Soncini and Van den Heuvel showed in ref. that for a canonical ensemble of a quantum mechanical system with arbitrary degenerate electronic energy levels, the NMR chemical shielding of nucleus K can be derived as the partial bi-derivative of the Helmholtz free energy A with respect to the external magnetic field B and the nuclear magnetic moment µ K . Indeed, in the presence of a strong magnetic field the expression of A = U -T S -MB (U is the internal energy, S is the entropy and T is the temperature and M is the magnetization of the system) and by the second law of thermodynamics, the first derivative ∂ A ∂ B T = -M can be formulated with a simpler derivation accordingly,