Overlap integral Pauli exclusion principle

The symbol 0(5 ) stands for all the terms that vanish with the fourth power of the overlap integrals or faster. The valence repulsion already appears in the first order of the perturbation theory (besides the electrostatic energy as a result of the Pauli exclusion principle. ... 

The orbital phase continuity conditions stem from the intrinsic property of electrons. Electrons are fermions, and are described by wavefnnctions antisymmetric (change plus and minus signs) with respect to an interchange of the coordinates of an pair of particles. The antisymmetry principle is a more fnndamental principle than Pauli s exclusion principle. Slater determinants are antisymmetric, which is why the overlap integral between t(a c) given above has a negative... 

Curve (a) in Fig. 2.11 gives an example for the form of u r) which results from the superposition of the repulsive hard core interaction, originating from Pauli s exclusion principle opposing an overlap of electron wave functions, and the attractive dispersive forces of the van-der-Waals type. At the Boyle-temperature this integral vanishes. 

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