CONICET | Buscador de Institutos y Recursos Humanos

The intertheoretical relationships between quantum mechanics and chemistry have receive careful attention in the history and the philosophy of chemistry of the last decades (cfr., for example, Gavroglu and Simoes 1994, Park 2000, Scerri 2006). As part of this debate, there is broad agreement on the impossibility of epistemological reduction, and ontological matters have begun to gain ground in recent years (cfr. for instance, Labarca and Lombardi 2005). Despite this fact, there is still much to be said about the ontology of quantum chemistry. In order to advance in this area, the aim of this work is to analyze the ontological relationships between molecular chemistry and quantum mechanics, considering for this purpose two theories of chemical bonding: valence bond (VB) and molecular orbital (MO).These two quantum-chemical theories of chemical bonding are methods for approximately solving the Schrödinger equation for molecular systems. Each method involves a particular algorithm to generate the wave function, and the conceptual justification for choosing this algorithm is based on qualitative assumptions about the form that the molecular wave function must have. For VB, the molecule is composed of its constituent atoms, in which electrons have a specific localized position and are associated with a particular nucleus; the wave function of the molecule should reflect this feature through a linear combination of the atomic wave functions corresponding to the atoms within the molecule. On the other hand, MO conceives a molecular system as a whole, where electrons do not have specific locations around a particular nucleus; thus, electrons are delocalized throughout the molecule, in such a way that the molecular wave function associates electrons with more than one nucleus.The purpose of this work is to show that, unlike a widespread assumption, quantum-chemical theories of chemical bonding refer neither to the ontology of molecular chemistry nor the ontology of quantum mechanics. We will argue that they can be understood as a certain kind of model, which we will call "bridge-models" that connect the two ontologies without describing, strictly speaking, any of them. In addition, the model that most closely matches the chemical ontology is farther away from the quantum ontology, while the model that best satisfies the constraints of the quantum world loses specifically chemical features.

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