Quantization theoretical background

The study of electron correlation, in terms of reduced density matrices (RDM s) may be approached from many different points of view. My first purpose here is to connect the main relations involving RDM s with some concepts appearing in the theory developed by Prof. Ede Kapuy [1-5]. Thus, after an introductory theoretical background (Sect, two), I look again at the concepts of independence and of strong orthogonality within an extended second quantization framework. 

In this first Appendix we consider the quantum field theoretical description of noninteracting spin-1/2 particles. In particular, we sununarize the quantization procedure, emphasizing the close relation between the minimum principle for the ground state energy and the renormalization scheme. At the same time this Appendix provides the background for the field theoretical treatment of the KS system, i.e. Eqs.(50)-(66). 

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