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:
           2018-08-06 - Written - Bovy (UofT)
        """
        x,y,z= bovy_coords.cyl_to_rect(R,phi,z)
        if self._aligned:
            xp, yp, zp= x, y, z
        else:
            xyzp= numpy.dot(self._rot,numpy.array([x,y,z]))
            xp, yp, zp= xyzp[0], xyzp[1], xyzp[2]
        m= numpy.sqrt(xp**2.+yp**2./self._b2+zp**2./self._c2)
        return self._mdens(m)