  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.
