The ontological interpretation of the concept of electron density

SEBASTIAN FORTIN; OLIMPIA LOMBARDI

Buenos Aires

Congreso; 26th Meeting of the International Society for the Philosophy of Chemistry ISPC2023; 2023

International Society for the Philosophy of Chemistry

The concept of electron density is central in quantumchemistry. There is no doubt in the discipline about how the electron densityfunction 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 isno generalized agreement. Frequently, the concept is defined as the probabilitydensity of finding an electron at each specific location in space. However, sometimesit is also defined as a measure of the number ofelectrons per unit volume of space. This meaning, in turn, leads to theinterpretation of the concept of electron density as referring to a “cloud” ofnegative charge spread out in physical space. This is the interpretationadopted by Richard Bader and Chérif Matta in the framework of the QuantumTheory of Atoms in Molecules (QTAIM) when they say: “The electronic charge, unlike that of the more massive nuclei, isspread throughout space, and matter is made up of point-like nuclei embedded inthe relatively diffuse spatial distribution of electronic charge. Thedistribution of electronic charge is described by the electron density thatdetermines the amount of negative charge per unit volume.” (Bader and Matta2013: 255). According to this interpretation,the electron density is not longer a probability density, but rather acquiresmateriality: it describes the kind of stuff of which the electronic part of theatoms is made.The aim of the present work is to examinethe stuff-interpretation of the concept of electronic density in the light ofquantum mechanics. This task is especially relevant in the context of atheoretical proposal as QTAIM, which claims to have offered a completereduction of molecular chemistry to quantum mechanics. In particular, it willbe shown in detail how the shift from a probability density to a distributed stuffis alien to orthodox quantum mechanics. Moreover, it will be argued that, ifone insists in adopting the stuff-interpretation of the electronic density in aquantum framework, in the manner of the original Schrödinger’s view, one islead to a many-worlds interpretation of quantum mechanics. The work willconclude with a plea in favor of a specific ontology for quantum chemistry,free from the constraints imposed by quantum mechanics in its differentversions.