Generalized multi-structural wavefunction

The main focus in this chapter is on two themes On the one hand, the discussion aims at presenting, without many formulae, certain critical characteristics and properties of unstable states in general, and, in particular, of states that are normally represented by multi-particle wavefunctions, such as the ones that are found in the spectra of atoms and small molecules. On the other hand, the discussion aims at demonstrating why and how it is possible to tackle, from first principles and from a many-electron point of view, a large number of problems that have to do with field-free and field-induced unstable states, for which reliable electronic structure calculations are, in principle, feasible. 

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