code: def _R2deriv(self,R,z,phi=0.,t=0.):
          
          r2= R**2.+z**2.
          rb= nu.sqrt(r2+self.b2)
          return -(-self.b**3.-self.b*z**2.+(2.*R**2.-z**2.-self.b**2.)*rb)/\
              rb**3./(self.b+rb)**3.
docs: NAME:
             _Rderiv
          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:
             2013-09-08 - Written - Bovy (IAS)