def _evaluate(self,R,z,phi=0.,t=0.):
        """
        NAME:
           _evaluate
        PURPOSE:
           evaluate the potential at R,z
        INPUT:
           R - Galactocentric cylindrical radius
           z - vertical height
           phi - azimuth
           t - time
        OUTPUT:
           Phi(R,z)
        HISTORY:
           2018-08-04 - Written - Bovy (UofT)
        """
        r2= R**2+z**2
        if r2 <= self.a2:
            return -1./self.a
        else:
            return -1./nu.sqrt(r2)