def _dens(self,R,z,phi=0.,t=0.):
        """
        NAME:
           _dens
        PURPOSE:
           evaluate the density for this potential
        INPUT:
           R - Galactocentric cylindrical radius
           z - vertical height
           phi - azimuth
           t - time
        OUTPUT:
           the density
        HISTORY:
           2015-12-04 - Started - Bovy (UofT)
        """
        return 1./(1.+(R**2.+z**2.)/self._a2)/4./nu.pi/self._a3
NAME:
           _dens
        PURPOSE:
           evaluate the density for this potential
        INPUT:
           R - Galactocentric cylindrical radius
           z - vertical height
           phi - azimuth
           t - time
        OUTPUT:
           the density
        HISTORY:
           2015-12-04 - Started - Bovy (UofT)