code: def _R2deriv(self,R,z,phi=0.,t=0.):
          
          return 1./(R**2.+z**2.)**(self.alpha/2.)\
              -self.alpha*R**2./(R**2.+z**2.)**(self.alpha/2.+1.)
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:
             2011-10-09 - Written - Bovy (NYU)