code: def _R2deriv(self,R,z,phi=0.,t=0.):
          
          r2= R**2.+z**2.
          r= nu.sqrt(r2)
          return (1./r2*(1.-R**2./r2*(3.*self._a2+2.*r2)/(self._a2+r2))\
                      +self._a/r2/r*(3.*R**2./r2-1.)*nu.arctan(r/self._a))\
                      /self._a
docs: NAME:
             _R2deriv
          PURPOSE:
             evaluate the second radial derivative for this potential
          INPUT:
             R - Galactocentric cylindrical radius
             z - vertical height
             phi - azimuth
             t - time
          OUTPUT:
             the second radial derivative
          HISTORY:
             2011-10-09 - Written - Bovy (IAS)