[Pw_forum] ibrav for Bmmb group

[giacsport at libero.it]

Dear All,
             I would like to calculate a structure whose symmetry group is 
Bmmb. I know that is a orthorombic structure with the ab-plane adopted as a 2D  
plane.
The unit cell is 1 atom of Ti and 2 of oxygen.
   Cartesian axes
     site n.     atom                  positions (a_0 units)
         1           Ti  tau(  1) = (   0.0000000   0.2500000   0.6800900  )
         2           O   tau(  2) = (   0.0000000   0.2500000   0.3832200  )
         3           O   tau(  3) = (   0.0000000   0.2500000   0.2180200  )
 
I tried setting ibrav = 8,     
                         celldm(1)=a (bohr),
                         celldm(2)=b/a,
                         celldm(3)=c/a

and I got this message in the output

     ortho sub-group =    2*   2 procs

     warning: symmetry operation #  2 not allowed.   fractional translation:
       0.0000000  0.3862661  0.0000000  in crystal coordinates
     warning: symmetry operation #  7 not allowed.   fractional translation:
       0.0000000  0.3862661  0.0000000  in crystal coordinates

Is the ibrav value not appropriate for the Bmmb space group? Can anybody 
kindly help me?
Very best,

Giacomo Giorgi,
PhD. Tokyo University.