Structural trends within the sp-valent elements

We have seen in the previous chapter that the total binding energy per atom of an elemental sp-valent system may be written within the approximation as the sum of three terms, namely 

The energy difference between two structure types is then given by the structural energy difference theorem as 

Further, assuming that the repulsive pair potential varies as the square of the bond integrals, as in eqn (8.4), we have 

6 The number of different contributions to the fourth moment about a given atom on a Bethe lattice with local coordination,.  

The relative stability of structural types with even-membered rings can often be inferred from the dimensionless shape parameters, s, which from eqn (4.58) is given by 

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