[Pw_forum] How to extract the new lattice parameter from the output file?

Shujun Hu hushujun at mail.sdu.edu.cn
Fri Mar 24 07:20:24 CET 2006

Dear Drs,

After the vc-relax calculation new lattice vectors are listed at the end of the pw
output file:
     Final estimate of lattice vectors (input alat units)
   0.995512018   0.003513654   0.003278596
   0.003513654   0.995512026   0.003278606
   0.006573586   0.006573605   2.028720672
It looks like a distorted crystal cell since the input coordinates in tetragonal
symmetry are not too nice. I just want to reconstruct the system depending on the
new coordinate to do further investigation (scf calculation, not ve-relax again)
Fe1     -0.007211587  -0.007210580   0.011331385
Sn       0.492929291   0.492930243   0.346908752
O        0.715072885   0.300388490   0.019456747
O        0.300387360   0.715073779   0.019456807
O        0.834228885   0.834229971   0.329978526
H        0.211445321   0.211446138   0.323097848
I found that only the diagonal elements in the new lattice vector matrix above are
not enough to reconstructed a healthy suppercell, since if so the output force and
stress after a scf calculation are so large, especially for the non-diagonal
elements of the stress. But if triclinic symmetry is selected and
a1,a2,a3,angle1,angle2 and angle3 are evaluated based on the lattice vector
matrix, the final force and stress is small. It seems that the symmetry has been
changed. Why does the program not hold the symmetry? 
    The symmetry option in the input file is set as:
          nosym = .true.
An old message from one of our group members had been submitted several days ago,
named "How to read lattice constant from the output file of vc-relax". Some help
information about the symmetry option from the user's document was posted. Does it
means that if .false. is selected, the input symmetry will be maintained? We
appreciate for some detailed explanations.

  Best wishes.

                                         Shujun Hu

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