In this study, we focus our attention on the SOC operator, which enters the CASCI-SO step. Spin-orbit interaction plays an important role in the case of lanthanide complexes. It causes the splitting and mixing of the atomic terms. The SOC Hamiltonians arise naturally from the reduction of the Dirac equation and its many electron extensions to two components theory . This reduction can be achieved in many different ways, and the most renowned spin-orbit Hamiltonian thus achieved is the Breit-Pauli Hamiltonian , including both a one-electron (1e-SOC) term, and a two-electron (2e-SOC) term. However, the Breit-Pauli operator is quite complex, and different approximations are often used in the literature . While many studies have been published on the performances of the different SOC approximations in the accurate modeling of spectroscopic transitions in organic molecules and inorganic complexes, including inter-multiplet transitions in lanthanide systems , to our knowledge we present here the first extensive comparison of the performance of different approximations to the full Breit-Pauli Hamiltonian in the calculation of the crystal field splitting of the ground multiplets of lanthanide complexes, i.e. of their magnetic excitations. Moreover, two of us have recently presented a new development that makes the evaluation of all the two-electron integrals entering the the Breit-Pauli Hamiltonian more efficient by the use of the Cholesky decomposition technique . The aim of this work is then to (i) introduce working equations for the use of the Cholesky representation of the 2e-SOC integrals within the Breit-Pauli Hamiltonian and various mean-field approximations to it, (ii) apply our developments to the explicit ab initio calculation of the crystal field levels in a family of Ln(III) complexes, and finally (iii) compare the performances of the bare 1e-SOC and different mean-field approximations to the spin-orbit coupling Hamiltonian with respect to that of the full Breit-Pauli Hamiltonian, via a detailed analysis of the ensuing errors affecting the crystal field energy levels in the lowest energy multiplets for a series of Ln(III) complexes.