def verticalfreq(self,R):
        """
        
        NAME:
        
           verticalfreq
        
        PURPOSE:
        
           calculate the vertical frequency at R in this potential
        
        INPUT:
        
           R - Galactocentric radius (can be Quantity)
        
        OUTPUT:
        
           vertical frequency
        
        HISTORY:
        
           2012-07-25 - Written - Bovy (IAS@MPIA)
        
        """
        return nu.sqrt(self.z2deriv(R,0.,use_physical=False))