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:
           2013-06-28 - Written - Bovy (IAS)
        """
        r= nu.sqrt(R**2.+z**2.)
        return 1./r**self.alpha*nu.exp(-(r/self.rc)**2.)
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:
           2013-06-28 - Written - Bovy (IAS)