We presented a novel multiconfigurational non-orthogonal group functions (MC-NOGF) approach to the calculation of the crystal field energies and magnetic properties of trivalent lanthanide complexes. The novel MC-NOGF approach has been implemented in the ab initio program Computational Emulator of Rare Earth Systems (CERES) . By direct comparison of two different fragments approaches of the crystal field energies of 44 complexes, namely the FAIMP and the here propsed MC-NOGF, we showed that the MC-NOGF model offers a better performing approach to an ab initio non-covalent description of metal-ligand interactions in trivalent lanthanide complexes.