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