Quantum chemistry of atoms

Molecular mechanics has proved very useful especially for large molecules, where more exact methods would require impractical amounts of computational resource. There are problems that molecular mechanics cannot tackle, however, for example the nature of transition states in chemical reactions or magnetic and spectroscopic properties of molecules. Even for such a simple molecule as dioxygen, O2, we need quantum theory to explain why it has two unpaired electrons. 

The advances in computer software and hardware since then have been such that by 1998, experimental chemists routinely used quantum calculations performed on a desktop computer as a complement to their work. The Nobel Prize for Chemistry in that year was awarded to two pioneers of quantum chemistry — John Pople and Walter Kohn (Box 3.2). 

John Pople was bom in Bumham-on-Sea, Somerset in 1925, the son of a men s clothing storeowner. His parents considered education important and sent him to Bristol Grammar School. Here he developed an interest in mathematics and in 1943 went to Trinity College, Cambridge to read mathematics. As it was wartime he had to finish his degree in two years and in 1945 went to work for the Bristol Aeroplane Company. In 1947 he returned to Cambridge as a maths student but developed an interest in theoretical science. He went on to work with Sir John Lennard-Jones after whom one of the interatomic potentials used in calculations is named. At about the same time Pople decided to learn to play the piano and went on to marry his piano teacher, Joy Bowers. 

It was after moving to Pittsburgh, USA in 1964 that John Pople began the work in computational chemistry that won him the Nobel Prize. He remained there until 1993. 

Walter Kohn was bom in Austria in 1923 to middle-class Jewish parents. 

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P. S. C. Matthews (1986) Quantum Chemistry of Atoms and Molecules, Cambridge Univ. Press, Cambridge. 

The quantum chemistry of atomic states and bond formation is discussed in more detail by ... 

The quantum theory of atoms in molecules is described in texts and several reviews [1-4]. A qualitative survey of the essential definitions and their application to problems in the field of medicinal chemistry are given here with two purposes ... 

This chapter has provided an introduction to the ideas underlying the quantum theory of atoms in molecules, the theory that gives theoretical expression to chemical concepts and enables one to employ these concepts in a quantitative manner for prediction and for understanding of chemical problems. The theory is particularly well-suited to problems in medicinal chemistry where the important role of building block molecules enables one to make maximum use of the transferability of atoms and groups defined as open quantum systems. 

Spatially localized functions are an extremely useful framework for thinking about the quantum chemistry of isolated molecules because the wave functions of isolated molecules really do decay to zero far away from the molecule. But what if we are interested in a bulk material such as the atoms in solid silicon or the atoms beneath the surface of a metal catalyst We could still use spatially localized functions to describe each atom and add up these functions to describe the overall material, but this is certainly not the only way forward. A useful alternative is to use periodic functions to describe the wave functions or electron densities. Figure 1.2 shows a simple example of this idea by plotting... 

The theory of atoms in molecules192 recovers all the fundamental concepts of chemistry, of atoms and functional groups with characteristic properties, of bonds, of molecular structure and structural stability, and of electron pairs and their role in molecular geometry and reactivity. The atomic principle of stationary action extends the predictions of quantum mechanics to the atomic constituents of all matter, the proper open systems of quantum mechanics. All facets of the theory are predictive and, as a consequence, the theory can be employed in many fields of research at the atomic level, from the design and synthesis of new drugs and catalysts, to the understanding and prediction of the properties of alloys. 

A rational deduction of elemental abundance from solar and stellar spectra had to be based on quantum theory, and the necessary foundation was laid with the Indian physicist Meghnad Saha s theory of 1920. Saha, who as part of his postdoctoral work had stayed with Nernst in Berlin, combined Bohr s quantum theory of atoms with statistical thermodynamics and chemical equilibrium theory. Making an analogy between the thermal dissociation of molecules and the ionization of atoms, he carried the van t Hoff-Nernst theory of reaction-isochores over from the laboratory to the stars. Although his work clearly belonged to astrophysics, and not chemistry, it relied heavily on theoretical methods introduced by and associated with physical chemistry. This influence from physical chemistry, and probably from his stay with Nernst, is clear from his 1920 paper where he described ionization as a sort of chemical reaction, in which we have to substitute ionization for chemical decomposition. [81] The influence was even more evident in a second paper of 1922 where he extended his analysis. [82]... 

