[Pw_forum] Fwd: different symmetry when using equivalent representation

[Jess Kondor]

Dear all,

 I have some trouble in understanding why equivalent (if I am not
wrong) representation of the crystal structure leads to different
symmetry (as shown in the output from pw.x). All input/output
files are attached.

I work with rhombohedral unit cell, alat = 5.46 Ang, gamma =60.36.
First, I let PWscf to build lattice vectors using the following
settings:

                      ibrav = 5 ,
                  celldm(1) = 10.317903898
                  celldm(4) = 0.494548768
......

The code generates lattice vectors:

    celldm(1)=  10.317904  celldm(2)=   0.000000  celldm(3)=   0.000000
    celldm(4)=   0.494549  celldm(5)=   0.000000  celldm(6)=   0.000000

    crystal axes: (cart. coord. in units of a_0)
              a(1) = (  0.502718 -0.290245  0.814268 )
              a(2) = (  0.000000  0.580489  0.814268 )
              a(3) = ( -0.502718 -0.290245  0.814268 )

    reciprocal axes: (cart. coord. in units 2 pi/a_0)
              b(1) = (  0.994593 -0.574229  0.409366 )
              b(2) = (  0.000000  1.148457  0.409366 )
              b(3) = ( -0.994593 -0.574229  0.409366 )

and shows that

   6 Sym.Ops. (no inversion)

    point group C_3v (3m)
    there are  3 classes
    the character table:

      E     2C3   3s_v
A_1    1.00  1.00  1.00
A_2    1.00  1.00 -1.00
E      2.00 -1.00  0.00

    the symmetry operations in each class:
    E        1
    2C3      2    3
    3s_v     4    5    6


However, I prefer to work with lattice vectors directly and then
changed my settings

from :

                      ibrav = 5 ,
                  celldm(1) = 10.317903898
                  celldm(4) = 0.494548768


to:

                      ibrav = 0 ,
                  celldm(1) = 10.317903898
                   .......

   CELL_PARAMETERS
               0.502718 -0.290245  0.814268
               0.000000  0.580489  0.814268
              -0.502718 -0.290245  0.814268

CELL_PARAMETERS were taken from the output when I use first settings
(see above, crystal axes: (cart. coord. in units of a_0)).  However,
now the code shows different symmetry:

2 Sym.Ops. (no inversion)
    point group C_s (m)
    there are  2 classes
    the character table:

      E     s
A'     1.00  1.00
A''    1.00 -1.00

Frankly speaking, I cannot identify what I did wrong. I think, there
is a simple explanation, but I failed to find one. Any ideas?

Best wishes,


-- 
=====================================
Jess Kondor

NSTI
=====================================
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