code: def _R2deriv(self,R,z,phi=0.,t=0.):
          
          if self.alpha == 0.:
              denom= 1./(R**2.+z**2./self.q2+self.core2)
              return denom-2.*R**2.*denom**2.
          else:
              m2= self.core2+R**2.+z**2./self.q2
              return -m2**(-self.alpha/2.-1.)*((self.alpha+2)*R**2./m2-1.)
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)