In this article, we have studied the complexation phenomena of bare Nd(III) (representative of light REEs) and Yb(III) (representative of heavy REEs) with an RTIL using density functional theory (DFT) methods to learn more about the possible interactions between the RE cations and RTIL. While more extensive modeling with these systems will be required to have a complete picture of the separation, this study is focused on understanding the metal-RTIL interaction. To carry out the study, we have considered the methyl derivative of 1-methyl-3-butylimidazolium + , PF6 -, i.e., 1,3-dimethyl-imidazolium + , PF6 -, defined as [MMI][PF6], to reduce the computational cost. As evident from the work of Chaumont et al., the PF6 -can coordinate with the metal center in monodentate, bidentate and tridentate fashions (Figure ) depending on the size of the RE cations and the overall CN. Following the literature , we anticipated a 9-coordinated complex for Nd(III) and an 8-coordinated complex for Yb(III) as the starting complex; however, we explored the maximum CN that can be achieved by each RE cation. The entire complexation is first studied in the gas phase and further explored with the continuum solvation model employing RTIL parameters to get a better understanding of bonding in practical scenarios. The number of PF6 -ligands added to the RE cations begins with three considering all possible combinations of coordinating modes of PF6 -and CN, and the sequential additions of PF6 -continue until the maximum number of PF6 -coordination to each RE cation is achieved. This was determined by calculating the binding energy (BE) value after each PF6 -addition. Further, we have considered the second solvation sphere constructed with [MMI] + for the maximum PF6 -coordinated RE complexes. The comparison of the preference of the RTIL between the two RE cations is based on geometrical parameters and BE values.