Ligand field effects in lanthanide ions compounds have consequences for optical and magnetic spectroscopy. In the analysis of electron paramagnetic resonance spectra of Gd3+ complexes, a major role is played by the zero field splitting (ZFS), which is a high order consequence of the ligand field and the spin–orbit coupling. We present a general parameterized method and a computer program for the study of the excited states of lanthanide complexes. We apply it for the first principles determination of the ZFS of the [Gd(H2O)8]3+ aqua ion and the corresponding EPR peak-to-peak width in solution. We calculate the influence of the various contributions to the effective Hamiltonian on the splitting of the ground state multiplet.