code: def _R2deriv(self,R,z,phi=0.,t=0.):
          
          l,n    = bovy_coords.Rz_to_lambdanu     (R,z,ac=self._ac,Delta=self._Delta)
          jac    = bovy_coords.Rz_to_lambdanu_jac (R,z,            Delta=self._Delta)
          hess   = bovy_coords.Rz_to_lambdanu_hess(R,z,            Delta=self._Delta)
          dldR   = jac[0,0]
          dndR   = jac[1,0]
          d2ldR2 = hess[0,0,0]
          d2ndR2 = hess[1,0,0]
          return d2ldR2       * self._lderiv(l,n)  + \
                 d2ndR2       * self._nderiv(l,n)  + \
                 (dldR)**2    * self._l2deriv(l,n) + \
                 (dndR)**2    * self._n2deriv(l,n) + \
                 2.*dldR*dndR * self._lnderiv(l,n)
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:
              2015-02-13 - Written - Trick (MPIA)