The ground-state structures of 13-molecule clusters of carbon dioxide, methane, benzene, cyclohexane, and naphthalene have been calculated by potential energy minimization, employing -12-6-1 nonbonded atom–atom potential functions. In addition, seven-molecule clusters were calculated for carbon dioxide and benzene. In all cases, noncrystallographic pentagonal motifs were found to stabilize the structures. The coordination polyhedra of the 13-molecule clusters are close to regular icosahedra, much the same as has been reported earlier for noble-gas clusters. The factors, determining the ability of the clusters to adopt this conformation, are discussed.

The structures of small benzene clusters (C,H,),. n = 2-7, have been calculated employing potential-energy minimization with respect to molecular translational and rotational coordinates, using exp-6-l non-bonded atom-atom potential functions. The influence of the adopted point-charge model is discussed for the dimer structure.

Different structural models for solid argon clusters with more than 500 atoms, that have been proposed previously on the basis of energy considerations, but whose origin and growth histories are not well understood, are compared with other models that have been devised mainly to overcome these difficulties, at the expense, however, of a smaller binding energy. Diffraction functions have been calculated for a variety of structures and sizes, and are compared with the observed electron-diffraction patterns obtained by Farges et al. [Adv. Chem. Phys. 70, 45 (1988)]. Since the nonsplitting of the first peak in these patterns up to N∼3000 suggests the absence of the— energetically most favorable— decahedra, and other features reflect an increasing fraction of clusters with— at that size unfavorable— bulklike fcc structure, it seems appropriate to relax the energy criterion and to consider other properties as well, notably the rate of growth. Defect-fcc clusters, obtained from perfect fcc crystallites by introducing at least two crossing stacking faults, derive this property from a modified surface structure, involving immobile, nonvanishing, stacking fault resisting steps. Calculated diffraction functions compare favorably with observed patterns for N∼600 and N∼3000. A growth mechanism is proposed which can explain the disappearance of multishell icosahedra and the appearance of (defect-) fcc clusters at N∼500, with no structural transition involved.

The structure of the Lennard-Jones solid, obtained by molecular-dynamics simulation of crystallization in the supercooled liquid, may be fee, although the hep structure is energetically more favorable. This could derive from the cubic symmetry of the fee lattice, allowing lattice defects that are not possible in the hep arrangement, but are essential to crystal growth in the simulated liquid. Two crossing stacking faults in a small fee crystallite can produce nonvanishing, growth-promoting, but stacking-faultresisting, surface steps. PACS numbers: 61.20.Ja, 36.40. +d, 61.50.Cj It is well known that the Lennard-Jones (LJ) potential favors the hep structure over the fee structure for the solid [1]. The difference in cohesive energy (^0.01%) appears to be too small, however, to provide a basis for an explanation of an observed preference for one of the two structures in molecular-dynamics (MD) simulations of crystallization in the supercooled LJ liquid. In fact, until very recently, such a preference has never been found [2][3][4][5][6][7], suggesting the inadequacy of the LJ potential to model the interatomic interactions in a simulation of either fee or hep crystal growth.It is the purpose of this Letter to investigate the role that lattice defects may play in the simulated crystallization process, and, in particular, to demonstrate that growth-stimulating defects are much more probable to occur in fee crystallites than in hep crystallites. Moreover, it will be shown that such defects exclusively stimulate fee growth, without further assumptions regarding the interatomic potential other than that it is isotropic and short ranged.That growth characteristics can be decisive in the choice of crystal structure of a substance, rather than a difference in cohesive energy, can be illustrated by a comparison with the method of static lattice energy calculations, aimed at structure prediction. Here, the evaluation and subsequent minimization of lattice sums involves inclusion of all interactions between a representative central atom (the reference atom) with all its close and more distant neighbors within a limiting sphere. However, the reference atom has been incorporated in the crystal lattice under completely different conditions, notably in the absence of at least half of its ultimate neighbors. Moreover, according to accepted theories of crystal growth [8], the motions of surface-migrating atoms are governed by short-range forces and are rather insensitive to the detailed shape of the potential. Trapping sites that are (almost) equally favorable (but, possibly, not equivalent, as on close-packed faces of fee or hep crystals) will have equal a priori occupation probability. The possibility of complete layers of atoms shifting to "better" positions in response to the arrival of new neighbors can be ruled out.Both the fee and the hep structures consist of plane hexagonal arrays of atoms that are stacked in an orderly way, with atoms in one layer over three-coordinated sites in the preceding layer. Consequently, each l...

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