[Pw_forum] Questions about vc-relax

[Shaofeng Wang]

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

-- 
Shaofeng Wang, Ph.D
Environmental Geochemistry,
Institute of Applied Ecology, Chinese Academy of Sciences,
No. 72, Wenhua Road, Shenyang, China
Email: wangshaofeng at iae.ac.cn