Quantum chemical shape: new density domain relations for the topology of molecular bodies, functional groups, and chemical bonding

A density domain (DD) is the formal body enclosed by a molecular isodensity contour (MIDCO) surface. Individual nuclear neighborhoods and various formal molecular fragments can be regarded as fuzzy moieties of electron densities, dominated by one or several nuclei. Such a fuzzy fragment involves a whole range of density values, hence it cannot be described by a single MIDCO, but it can be represented by a sequence of density domains. Within the chemically important range of density values, there are only a finite number of topologically different bodies of density domains. In the Density Domain Approach, chemical bonding is described by the interfacing and mutual interpenetration of local fuzzy charge density clouds. The bonding between fragments of a molecule is characterized by a finite sequence of density domains within a wide range of density values and by the correponding sequence of topological patterns of the mutual interpenetration of these fragments. In earlier works, the DD approach was advocated as an alternative to the conventional "skeletal model" of chemical bonding. The classically motivated line diagrams as representatives of bonding are replaced by the pattern of interpenetration of fuzzy fragment bodies at various density thresholds. In this study, novel DD relations are described, suitable for a quantum chemical characterization of functional groups, the local shape properties of such groups, and their contributions to global molecular shape.