General Index and Its Application in MD Simulations

In the long-term practice, it is recognized that the properties of materials are not uniquely determined by their average chemical composition but also, to a large extent, influenced by their structures. The impurities and defects in metal will hinder the movement of free electrons and reduce their conduction, therefore, the thermal conductivity of alloy is significantly smaller than that of pure metal. The yield strength, fracture strength, fatigue toughness and other mechanical properties of metal are influenced by defects, such as dislocations, grain boundaries, micro voids and cracks. In weak external magnetic field, due to the existence of spontaneous magnetization within a small area, e.g. magnetic domains, ferromagnet shows strong magnetism. The bonding strength and density of crystalline phases dramatically influence the strength of ceramic. Due to the existence of independent molecules, linear structure (including the branched-chain structure) polymers are flexible, malleable, less hard and brittle, and can be dissolved in a solvent or be heated to melt. However, in three-dimensional polymers, as there are no independent molecules, they are hard and brittle, can swell but cannot be dissolved or melt, and are less flexible. In nematic liquid crystal, the rod-like molecules are arranged parallelly to each other, but their centres of gravity are in disorder. Under external force, molecules can flow easily along the longitudinal direction, and consequently have a considerable mobility. In smectic liquid crystal, the molecules align in a layered structure via lateral interaction of molecule and interaction of functional groups contained by molecules. Two-dimensional layers can slide between each other, but the flow perpendicular to layers is difficult.