Note that we leave out hyperfine coupling when diagonalizing this Hamiltonian. In principle, hyperfine coupling leads to an additional contribution to the induced magnetic field (next to the one due to the external field). As a consequence of leaving out this contribution, indirect nuclear spin-spin coupling is not described here. Also note that B 0 and S as well as the spin Hamiltonian parameters refer to the molecule-fixed frame. The field-dependent states, energies, and partition functions obtained after diagonalization of Eq. ( ) are inserted into Eq. ( ) to compute the electronically-averaged induced field in the molecule-fixed frame.