code: def _2ndderiv_xyz(self,x,y,z,i,j):
          
          return 4.*numpy.pi*self._b*self._c\
              *_2ndDerivInt(x,y,z,
                            lambda m: self._mdens(m),
                            lambda m: self._mdens_deriv(m),
                            self._b2,self._c2,i,j,glx=self._glx,glw=self._glw)
docs: General 2nd derivative of the potential as a function of (x,y,z)
          in the aligned coordinate frame