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:
            2015-02-15 - Written - Trick (MPIA)
        """
        l,n = bovy_coords.Rz_to_lambdanu(R,z,ac=self._ac,Delta=self._Delta)
        return -1./(nu.sqrt(l) + nu.sqrt(n))