Anisotropy Effect Of Multi-Center Lennard-Jones Molecular Clusters

Random packing problems of rigid geometries have been studied by materials scientists, biologists, chemists, engineers and physicists to understand the structure of living cells, liquids, granular media, glasses, and amorphous solids. A variety of geometries who are packed are paid attention to. In the field of chemistry, packing of polygons, molecule and atom are most favored. LJ clusters are the largest number of studies. With in-depth research, people regarded the Lennard-Jones model as standard and basic, moreover the results obtained under optimized was compared with Lennard-Jones clusters, so as to get the law of accumulation about the research object.When the packing of particles, every particle can be rotated in three-dimensional space. If the particles are not sphere, when it rotated, then the cluster structure has change. So the orientation is a problem in packing of particles, the anisotropy effect is orientation problem of packing of particles.In this paper, the four highly symmetrical molecules are modeled:4-atom tetrahedron (TLJ),6-atom octahedron (OLJ),8-atom cube (CLJ), and 12-atom icosahedron (ILJ). The intermolecular interactions are described using the MCLJ model. Using the funnel hopping algorithm, we located the putative global minimum structures of these four MCLJ molecular clusters up to cluster size N=80.The main contents are as follows:1、Research of TLJ clusters and OLJ clustersThe anisotropy effect has great influence on the packing of the TLJ clusters. The structures of TLJ clusters are most irregular due to the highest anisotropy effect of the tetrahedral molecule, and symmetry is not satisfactory. For the structure of magic number, four typical structures are unique geometry. Compared with methane molecule clusters, we found they are analogous when the sizes between 22 and 26, and they are cigar-like shape.OLJ clusters structures is very interesting. They also have strong anisotropy effects, but compared with TLJ clusters, anisotropy effects is weaker. OLJ clusters have rule of itself due to anisotropic effect. For OLJ clusters, a new structural pattern, rhombic dodecahedron, is viewed. When the cluster size is large, the other global minimum structures are formed of one or several small OLJ clusters. OLJ15, OLJ65 and OLJ175 are new onion-like layered structures, the rule is like LJ13, LJ55 and LJ147 of LJ clusters.2、Research of CLJ clusters and ILJ clustersBecause of anisotropy, CLJ clusters structures is different. For CLJ clusters, a new packing, the face-centered cubic, are favored in energy. CLJ8 is the simplest face--centered cubic. When the cluster sizes N< 19, simple geometries and chaotic geometries are favored more. When N> 19, all of the structure are formed of several CLJ8, and CLJ8 is a unit cell of these structures.For icosahedron molecular clusters, icosahedron and face-centered cubic motifs are most favored. When N< 19, the global minimum structures of ILJ clusters are similar to structures of LJ clusters, the most stable motifs is icosahedron. When N> 19, face-centered cubic packing is favored at ILJ clusters, LJ clusters are different. But ILJ38 and LJ38 are truncated octahedron, which is worth noting. ILJ clusters and C6o clusters also are different. When the cluster size is small, they are the same. When the size is large, the hexagonal close-packed and decahedron configuration which C60 molecule clusters are both favored was not found in the clusters of ILJ.