This block is needed only when
TDPolarizationEquivAxes is set to 3. In such a case,
the three directions (pol1, pol2, and pol3) defined in
the TDPolarization block should be related by symmetry
operations. If A is the symmetry operation that takes you
from pol1 to pol2, then TDPolarizationWprime
should be set to the direction defined by A$^{-1}$pol3.
For more information see MJT Oliveira
et al., J. Nanoscience and Nanotechnology8,
3392 (2008).