def vcirc(self,R,phi=None):
        """
        
        NAME:
        
            vcirc
        
        PURPOSE:
        
            calculate the circular velocity at R in potential Pot

        INPUT:
        
            Pot - Potential instance or list of such instances
        
            R - Galactocentric radius (can be Quantity)
        
            phi= (None) azimuth to use for non-axisymmetric potentials

        OUTPUT:
        
            circular rotation velocity
        
        HISTORY:
        
            2011-10-09 - Written - Bovy (IAS)
        
            2016-06-15 - Added phi= keyword for non-axisymmetric potential - Bovy (UofT)

        """
        return nu.sqrt(R*-self.Rforce(R,phi=phi,use_physical=False))