Stretching, of bonds

The next step is to assume particular functional forms for the various terms. These can be extremely elaborate, but most are usually based on a simple, chemically intuitive model of the interactions, e.g. stretching of bonds, changes in bond and torsion angles, Coulomb forces and van der Waals intermolecular interactions. Thus, for a molecular solid comprised of discrete molecules, we might well use... 

Off-diagonal elastic constants for stretching of bonds incident to the central... 

Forces involving two atoms these include stretching of bonds, interactions between atoms bound to a common atom (Urey-Bradley forces), and the common non-bonded interactions (van der Waals-London forces). 

The deformation of elastic solids occurs because of the stretching of intermo-lecular bonds to a point where internal stresses balance the externally applied stress (11,13). At this point, an equilibrium deformation is established. As there is little motion involved in the stretching of bonds, this occurs rapidly, and the equilibrium deformation is established infinitesimally. Deviations from ideal identity occur whenever the elastic limit of the solid material is exceeded and irreversible sample deformation results, i.e., breakage of chemical bonds (2,14). Irreversible sample deformation leading to fracture forms the basis of tensile testing (11). 

This implies that the stretching of bonds between adjacent atoms or the distortion of the valency angles are the only important factors. It neglects the consideration that a stretched bond probably has a different bending constant and that a number of other mutual influences are at work. 

The physical origin of energies of activation has to be discussed individually in the case of every situation. In general, however, it can be stated that when the reactants arrive at the interface for reaction, most of them are not in an energetic state appropriate to electron transfer. Such a state, which involves rearrangement of the atoms within the molecule, and perhaps some stretching of bonds, is only attained if one waits a certain time (of the order of magnitude of s) while some of the... 

When a "smair atom such as O is highly coordinated by large atoms such as the more electropositive metals, the effect of non-bonded repulsions can be very severe (McGuire O Keeffe, 1984) and it is natural to ascribe the effect to cation-cation non-bonded repulsions. The stretching of bonds due to this effect is also mirrored in anomalously low bond energies (O Keeffe Hyde, 1984). A particularly revealing example is that of the A-type rare earth oxide structure exemplified by LaaOs -viiLa2 (1) 0(2)2. Here the apparent valences from bond valence sums are ... 

In principle, the force field contains terms not only for stretching of bonds, and bending of bonds, but also for simultaneously stretching and bending bonds and angles (a... 

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