By taking molecules and atoms as models for wholes and parts, I explore the relation between the atomic and molecular properties in this article. Molecular properties do not reduce to atomic properties in the sense the former cannot be obtained by adding or multiplying the latter. However, quantum states of molecules are obtained from the quantum states of atoms by addition and multiplication. Because molecular properties are calculated reliably from the atomic states in quantum theory, it may be claimed that property reduction has been accomplished successfully. I show that such reduction entails underlying holism.
By taking molecules and atoms as models for wholes and parts, I explore the relation between the atomic and molecular properties in this article. Molecular properties do not reduce to atomic properties in the sense the former cannot be obtained by adding or multiplying the latter. However, quantum states of molecules are obtained from the quantum states of atoms by addition and multiplication. Because molecular properties are calculated reliably from the atomic states in quantum theory, it may be claimed that property reduction has been accomplished successfully. I show that such reduction entails underlying holism.
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.
In quantum mechanics the properties of a composite system can be divided into properties which are reducible to the properties of the subsystems of the composite and properties which are irreducible to those subsystem properties. The irreducible properties are called holistic and emerge when one describes the system as a whole. In this chapter I show that the bi and spectral modal interpretations can ascribe the holistic properties to composites but, in general, fail to reproduce the relations between the reducible properties of a composite and the properties of the subsystems. The atomic modal interpretation, on the other hand, fails to ascribe the holistic properties but is much more successful in reproducing the relations between the reducible properties and the subsystem properties. I discuss whether these failures harm the empirical adequacy and the metaphysical tenability of modal interpretations. Holistic properties of composite systems The basic principle of reductionism, that all the properties of composite systems are amalgamates of the properties of the subsystems of these composites, has triumphed so much in classical physics that I find it difficult to come up with a clear example of a classical property that cannot be taken as a complex of the subsystem properties. So, in order to make a case for the idea that composites can also possess, in addition to reducible properties, so-called holistic properties which cannot be reduced to the properties of the subsystems, one has to move outside physics or to draw upon some counterfactual development of physics.
Whether or not quantum physics can account for molecular structure is a matter of considerable controversy. Three of the problems raised in this regard are the problems of molecular structure. We argue that these problems are just special cases of the measurement problem of quantum mechanics: insofar as the measurement problem is solved, the problems of molecular structure are resolved as well. In addition, we explore one consequence of our argument: that claims about the reduction or emergence of molecular structure cannot be settled independently of the choice of a particular resolution to the measurement problem. Specifically, we consider how three standard putative solutions to the measurement problem inform our understanding of a molecule in isolation, as well as of chemistry’s relation to quantum physics.
This introductory chapter provides an overview of quantum mechanics, which is a subject of central importance in chemistry, and is by nature a mathematical theory. The chemist's approach to the understanding of matter and its chemical transformations is to take a microscopic view, connecting experimental observation with the properties of constituent molecules. However, at the microscopic level, atoms, molecules, and their constituent particles do not obey the familiar laws of mechanics that pertain to the everyday macroscopic world. For example, the existence of molecular line spectra indicates that molecules can be found only at certain well-defined energies, unlike objects at our own scale whose energy appears to be continuously variable. This book aims to explain the fundamentals of quantum mechanics from the point of view of chemistry; describe the areas of chemistry where quantum mechanics is most important; and show how quantum mechanics is applied to chemical problems.
Mature theories of physical science associate states with systems, for instance with particles or waves in classical physics. The quantities whose values comprise states in classical mechanics, namely position and momentum, are chosen, first of all, for predictive purposes. Everything that one would want to say about classical systems can, it seems, be said in terms of properties: specification of the properties of a system gives a complete specification of its state, and conversely. Quantum mechanics associates states with physical systems; whatever else these might be, systems are the particulars which bear physical properties. The former is non-reduced state, the quantum state proper, while the latter is the reduced state, the class of quantitative properties possessed by the system. The state therefore determines which quantities and which properties are instantiated. The non-reductive account of states was initially appealing because it seemed to promise some additional explanatory scope.
Reductionism and holism It was a fad in certain circles in the 1990s to announce that “reductionism is dead,” and that holism is necessary in order to understand biology and social systems. To discuss this we must first define the terms (The American Heritage Dictionary). Holism is a theory or belief emphasizing the importance of the whole and the interdependence of its parts. In classical mechanics, for example, we understand the solar system in terms of its parts (the sun and the planets), and the interactions (“relationships”) are nonlinear. In quantum mechanics we understand complicated molecules like DNA in terms of the binding of smaller molecules. The genetic code is understood in terms of its building blocks: the four letter (computer) alphabet constructed from the bases A, C, G, and T. One understands the bases in terms of simpler molecules, the molecules in terms of atoms, and the atoms in terms of electrons and nuclei. This is reductionism: everything can be understood to be constructed systematically from electrons and positively charged nuclei. Yet, there is something that we cannot calculate from first principles using quantum mechanics: the behavior or the genes as a classical computer with three-letter words constructed from a four-letter alphabet. Quantum theory is linear, and the phase coherence cannot be destroyed from within, meaning that classical behavior cannot be derived from quantum theory without introducing a nonquantum assumption of destruction of phase coherence. This leads into the famous measurement problem clarified by von Neumann.