Solid state applications of QTAIM and the Source Function: Molecular Crystals, Surfaces, Host-Guest Systems and Molecular Complexes

This chapter deals with the application of QTAIM to the solid state, except for the last section, in which the source function - a recently developed tool for the QTAIM study of the chemical bond from a somewhat original viewpoint - is introduced. The chapter starts with an illustration of TOPOND software, which implements QTAIM for systems periodic in 0 to 3 dimensions, which covers polymers, surfaces, and crystals, besides molecules. The interface of TOPOND to the multipolar package XD is also mentioned, because it enables QTAIM analysis of experimentally derived electron densities. The chapter continues with an example of a didactic application of TOPOND to a study of crystal field effects on bonding and on molecular properties in the urea molecular crystal. Clean and chemisorbed semiconductor surfaces then serve as an example of the wealth of information provided by QTAIM about the effect of surface formation and reconstruction on the bonding and atomic properties of first surface layers. Guest-host systems are discussed as a last example, with emphasis on guest to/from host electron transfer and on the peculiar features of guest-host bonding interactions. The relevance of these key issues to materials science applications is briefly touched upon for thermoelectric materials. In the last section, the source function (SF) is introduced and examples of its preliminary and future potential applications and extensions are presented. This function enables the value of the density at any point within a system to be equated to a sum of atomic contributions, thus enabling the properties of the density to be viewed from a totally new perspective. Although depending on the whole set of interactions within a system, a bond path is topologically associated only with the two atoms it connects. In contrast, the source function details how all the other atoms in a system, in addition to the two linked atoms, contribute to the accumulation of electron density along a bond path and, in particular, to BCP. It thus discloses nonlocal information on bonding and on complex bonding patterns, analogously to the QTAIM delocalization index or the synaptic order of an ELF valence basin. One advantage of the SF over these two powerful interpretive tools is that it is directly amenable to experimental determination, since to evaluate it only the knowledge of the system's electron density and Laplacian is required.