The ontological interpretation of the concept of electron density

S. FORTIN; O. LOMBARDI

Buenos Aires

Simposio; International Society for the Philosophy of Chemistry Summer Symposium 2023 (ISPC 2023); 2023

International Society for the Philosophy of Chemistry

The concept of electron density is central in quantum chemistry. There is no doubt in the discipline about how the electron density function is computed in terms of the electronic wave function of a molecule. However, when the issue at stake is the interpretation of the concept, there is no generalized agreement. Frequently, the concept is defined as the probability density of finding an electron at each specific location in space. However, sometimes it is also defined as a measure of the number of electrons per unit volume of space. This meaning, in turn, leads to the interpretation of the concept of electron density as referring to a “cloud” of negative charge spread out in physical space. This is the interpretation adopted by Richard Bader and Chérif Matta in the framework of the Quantum Theory of Atoms in Molecules (QTAIM) when they say: “The electronic charge, unlike that of the more massive nuclei, is spread throughout space, and matter is made up of point-like nuclei embedded in the relatively diffuse spatial distribution of electronic charge. The distribution of electronic charge is described by the electron density that determines the amount of negative charge per unit volume.” (Bader and Matta 2013: 255). According to this interpretation, the electron density is not longer a probability density, but rather acquires materiality: it describes the kind of stuff of which the electronic part of the atoms is made.The aim of the present work is to examine the stuff-interpretation of the concept of electronic density in the light of quantum mechanics. This task is especially relevant in the context of a theoretical proposal as QTAIM, which claims to have offered a complete reduction of molecular chemistry to quantum mechanics. In particular, it will be shown in detail how the shift from a probability density to a distributed stuff is alien to orthodox quantum mechanics. Moreover, it will be argued that, if one insists in adopting the stuff-interpretation of the electronic density in a quantum framework, in the manner of the original Schrödinger’s view, one is lead to a many-worlds interpretation of quantum mechanics. The work will conclude with a plea in favor of a specific ontology for quantum chemistry, free from the constraints imposed by quantum mechanics in its different versions.