[Pw_forum] calculation of lattice parameter and angle of rhombohedral structure

[yedu kondalu]

Dear users,

    I did the optimization for a compound using variable cell approximation
using PWSCF, which belongs to the space group R3m(160) Rhombohedral
representation.  The primitive vectors in terms of lattice parameter a =
8.25791360 a.u.
               a(1) = (   0.619505  -0.357671   0.698774 )
               a(2) = (   0.000000   0.715343   0.698774 )
               a(3) = (  -0.619505  -0.357671   0.698774 )

after completion of optimization step, the primitive vectors

CELL_PARAMETERS (alat=  8.25791360)
   0.636439417  -0.367448469   0.640642896
   0.000000000   0.734896938   0.640642896
  -0.636439417  -0.367448469   0.640642896

can u please explain me

how can I calculate the lattice parameter *a* and the *angle (alpha)*  ???

Thanks in advance

         Regards
     Yedukondalu
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