code: def _surfdens(self,R,z,phi=0.,t=0.):
          
          Rb= R**2.+self._b2
          return self._b2*z*(3.*Rb+2.*z**2.)/Rb**2.*(Rb+z**2.)**-1.5/2./nu.pi
docs: NAME:
             _surfdens
          PURPOSE:
             evaluate the surface density for this potential
          INPUT:
             R - Galactocentric cylindrical radius
             z - vertical height
             phi - azimuth
             t - time
          OUTPUT:
             the density
          HISTORY:
             2018-08-19 - Written - Bovy (UofT)