We develop the Valence Effective Hamiltonian method to calculate the magnetic dipole (MD) and electric quadrupole (EQ) contributions to the hyperpolarizability beta of molecular systems. For both the C-60 and C-70 molecules, we find that the MD contribution leads to a beta value which is several times larger than that of the EQ contribution. Taking into account the local field correction factor for C-60 in solid state, we calculate chi((2)) susceptibility values that are in excellent agreement with recent experimental data.