Single atomic fragment

A single-atom fragment can only be (1) an isolating carbon atom, or (2) a hydrogen or hetero atom, all of whose bonds are isolating carbons. For example ... 

Single atomic fragments-, e.g., an isolating carbon, namely, a carbon atom not doubly or triply bonded to a heteroatom, or a hydrogen or heteroatom connected to an isolating carbon. 

Multiatomic derived fragments any combination of single atomic fragments or multiatomic fundamental fragments... 

Most 1,3-diazepine syntheses employ the insertion of a single carbon fragment between the two nitrogen atoms. This is further exemplified by the use of tosyl isocyanate and methyl isothiocyanate in the preparation of the bicyclic imidazo[4,5-e][l,3]diazepines 46 and the imidazo[4,5-rf][l,3]diazepines 47 respectively <96JHC855, 96JCS(P1)2257>. 

Atom-based Methods As for fragmental methods, the molecule is considered as composed of fragments, but these latter are single atoms instead of functional groups. Each atom is characterized by its own lipophilicity constant and the global lipophilicity is then a sum of each contribution (Equation 5.4). 

The flexibility of the valence bond self-consistent field (VBSCF) method can be exploited to calculate VB wave functions based on orbitals that are purely localized on a single atom or fragment. In such a case, the VBSCF wave function takes a classical VB form, which has the advantage of giving a very detailed description of an electronic system, as, for example, the interplay between the various covalent and ionic structures in a reaction. On the other hand, since covalent and ionic structures have to be explicitly considered for... 

An important feature of the BOVB method is that the active orbitals are chosen to be strictly localized on a single atom or fragment, without any delocalization tails. If this were not the case, a so-called "covalent" structure, defined with more or less delocalized orbitals like, e.g., Coulson-Fischer orbitals, would implicitly contain some ionic contributions, which would make the interpretation of the wave function questionable [27]. The use of pure AOs is therefore a way to ensure an unambiguous correspondence between the concept of Lewis structural scheme and its mathematical formulation. Another reason for the choice of local orbitals is that the breathing orbital effect is... 

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