Can Quantum Mechanics Account for Chemical Structures?

O. Tapia

Brian T. Sutcliffe, R. Guy Woolley

Transition state theory was introduced in 1930s to account for chemical reactions. Central to this theory is the idea of a potential energy surface (PES). It was assumed that such a surface could be constructed using eigensolutions of the Schrödinger equation for the molecular (Coulomb) Hamiltonian but at that time such calculations were not possible. Nowadays quantum mechanical ab initio electronic structure calculations are routine and from their results PESs can be constructed which are believed to approximate those assumed derivable from the eigensolutions. It is argued here that this belief is unfounded. It is suggested that the potential energy surface construction is more appropriately regarded as a legitimate and effective modification of quantum mechanics for chemical purposes.

Thomas Freeman Babson

Jesus Alberto Jaimes-Arriaga

Abstract The Quantum Theory of Atoms in Molecules (QTAIM), developed by Richard F. W. Bader, takes the electron density as its foundational quantity and derives from it a quantum-mechanical account of atoms, bonds, and molecular structure. Bader's characterization of electron density exhibits a persistent philosophical tension: his discourse simultaneously presents it as an observable property — a measurable quantity accessible through diffraction experiments — and as the material stuff of which atoms and molecules are constituted. Since property and stuff are mutually exclusive ontological categories, no entity can occupy both at once, yet Bader's framework requires precisely this. This paper argues that the tension cannot be resolved within either dominant position in the metaphysics of properties: categoricalism captures the intrinsic, geometrically representable dimension of electron density but cannot account for its constitutive causal role, while pandispositionalism captures the observable, causal dimension but cannot account for its intrinsic topological structure. The paper argues electron density should be conceived as a powerful quality in Ingthorsson’s sense — a property simultaneously and irreducibly both qualitative and dispositional. Its qualitative aspect is approximated by the objective topology of the electron density distribution, distinguished from the chemical reading of that topology as an interpretive layer dependent on structural chemistry. Its dispositional aspect is expressed through two heterogeneous but ontologically unified manifestations: observability in diffraction experiments and chemical reactivity underwritten by the Laplacian's complementarity principle. The paper draws out implications for the powerful quality framework, the reducibility of chemistry to physics, and scientific realism about quantum-chemical entities.

Seogjoo Jang

Ryan Michael Miller

Abstract The question of whether chemical structure is reducible to Everettian Quantum Mechanics (EQM) should be of interest to philosophers of chemistry and philosophers of physics alike. Among the three realist interpretations of quantum mechanics, EQM resolves the measurement problem by claiming that measurements (now interpreted as instances of decoherence) have indeterminate outcomes absolutely speaking, but determinate outcomes relative to emergent worlds—Maudlin (Topoi, 14:7-15, 1995). Philosophers who wish to be sensitive to the practice of quantum chemistry e.g. Scerri (The changing views of a philosopher of chemistry on the question of reduction, 2016) should be interested in EQM because Franklin and Seifert (J. Philos. Sci, 2020) claim that resolving the measurement problem also resolves the reducibility of chemical structure, and EQM is the interpretation which involves no mathematical structure beyond that used by practicing scientists. Philosophers interested in the quantum interpretation debate should be interested in the reducibility of chemistry because chemical structure is precisely the kind of determinate three-dimensional fact which EQM should be able to ground if it is to be empirically coherent—see Allori (Quantum Rep, 5:80-101, 2023). The prospects for reduction of chemical structure are poor if it cannot succeed in EQM; the prospects for EQM as a guide to ontology are poor if it cannot reduce chemical structure. Unfortunately for proponents of chemical reduction and EQM, there are three serious barriers to the reduction of chemistry to EQM. The first concern is that quantum treatments of chemical structure rely on the Born-Oppenheimer approximation, which holds nuclear locations fixed while minimizing the energy of the electronic configuration—Hendry (Philosophical Perspectives in Quantum Chemistry, 147-172, 2022), but this approximation is not licensed by EQM. The Born-Oppenheimer approximation relies on nuclei and molecular orbitals being simultaneously present, but in the three-dimensional ontology following from the Everett interpretation these only emerge at different energy scales and are not simultaneously present—Miller (Found. Chem, 25:405-417, 2023). The second concern is that the emergent worlds of EQM are supposed to be decoherent at the macro-scale—Wilson (The Nature of Contingency: Quantum Physics as Modal Realism 2020), but the recent development of superchemistry suggests that chemical reactions can occur in coherent states—Zhang et al. (Nat. Phys, 1-5, 2023). The third concern is that emergent worlds are only pragmatic pseudo-processes—Wallace (The Emergent Multiverse: Quantum Theory According to the Everett Interpretation, 2012b), but this means EQM trades realist physics for mere instrumentalism about chemistry. Absent a commitment to chemical realism, reduction is an empty promise. The prospects for reduction of chemical structure to EQM are therefore poor.