Spheroid Model for Molecular Packing in Crystalline Phase

Abstract

Dense packing of particles has provided important models to study the structure of matter in various systems such as liquid, glassy and crystalline phase, etc. The simplest sphere packing models are able to represent and capture salient properties of the building blocks for covalent, metallic and ionic crystals; it however becomes insufficient to reflect the broken symmetry of the commonly anisotropic molecules in complex molecular crystals. Here we develop spheroid models with the minimal degree of anisotropy, which serve as a simple geometrical representation for a rich spectrum of molecules--including both isotropic and anisotropic, convex and concave ones--in crystalline phases. Our models are determined via an inverse packing approach: given a molecular crystal, an optimal spheroid model is constructed using a contact diagram, which depicts packing relationship between neighboring molecules within the crystal. The spheroid models are capable of accurately capturing the broken symmetry and characterizing the equivalent volume of molecules in the crystalline phases. Our model also allows to retrieve such molecular information from poor-quality crystal X-ray diffraction data that otherwise would be simply discarded.