In this work we investigate fragment ab initio modelling of crystal field levels in Ln(III) systems from an innovative point of view. Because it is considered one of the most rigorous non-covalent embedding of a metal ion in an ab initio potential created by the ligand, we start briefly reviewing the FAIMP model as an ab initio electrostatic description. Then we introduce a novel ab initio approach based on the theory of Multi-Configurational Non-Orthogonal Group Functions (MC-NOGF), with the aim of modelling the strongly ionic metal-ligand interactions in terms of a systematic ab initio description based on purely intermolecular non-covalent interactions. This approach allows us to treat the metal-ligands interactions as a perturbation acting on the wave functions of the isolated fragments, which accounts in principle for both purely non-covalent (electrostatics, induction and dispersion) interactions, and for overlap effects which arise from the non-orthogonal theoretical framework. After presenting the equations for an ab initio multiconfiguration development involving matrix elements between non-orthogonal sets of orbitals, as we implemented in our own quantum chemistry code CERES , we study how different approximations to the ligands' wave function perform within the MC-NOGF method in the simulation of the crystal field level and magnetic proprieties of a set of Ln(III) complexes.