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:
           2010-07-09 - Started - Bovy (NYU)
        """
        return -1./nu.sqrt(R**2.+(self._a+nu.sqrt(z**2.+self._b2))**2.)