Intermolecular interaction energy in a crystal composed of helical polymers was calculated analytically without specifying a material. It was found that the interaction energy is lowered when the molecular conformation is a commensurate helix (helical conformation with translational symmetry along the helix axis) with small translational period. Even a small change of the molecular conformation from the commensurate one, which is unimportant for intramolecular interaction, increases the intermolecular interaction energy abruptly. Because of this abrupt change of the intermolecular interaction energy, the actual molecular conformation in the crystal will not be what gives the lowest intramolecular interaction energy but what corresponds to one of the minimums of the intermolecular interaction energy. The obtained results assure translational periodicity of the crystal structure along the molecular axis; the translational periodicity is not directly required by the postulates of the molecular conformation presented by Natta and Corradini.
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