ABSTRACT
The separation of σ and π energies represents a fundamental concept in chemistry (see, e.g., [1,2] and references therein). It can be traced back to the very early years of quantum chemistry [3]. Although the arbitrary nature of such a separation is well recognized, it has proven extremely useful for the rationalization of many experimental observations, such as concerted reactions [4,5] or π back-donations [6]. In a recent review [1], several methods for all-electron σ – π energy separations are discussed. A fundamental problem for the direct separation of σ and π energies represents the nuclear repulsion term. Usually, this term is incorporated into the σ energy term. However, such a partitioning divides the systems into charged fragments. In this case, the σ energy term includes all nuclear charges but only a reduced number of electrons, namely the σ electrons. On the contrary, the π energy term is free of the nuclear repulsion and, therefore, possesses a negative charge corresponding to the number of π electrons. It is obvious that this separation is unbalanced and cannot be supported by reasonable physical arguments [1].
