Calculating Hirshfeld-I charges in solids: implementation, pitfalls and applications

The concept of “Atoms in molecules” (AIM) is one of the most successful concepts in chemistry. It states that the properties of a molecule can be considered as simple sums of the properties of its constituent atoms. Although a precise definition of an AIM remains elusive, an impressive amount of insight has, over the years, been gained through its usage. The central question in the AIM concept is how one should divide the electrons of the system over its constituent “atoms”. Such a division can be done either in Hilbert-space (e.g. Mulliken population analysis) or in real space (e.g. Bader’s QTAIM). Over the years, many different partitioning schemes have been developed and have been employed in the study of molecules. In recent years, the solid state community has started to show an increasing interest in using the AIM concept for the investigation of solids. Bader’s QTAIM approach has received much interest in this context, since this approach is well suited for the close packed nature of solids. Recently, also the Hirshfeld and Hirshfeld-I (HI)1 partitioning schemes have been implemented for solids. In this scheme the AIM do not have a sharp boundary, as in the QTAIM case, instead they have a diffuse boundary allowing for overlap between neighboring AIM’s.