def _surfdens(self,R,z,phi=0.,t=0.):
        """
        NAME:
           _surfdens
        PURPOSE:
           evaluate the surface density for this potential
        INPUT:
           R - Galactocentric cylindrical radius
           z - vertical height
           phi - azimuth
           t - time
        OUTPUT:
           the surface density
        HISTORY:
           2018-08-19 - Written - Bovy (UofT)
        """
        r2= R**2.+z**2.
        rb= nu.sqrt(r2+self.b2)
        return self.b*((R*z)/r2-(self.b*R*z*(self.b**2+2.*R**2+z**2))
                       /((self.b**2+R**2)*r2*rb)
                       +nu.arctan(z/R)-nu.arctan(self.b*z/R/rb))/R**3/2./nu.pi
NAME:
           _surfdens
        PURPOSE:
           evaluate the surface density for this potential
        INPUT:
           R - Galactocentric cylindrical radius
           z - vertical height
           phi - azimuth
           t - time
        OUTPUT:
           the surface density
        HISTORY:
           2018-08-19 - Written - Bovy (UofT)