code: def verticalfreq(Pot,R):
      
      from .planarPotential import planarPotential
      if isinstance(Pot,(Potential,planarPotential)):
          return Pot.verticalfreq(R,use_physical=False)
      return nu.sqrt(evaluatez2derivs(Pot,R,0.,use_physical=False))
docs: NAME:
      
         verticalfreq
          
      PURPOSE:
      
          calculate the vertical frequency at R in the potential Pot
      
      INPUT:
  
         Pot - Potential instance or list thereof
      
         R - Galactocentric radius (can be Quantity)
      
      OUTPUT:
      
          vertical frequency
      
      HISTORY:
      
          2012-07-25 - Written - Bovy (IAS@MPIA)