[Pw_forum] question on random_matrix

[Virginie Trinité]

Dear all

I have a question about the subroutine random_matrix in PH. here a 
simplify version of the code:

**************************
wdyn (:, :, :, : )= (0d0, 0d0)
do na = 1, nat
    wdyn (ipol, ipol, na, na) = random
    do ipol = 1, 3
    wdyn (ipol, jpol, na, na) = random
    wdyn (jpol, ipol, na, na) = CONJG(wdyn (ipol, jpol, na, na) )
    enddo
    do nb = na + 1, nat
       do isymq = 1, nsymq
          irot = irgq (isymq)
          ira = irt (irot, na)
           if ( (nb == ira)) then
               do jpol = 1, 3
                  wdyn (ipol, jpol, na, nb) = random
                  wdyn(jpol, ipol, nb, na) = CONJG(wdyn(ipol, jpol, na, 
nb))
               enddo
                goto 10
             endif
          enddo
10         continue
***************************

I didn't understand why if nb is never the transform of na by some 
symmetry then the dynamical matrix is zero?
Best regards

Virginie