code: def verticalfreq(self,R):
          
          return nu.sqrt(self.z2deriv(R,0.,use_physical=False))
docs: 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)