function. This leaves only O(N) significant Coulomb interactions (how to calculate these interactions in only O(N) work is covered in Chapter 7).
While Coulomb attenuation does have a major effect on the total energy of a system, the wavefunction is largely unaffected. Most relative energies are also reproduced quite well, as long as the number of particles is conserved in the reaction. Correlation energies, being the difference between two integrals, are also largely unaffected, something which may allow inexpensive correlation treatments in the future.
All of these properties can be ascribed to the blandness of the excluded background term, giving it little chemical significance. As well as being computationally useful, the CASE approximation is a comfortable idea for chemists as it affirms the well known idea that the behaviour of an atom within a molecule is principally governed by its immediate vicinity.