Dimerization and optical gaps

Comparing this with eqn (2.40) we identify 2aW with / . The other component represents the changes in the nearest neighbour electron-electron interaction from the change in bond length, 

The first term on the right-hand side of eqn (7.1) is just the electron-phonon coupling arising from the change in the kinetic energy, which is the first term in 

As described in Chapter 4, by using the Hellmann-Feynman theorem we can derive a self-consistent equation for A for any state, 

We note that constant chain lengths, implying that A = 0, means that 

In this section we describe how electron-electron interactions modify the noninteracting predictions both for the bond alternation of the groimd state and the optical gap. This subject has been studied by a number of authors (Ukranskii 1978 Horsch 1981 Hirsch 1983 Mazumdar and Dixit 1983 Baeriswyl and Maki 1985 Konig and Stollhoff 1990 Baeriswyl et al. 1992). 

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