code: def _R2deriv(self,R,z,phi=0.,t=0.):
          
          Rz= R**2.+z**2.
          sqrtRz= numpy.sqrt(Rz)
          return (3.*R**4.+2.*R**2.*(z**2.+self.a*sqrtRz)\
                      -z**2.*(z**2.+self.a*sqrtRz)\
                      -(2.*R**2.-z**2.)*(self.a**2.+R**2.+z**2.+2.*self.a*sqrtRz)\
                      *numpy.log(1.+sqrtRz/self.a))\
                      /Rz**2.5/(self.a+sqrtRz)**2.
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)