Ab initio studies on polymers. VI. Torsional potential in regular polyethylene chains

Ab initio crystal orbital calculations have been performed on regular polyethylene chains applying basis sets of minimal and double‐zeta quality. Relative stabilities of periodic all‐trans, all‐gauche, and alternating trans–gauche conformers have been evaluated, including extensive geometry optimization. Potential curves for a simultaneous rotation around CC single bonds from the all‐trans to the all‐gauche conformation have been computed applying the rigid‐rotor approximation, the flexible‐rotor approximation, and an additional reoptimization of CC distances. A rigid‐rotor potential curve from the all‐trans to the alternating trans‐gauche conformation has been computed as well. Results obtained are compared with ab initio calculations on butane and pentane and with semiempirical and empirical force‐field studies on polyethylene.