The conformational changes in tetraoxacrown ethers (ring size 12–16) during hypothetical reaction with alkali-metal ions were investigated by the method of penalty functions. An analysis is made of the variation of the selectivity curves of crown ethers, calculated by the method of atom—atom potential functions, with variation in the length of the methylene chains between the donor atoms. It is shown that the important factor for the effective complexation of macroheterocycles is not the formal correspondence between the size of the ion and the cavity of the ligand but the possible adjustment of the macrocycle for complementary combination with the cation with minimal expenditures of conformotional ene