The density functional calculations were performed to investigate why Li + is selectively transported by the aza-12crown-4 with an amine pendant arm. Two type of complexes, <[M + ] and [>M + ] with four or five donor atoms coordinating to a metal, were optimized where <[] and < denote a free ligand and its pendant arm, respectively. It is ascertained that a donor atom in both amine and ether arms can coordinate to the alkali cations (Li + , Na + , and K + ) as the fifth ligand in [>M + ]. This result is inconsistent with that observed, i.e., only the amine arm coordinates to Li + and form [>M + ] in the solution. In order to include solvent effect in the calculations, we optimized the geometries of Li + and Na + complexes with one or two waters, [>M + ](OH 2 ) and <[M + ] (OH 2 ) 2 , which have a cation with the hexa-coordinated environment. Only the combination of an amine arm and Li + has similar stabilization energies for formation of such complexes. The <[M + ](OH 2 ) 2 complex releases larger stabilization energies than those for the [>M + ](OH 2 ) in the other combination of the arms and the cations.