Physical Interpretation of Koopmans' Theorem: A Criticism of the Current Didactic Presentation

Abstract

Koopmans' theorem is one of the basic theorems learned in the preliminary courses of theoretical quantum chemistry. Its importance is not only related to ionization processes: it also gives the justification to the concept of "orbital energies". This concept is widely used in all qualitative description of electronic structure of atoms and molecules. In spite of such a widespread interest, the current didactic presentation of Koopmans' theorem in some widely used textbooks does not completely clarify the particular role played in its demonstration by the canonical Hartree-Fock orbitals. This step is essential to correctly define the orbital energies. In this paper we present a criticism of current didactic presentation of Koopmans' theorem and we propose a new didactic approach, which seems to us interesting from a pedagogical point of view. This paper is addressed to teachers and students of university courses in theoretical chemistry.