Using Computational Quantum Chemistry as a Tool to Understand the Structure of Molecular Crystals and the Nature of their Intermolecular Interactions

Abstract

The linear increase in the performance of computers that has taken place year-after-year during the last five decades, nowadays makes possible the accurate computation of the strength of all the symmetry-unique intermolecular interactions present in a molecular crystal in a reasonable amount of time (both human and CPU time). This possibility opens the door to the rationalization of the structure of molecular crystals based on solid quantitative energetic considerations, that is, from the analysis of the strengths of the symmetry-unique intermolecular interactions, which in our procedure are evaluated using a quantum chemical method. The selection of a proper model, computational method and monoelectronic basis set capable of accurately describing all the intermolecular interactions present in that crystal requires of a basic knowledge about these interactions. Therefore, we start this chapter by describing the state-of-the-art regarding the properties of van der Waals and hydrogen bonded interactions. This part will be followed by a description of the computational methods and basis sets most commonly employed in the study of intermolecular interactions. This methodological section will be ended by presenting and discussing a few relevant considerations about how to select a model system that reproduces the environment of the intermolecular interaction in the crystal. Finally, two examples illustrating how to carry out crystal packing analysis will be described in detail on two illustrative cases, a neutral molecular crystal, and an ionic molecular crystal.