.95284 a.u.^3 (    44.00387 Ang^3 )
>       new unit-cell volume =    298.30798 a.u.^3 (    44.20468 Ang^3 )
>       new unit-cell volume =    300.62688 a.u.^3 (    44.54831 Ang^3 )
>       new unit-cell volume =    301.83711 a.u.^3 (    44.72764 Ang^3 )
>       new unit-cell volume =    302.26812 a.u.^3 (    44.79151 Ang^3 )
>       new unit-cell volume =    302.39881 a.u.^3 (    44.81088 Ang^3 )
>       new unit-cell volume =    302.39881 a.u.^3 (    44.81088 Ang^3 )
>       unit-cell volume          =     302.3988 (a.u.)^3
> when I used As.pbe-n-rrkjus_psl.0.2.UPF, the results are similar to the
> above one.
>       unit-cell volume          =     245.3705 (a.u.)^3
>       new unit-cell volume =    284.78238 a.u.^3 (    42.20039 Ang^3 )
>       new unit-cell volume =    297.19206 a.u.^3 (    44.03932 Ang^3 )
>       new unit-cell volume =    302.09032 a.u.^3 (    44.76517 Ang^3 )
>       new unit-cell volume =    301.70966 a.u.^3 (    44.70876 Ang^3 )
>       new unit-cell volume =    298.43756 a.u.^3 (    44.22388 Ang^3 )
>       new unit-cell volume =    296.84707 a.u.^3 (    43.98820 Ang^3 )
>       new unit-cell volume =    296.98526 a.u.^3 (    44.00867 Ang^3 )
>       new unit-cell volume =    298.33351 a.u.^3 (    44.20846 Ang^3 )
>       new unit-cell volume =    300.56076 a.u.^3 (    44.53851 Ang^3 )
>       new unit-cell volume =    301.94256 a.u.^3 (    44.74327 Ang^3 )
>       new unit-cell volume =    302.43141 a.u.^3 (    44.81571 Ang^3 )
>       new unit-cell volume =    302.63485 a.u.^3 (    44.84586 Ang^3 )
>       new unit-cell volume =    302.63485 a.u.^3 (    44.84586 Ang^3 )
>       unit-cell volume          =     302.6348 (a.u.)^3
> 
> So, my questions are: (1) why the volume increase significantly, (2) what
> caused the difference between different pseudopotential, (3) how can I get
> correct reuslts?
> 
> Any comments or help are very appreciated.
> 
> Regards,
> 
> Shaofeng

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   Giuseppe Mattioli                            
   CNR - ISTITUTO DI STRUTTURA DELLA MATERIA   
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