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Molecular simulations are used to investigate the energetics and siting of linear and branched alkanes in the zeolite silicalite. The calculated heats of adsorption of the branched alkanes are in good agreement with the experimental data. The simulations show a striking di †erence between the behaviour of linear and branched alkanes. The linear alkanes are relatively free to move in all channels of the zeolites. The branched alkanes are trapped with their CH group in the intersection of the zig-zag and straight channels of silicalite. This trapping of the branched alkanes suggests that di †usion of these molecules is an activated process ; most of the time the molecule is located in the intersection but, occasionally, it hops from one intersection to another. The straight and zig-zag channels form a barrier for the di †usion. We present some preliminary calculations of this hopping rate, from which the di †usion coefficient can be calculated. These preliminary results are in reasonable agreement with experimental data.

A truly parallel algorithm for the Gibbs-ensemble simulation technique is given. The key components of this algorithm are a parallel displacement move based on the hybrid Monte Carlo method and a parallel exchange move based on a novel algorithm. For the parallel exchange move each processor generates additional trial conformations, and a trial conformation is selected with the most favourable energy. This introduces a bias, which is removed by a modification of the acceptance rules. A proof of the correctness of the parallel exchange move is given and the algorithm is verified for a Lennard-Jones system. Simulations on ten processors of an SP/2 parallel computer give a speedup of eight for large systems and a speedup of four for systems of typical size.

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