[Pw_forum] subroutine: star_q

[Katalin Gaal-Nagy]

Dear all,

I try to calculate for a given system the q points which are equvalent to 
a given one. For this purpose I try to use the routine star_q given in the 
ph directory of the quantum espresso package (espresso-3.1):

subroutine star_q (xq, at, bg, ibrav, symm_type, nat, tau, ityp, &
      nr1, nr2, nr3, nsym, s, invs, irt, rtau, nq, sxq, isq, imq, noinv, &
      modenum)

Most of the arguments are clear for me, I just have some problems in
understanding the input parameters noinv and modenum.

The parameter noinv sets the inversion symmetry of the system, thus, it
should be set to .true. in the case there is no inversion symmetry, am I
right?

The parameter modenum is the number of modes to be done, thus, in case of
the calculation of all phonon modes of the crystal, it should be 3*nat 
where nat is the number of atoms in the basis. Did I understood it 
correctly?

Another question (just for curiosity): what will change for the q in
star(q), if modenum<3*nat, e.g., modenum=0?

Of course, it is not nice to set noinv "by hand", therefore it would be 
nice if someone could give me a hint which subroutine I could utilize for
determining this parameter ... if not, I will search in the pw code and 
the pw directory ...

Thanks for your help!

Best wishes,
Katalin