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:
           2016-11-04 - Written - Bovy (UofT/CCA)
        """
        x,y,z= self._compute_xyz(R,phi,z,t)
        zc= numpy.sqrt(z**2.+self._c2)
        bzc2= (self._b+zc)**2.
        bigA= self._b*y**2.+(self._b+3.*zc)*bzc2
        bigC= y**2.+bzc2
        return self._c2/24./numpy.pi/self._a/bigC**2./zc**3.\
            *((x+self._a)*(3.*bigA*bigC+(2.*bigA+self._b*bigC)*(x+self._a)**2.)\
                  /(bigC+(x+self._a)**2.)**1.5\
             -(x-self._a)*(3.*bigA*bigC+(2.*bigA+self._b*bigC)*(x-self._a)**2.)\
                  /(bigC+(x-self._a)**2.)**1.5)
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:
           2016-11-04 - Written - Bovy (UofT/CCA)