code: def _R2deriv(self,R,z,phi=0.,t=0.):
          
          if not self.isNonAxi:
              phi= 0.
          x,y,z= bovy_coords.cyl_to_rect(R,phi,z)
          if not self._aligned:
              raise NotImplementedError("2nd potential derivatives of TwoPowerTriaxialPotential not implemented for rotated coordinated frames (non-trivial zvec and pa)")
          phixx= self._2ndderiv_xyz(x,y,z,0,0)
          phixy= self._2ndderiv_xyz(x,y,z,0,1)
          phiyy= self._2ndderiv_xyz(x,y,z,1,1)
          return numpy.cos(phi)**2.*phixx+numpy.sin(phi)**2.*phiyy\
              +2.*numpy.cos(phi)*numpy.sin(phi)*phixy
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:
             2016-06-15 - Written - Bovy (UofT)