Density domain chemical bond

Apparently, the concept of similarity plays an important role in the chemistry of functional groups. Motivated by the recent revival of interest in molecular similarity [7-39], we shall present a systematic approach towards a quantum chemical description of functional groups. There are two main components of the approach described in this report. The first component is shape-similarity, based on the topological shape groups and topological similarity measures of molecular electron densities[2,19-34], whereas the second component is the Density Domain approach to chemical bonding [4]. The topological Density Domain is a natural basis for a quantum... 

The density domain approach was first proposed [4] as a tool for the description of chemical bonding where the complete shape information of the molecular electron density was taken into account. Density domains are formal bodies of electron density clouds enclosed by MIDCOs defined by eq. (1) [or by eq. (2) if there is no need to specify the nuclear configuration K],... 

One of the exceptions, that offers an alternative to the conventional bond diagrams is the density domain approach [4,5] to chemical bonding. This approach is based on the following observation for a given molecule with a specified nuclear configuration K, the infinite family (DD(a,K) of density domains for the range (0, amax] of density thresholds,... 

The shapes of these density domains are characteristic to the set of nuclei enclosed by them, to the nuclear geometry, and also to the location of these density domains within the molecule, collectively represented by the configuration variable K, as well as to the actual density threshold a. The sequence of density domains as a function of density threshold a, augmented with the results of a local shape analysis of these density domains [2], provides a detailed description of chemical bonding within the methane molecule. 

The Density Domain Approach (DDA) to chemical bonding has been proposed [109] as a tool that is able to describe the global properties as well as the fine details of the full, three-dimensional bonding pattern within molecular bodies. 

The Density Domain Approach to chemical bonding is based on the topological analysis of the dominant shape variations of the molecular body DD(a) [or, equivalently, those of the G(a) contour surface] regarded as a function of the density parameter a. 

One may summarize the essence of the approach as follows. The Density Domain Approach is a 3D topological tool for a comprehensive description of chemical bonding [ 09], By decreasing the contour parameter a from high values to zero, the various density domains DDj(a), that is, the parts of the body DD(a) become connected. The isodensity surface threshold values aj at which such connections occur are characteristic to the given configuration of the nuclei and to the electronic state. In actual computations, if the density domains are calculated by some ab initio technique, then the calculated aj values and the associated shapes of... 

Figure 2.2 Selected families (DDj(aj)) of density domains of the water molecule, as calculated with the GAUSSIAN 90 ab initio program [253] and the GSHAPE 90 molecular shape analysis program [254], using a 6-3IG basis set. There are only two topologically different types of families of density domains either a single density domain, or a family of three density domains. The sequence of topologically distinct cases provides a topological description of chemical bonding.

According to a recent proposal [4,8,9], chemical bonding within formal molecular bodies can be described by molecular isodensity contour (MIDCO) surfaces and by density domains (DD) that are the formal bodies enclosed by... 

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