Translational and rotational disorder in solid n-alkanes: Constant temperature–constant pressure molecular dynamics calculations using infinitely long flexible chains

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Abstract
Molecular dynamics calculations are used to study the effect of temperature on the interchain packing in solid n‐alkanes. The model used consists of infinite chains initially arranged in a centered orthorhombic structure with the lateral packing found in the low temperature phase of n‐alkanes with an odd number of carbon atoms. An atom–atom interchain potential is employed and the chains have flexible backbones. The problem of equilibrating the inter‐ and intramolecular motions is overcome using the technique of massive stochastic collisions proposed by Andersen. The calculated isobaric lateral thermal expansion of the orthohombic a and b cell parameters is in excellent agreement with experimental data over a wide temperature range. For T> 250 K translational (jump) diffusion is observed along the chain axis (c direction) without any accompanying rigid body chain rotation. At higher temperatures, the diffusion becomes liquid‐ like and rotational diffusion sets in between four well defined sites. Possible implications of our results for understanding of real alkanes are explored.

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