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. (C) 2003 Elsevier B.V. All rights reserved.