11 - Molecular Crystals

Abstract

This chapter focuses on molecular crystals, reviews the contrast between molecular and ionic solids, and discusses the thermal effects and implications for target accuracy. Structures of ionic solids may be understood in terms of the packing of spheres, whereas molecular crystals result from the packing of diverse shapes. Central to the modeling of molecular crystal structures is the assumption that the forces between the molecules are very much weaker than the forces acting within the molecule. Thus the intermolecular forces do not change the molecular structure significantly. Many methods of predicting the structures of isolated molecules have been developed in recent years which are capable of identifying which aspects of the molecular structure (usually torsion angles) can change significantly within a population of gas phase molecules at finite temperature or on crystallization. Ab initio methods can determine the structure of most small polyatomics extremely accurately, and are quite reliable for most moderately sized molecules. The chapter also discusses the systematic search methods which are particularly appropriate for molecular crystals and which are becoming practicable because of the increase in computer power. All the methods discussed are designed for organic molecules and are only capable of generating the most common packing symmetries; moreover, they only consider structures where there is only one independent molecule in the unit cell and only certain space groups.