Summary of 7r-electron models

Even though the neglect of the a electronic d3mamics leads to considerable simplifications, the full TT-electron-nuclear Hamiltonian is still too complicated to solve exactly. In this section we introduce various approximations to the complete Hamiltonian that make it more tractable. These models are summarized in Table 2.1. Their physical properties are discussed in more detail in the chapters that follow, as indicated in Table 2.1. 

We also emphasize that even if accurate effective potentials could be derived from first principles, their utility would be limited because of the errors associated with the neglect of the dynamical influences of the a and core electrons. In practice, therefore, semiempirical parameters are often used, which are derived by fitting the predictions of model Hamiltonians to some known experimental results. This is often a very successful procedure. 

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