Ignoring it or Pretending to do so

This strategy, being rather radical in spirit, simply sets the last entry of Equation (2.5) to zero and therefore has the ideal consequence that each of the N electrons moves completely independently of each other thus, the Hamiltonian nicely simplifies to a sum of N one-electron Hamiltonians hi as in 

This will lead to a set of N orbitals tpi, and the total wave function Y is then constructed as a simple product of these one-electron wave functions. 

Although the simplification is rather drastic, it may still yield a qualitatively instructive picture for the solid state, the famous Sommerfeld model of the free electrons [53] with its many useful conclusions belongs to this class of theories. Quantitatively, however, a Hamilton operator as given in Equation (2.68) does not lead anywhere because it is far too primitive for chemical questions, which can be easily illustrated for, say, the caesium atom. The above hi obviously means that the 6s valence electron moves in a nuclear potential generated by 55 protons but this very electron does not sense the remaining 54 core-like electrons which (should) lie deeper in energy, which must be quantitatively incorrect. 

The most influential parametrization is probably given by extended Hiickel (EH) theory [54], a very useful semiempirical method which has found wide- 

21) A general approach using the same idea is covered in Section 2.15.2. 

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