code: def _compute_a22_factor(self, imt):
          
          if imt.name == 'PGV':
              return 0.0
          period = imt.period
          if period < 2.0:
              return 0.0
          else:
              return 0.0625 * (period - 2.0)
docs: Compute and return the a22 factor, equation 20, page 80.