[Pw_forum] symmetries

[Elwira]

Stefano de Gironcoli wrote:

 > you are using atomic positions in crystallographic coordinates
 > without specifing ltaucy=.true. in the namelist ...
 >


Thank you for your answer. I was mislead by the message in output file:

  site n.     atom                  positions (a_0 units)
          1        Cu     tau( 1) = (  0.0000000  0.0000000  0.0000000  )
          2        Cu     tau( 2) = (  0.3333333  0.3333333  0.5000000  )


I thought that it indicates the atom positions in lattice vectors I wanted.

But I'm afraid I still don't quite understand  how it works.

In example 2 when bulk Si is introduced there is no ltaucry=.true. but 
the atom positions are in lattice vectors. Why is it so?

This time I used ibrav=0, ltaucry=.true. and lattice vectors and atomic 
postions as follows:
1.0 0.0 0.0
0.0 0.8660254 0.0
0.0 0.0 1.0
hexagonal
0.00 0.00 0.00 1
0.33333333333333333 0.33333333333333333 0.50 1

In the output file the crystal is described by 12 symmetry operations 
which is ok, but the atomic positions are as follows:

site n.     atom                  positions (a_0 units)
          1        Cu     tau( 1) = (  0.0000000  0.0000000  0.0000000  )
          2        Cu     tau( 2) = (  0.3333333  0.2886751  0.5000000  )

I don't understand why the second coordinate of second atom was changed?

I hope you will be so kind to answer for these very basic questions.

Best wishes,

Elwira