[Pw_forum] About BFGS optimized structure

[David Foster]

Dear Users
I have two questions:
1) I use QE5.1, and optimized CeO2 primitive cell. We all know that after getting final optimized structure, QE does an extra scf with new cell parameters. My last energy (with new cell parameters) and optimized one difference is about 0.1RY. Now, do I need a new vc-relax calculation with previous parameters as input? 

this is my input cell parameter of vc-relax calculation:
=================================================
     celldm(1)=  10.225308  celldm(2)=   0.000000  celldm(3)=   0.000000
     celldm(4)=   0.000000  celldm(5)=   0.000000  celldm(6)=   0.000000

    crystal axes: (cart. coord. in units of alat)
               a(1) = (  -0.530413   0.000025   0.530403 )  
               a(2) = (   0.000052   0.530414   0.530414 )  
               a(3) = (  -0.530413   0.530403   0.000025 )  


      site n.     atom                  positions (cryst. coord.)
         1           O   tau(   1) = (  0.2900000  0.2700000  0.2900000  )
         2           O   tau(   2) = ( -0.2500000 -0.2500000  0.7500000  )
         3           Ce  tau(   3) = (  0.0000000  0.0000000  0.0000000  )
=================================================


and this is my optimized vc-relax output:
===============================
CELL_PARAMETERS (alat= 10.22530811)
  -0.530413331   0.000024549   0.530403418
   0.000052398   0.530413516   0.530413516
  -0.530413331   0.530403418   0.000024549

ATOMIC_POSITIONS (crystal)
O        0.263099905   0.256909491   0.263099905
O       -0.236877222  -0.243128052   0.763122778
Ce       0.013777317   0.006218561   0.013777317
End final coordinates
=================================


My last extra scf force and stress are (with the final optimized cell):
==============
Total force =     0.003745     Total SCF correction =     0.000000

     entering subroutine stress ...

          total   stress  (Ry/bohr**3)                   (kbar)     P=   26.51
   0.00017290  -0.00000007  -0.00000007         25.43     -0.01     -0.01
  -0.00000007   0.00018388  -0.00000007         -0.01     27.05     -0.01
  -0.00000007  -0.00000007   0.00018388         -0.01     -0.01     27.05
==============

2) I used ibrav=2 in above calculation, and changed the experimental positions of one oxygen atom to change the crystal structure from its stable form (I need to do this, because with experimental data, I always get the input structure after doing vc-relax). Now, I need to change the primitive cell to its conventional. I used xcrysden and VESTA, but they only show the primitive structure. How can I change the QE output to its conventional cell. 

Regards

David Foster

Ph.D. Student of Chemistry