PM is used for obtaining DISP file involving plus and minus
displacements. In this tag, only the plus and minus displacements
which are symmetrically different are output. Sometimes phonon
frequency is not well calculated even if equilibrium structure is
highly optimized. This is often because the structure in which an
atom is displaced in one direction is symmetrically different from
that in which the atom is displaced in the opposite direction. The
plus and minus displacements are useful, when you want to counteract
the finite displacement errors.
PM = .TRUE.