Section 2: Electronic States of Atoms and Molecules

  During the chemical reactions, the change of the electronic states, i.e, the change of electronic degeneracy causes one of the most essential change in the statistical weight.   For example, the combination reaction of two ground-state (⁴S state) nitrogen atoms to form ground-state (¹Σ_g⁺) N₂ molecule involves the change of the electronic degeneracy from 16 (=4×4) to 1.   For the practical statistical calculations, at least, the electronic degeneracy must be properly distinguished from the term designation (symbol) of the electronic states.   This section provides a concise summary to do it.
  Needless to say, better understanding on the electronic states will be needed in many cases, e.g., for quantum chemcal electronic structure calculations.

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Lecture note