def epifreq(self,R):
        """
        
        NAME:
        
           epifreq
        
        PURPOSE:
        
           calculate the epicycle frequency at R in this potential
        
        INPUT:
        
           R - Galactocentric radius (can be Quantity)
        
        OUTPUT:
        
           epicycle frequency
        
        HISTORY:
        
           2011-10-09 - Written - Bovy (IAS)
        
        """
        return nu.sqrt(self.R2deriv(R,0.,use_physical=False)\
                           -3./R*self.Rforce(R,0.,use_physical=False))