This paper provides a rigorous epistemological analysis of the boundaries between quantum chemistry and nuclear physics, focusing on the conceptual "isolation of the nucleus" from its chemical environment. It argues that the computational success of the Born–Oppenheimer approximation has inadvertently led to a rigid ontological commitment, treating the nucleus as effectively independent of surrounding chemical structures. Drawing upon Nancy Cartwright’s account of nomological machines, Quine's criterion of ontological commitment, and anti-reductionist philosophy of chemistry, the author affirms the relative autonomy of chemistry against physical imperialism. Through a historical re-reading of Ida Noddack’s 1934 intervention and the introduction of a novel thought experiment "Topological Nuclear Strain" (TNS) the paper demonstrates how mathematical approximations can morph into epistemological constraints. It challenges linear energy-scale assumptions by exploring how non-linear quantum tunneling could, in principle, be influenced by complex, asymmetric chemical articulation.
This paper provides a rigorous epistemological analysis of the boundaries between quantum chemistry and nuclear physics, focusing on the conceptual "isolation of the nucleus" from its chemical environment. It argues that the computational success of the Born–Oppenheimer approximation has inadvertently led to a rigid ontological commitment, treating the nucleus as effectively independent of surrounding chemical structures. Drawing upon Nancy Cartwright’s account of nomological machines, Quine's criterion of ontological commitment, and anti-reductionist philosophy of chemistry, the author affirms the relative autonomy of chemistry against physical imperialism. Through a historical re-reading of Ida Noddack’s 1934 intervention and the introduction of a novel thought experiment "Topological Nuclear Strain" (TNS) the paper demonstrates how mathematical approximations can morph into epistemological constraints. It challenges linear energy-scale assumptions by exploring how non-linear quantum tunneling could, in principle, be influenced by complex, asymmetric chemical articulation.
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.
Quantum chemistry is the branch of chemistry whose primary focus is the application of quantum mechanics to chemical systems at the molecular level. Precisely due to its peculiar position between chemistry and physics, in the last times it has begun to engage the interest of the philosophers of chemistry. Nevertheless, in this philosophical field, quantum chemistry has been studied mainly from a historical viewpoint or from a perspective interested on methodological issues. By contrast, the question that will guide this article is: what kind of ontic items are those studied by quantum chemistry? In order to develop the argumentation, first the relevance of the ontological questions will be addressed. Then, it will be considered in what measure the Born-Oppenheimer approximation and the quantum-chemistry concept of electron fit in the quantum theoretical context. Finally, some issues about what quantum chemistry refers to will be discussed.
Abstract Chemistry occupies a distinctive explanatory position: quantum mechanics governs electronic behaviour, yet molecular structure is definite, stable, and causally efficacious. This paper argues that the apparent tension between quantum indeterminacy and chemical definiteness dissolves once molecular identity is understood as constraint localised at finite scale. Drawing on the Triaxial Existential Field (TEF) framework, the analysis reinterprets the potential energy surface as R-register constraint operating at molecular scale, molecular identity as C-locus persistence under that constraint, and chemical reactions as constraint reconfiguration rather than mere rearrangement of substance. The Born-Oppenheimer approximation is reinterpreted not as a substance distinction between classical nuclei and quantum electrons but as a register distinction between constraint and participation. Chirality is analysed as topological asymmetry in the constraint landscape, with homochirality identified as a candidate threshold condition for biological admissibility. The paper concludes that chemistry is the regime where constraint first localises at finite scale, providing the template for all subsequent levels of structural organisation and bridging the foundational physics treated elsewhere in the TEF programme with the biological domain addressed in a companion paper.
Abstract Philosophers have claimed that: (a) Born–Oppenheimer approximation methods for solving molecular Schrödinger equations violate the Heisenberg uncertainty relations; therefore, (b) ‘quantum chemistry’ is not fully quantum; and (c) therefore chemistry does not reduce to physics. This paper analyses the reasoning behind Born–Oppenheimer methods and shows that they are internally consistent and fully quantum mechanical, contrary to (a)–(c). Our analysis addresses important issues of mathematical rigour, physical idealization, reduction, and classicality in the quantum theory of molecules, and we propose an agenda for the philosophy of quantum chemistry more grounded in scientific practice.
The accurate computational determination of chemical, materials, biological, and atmospheric properties has a critical impact on a wide range of health and environmental problems, but is deeply limited by the computational scaling of quantum mechanical methods. The complexity of quantum chemical studies arises from the steep algebraic scaling of electron correlation methods and the exponential scaling in studying nuclear dynamics and molecular flexibility. To date, efforts to apply quantum hardware to such quantum chemistry problems have focused primarily on electron correlation. Here, we provide a framework that allows for the solution of quantum chemical nuclear dynamics by mapping these to quantum spin-lattice simulators. Using the example case of a short-strong hydrogen-bonded system, we construct the Hamiltonian for the nuclear degrees of freedom on a single Born-Oppenheimer surface and show how it can be transformed to a generalized Ising model Hamiltonian. We then demonstrate a method to determine the local fields and spin-spin couplings needed to identically match the molecular and spin-lattice Hamiltonians. We describe a protocol to determine the on-site and intersite coupling parameters of this Ising Hamiltonian from the Born-Oppenheimer potential and nuclear kinetic energy operator. Our approach represents a paradigm shift in the methods used to study quantum nuclear dynamics, opening the possibility to solve both electronic structure and nuclear dynamics problems using quantum computing systems.
Chemical dynamics, in principle, should be understood by solving the time-dependent Schrödinger equation for a molecular system, describing motion of the nuclei and electrons. However, the computational efforts to solve this partial second-order differential equation scales exponentially with the system size, which prevents us from getting exact numerical solutions for systems larger than 4-5 atoms. Thus, approximations simplifying the picture are necessary. The so-called Born-Oppenheimer approximation, separating motion of the electrons and nuclei is the central one: solution to the electronic Schrödinger equation defines the potential energy surface on which the nuclear motion unfolds, and there are standard quantum chemistry software packages for solving the electronic Schrödinger equation. For the nuclear Schrödinger equation, however, there are no widely applicable quantum-mechanical approaches, and most simulations are performed using classical Newtonian mechanics which is often adequate due to large nuclear masses. However, the nuclear quantum effects are significant for chemical processes involving light nuclei at low energies, and including these effects into simulation, even approximately, is highly desirable. In this dissertation, an approximate methodology of including quantum-mechanical effects within the quantum trajectory or the de Broglie-Bohm formulation of the Schrödinger equations is developed. Use of the trajectory framework makes the approach scalable to hundreds of degrees of freedom. The methodology is applied to study high-dimensional systems (solid He4 and others) relevant to chemistry.