Density functional theoretical analysis was performed to explore the enhanced selectivity of the Cs+ ion over the Na+ ion with hybrid calix[4]-bis-crown inacrocyclic ligand compared to 18-crown-6 ether. The calculated Selectivity:data for Cs+/Na+ with hybrid calix[4]-bis-crown ligand using the free energy of extraction employing thermodynamical cycle was found to be in excellent agreement With the reported solvent extraction results. The present study further establishes that the selectivity for a specific metal ion between two competitive ligands is primarily due to the complexation free energy of the ligand to the metal ions and is independent of the aqueous solvent effect but strongly depends on the dielectricity of the organic solvents and the presence of the coanion.