The main purpose of this chapter is to present the basics of ab initio molecular dynamics, focusing on the practical aspects of the simulations, and in particular, on modeling chemical reactions. Although CP-MD is a general molecular dynamics scheme which potentially can be applied in combination with any electronic structure method, the Car-Parinello MD is usually implemented within the framework of density functional theory with plane-waves as the basis set. Such an approach is conceptually quite distant from the commonly applied static approaches of quantum-chemistry with atom-centered basis sets. Therefore, a main... 

Parallel to the exciting reports about new types of hydrogen-hydrogen interactions, a paradigm shift was (and is) taking place in interpretative theoretical chemistry. Since the publication of Bader s classic monograph in 1990 [63], the quantum theory of atoms in molecules (QTAIM) has become a standard tool for the interpretation of theoretical and experimental [65-69] electron density distribution maps. The theory and its applications have been reviewed on a number of occasions by its principal author [63, 70-78] and by others [65-67, 69, 79-84]. A brief reminder of some of the basic concepts of QTAIM will be presented here with the sole purpose of keeping this chapter self-contained, but the interested reader is referred to the previously cited literature for in-depth treatments. 

This looks far more plausible the whole enterprise of quantum chemistry, after all, is to use quantum mechanics to recover facts about chemical bonding that are well known to the chemist. If there is meshing between chemistry and quantum mechanics, do the real meshes and interfaces we saw in the last section really support strict physicalism Surely they cannot, if they are consistent with downward causation. In any case, it is not as if, in the explanation of the spectrum of carbon dioxide, there are two independent theoretical accounts that were compared and found to be consistent. Perhaps that would constitute an explanation of the less fundamental models. But neither chemistry nor the quantum mechanics of resultant Hamiltonians have the resources for independent accounts of the spectrum of carbon dioxide. Rather than an explanation of chemical structure by physical theory there was a joint venture the explanation of various facts by the use of quantum mechanics applied to a given molecular structure. There was no mesh or interface between the quantum mechanics and chemistry, at least none that required explanation. What we had was an instance of quantum chemistry, the quantum theory of atoms and molecules.14... 

The present article essentially focusses on the application of ECPs in standard molecular wavefunction theory (WFT), i.e., standard ab initio quantum chemistry using atomic and molecular orbitals as one-particle basis sets from which determinantal many-electron wavefunctions are constmcted. Applications to... 

Boeyens, J.C.A. (2010) Emergent Properties in Bohmian Chemistry, in M.V. Putz (ed.). Quantum Frontiers of Atoms and Molecules, NOVA, New York. 

A probability has been grounded by methods of quantum chemistry of the formation of two forms of coordination-bonded water molecules with silicon atoms of silanol groups [72,121,122]. At the initial stage of adsorption the complexes are formed with transposition of water molecules relatively to silanol group ... 

Simons, J., and J. Nichols, Quantum Mechanics in Chemistry, Oxford University Press, 1997. Slater, J. C, Quantum Theory of Atomic Structure, vols. I and n, McGraw-Hill, 1960. 

In the transition state of the dissociating alkane molecule, an unoccupied orbital asymmetric with respect to the a symmetric CH bond becomes partiaUy populated during the dissociation process. When parallel to the surface, it interacts with a partially occupied asymmetric d-valence atomic orbital of the transition metal. The increased population of the asymmetric CH orbital will weaken the CH bond and hence lowers the activation energy for bond cleavage. The quantum-chemistry of the surface oxidative addition process is illustrated for dissociative adsorption in Figure 10.38. 

Boeyens, J. C. A. (2008). Chemistry from First Principles, Springer, Heidelbeig-Berlin. Boeyens, J. C. A. (2011). Emergent properties in bohmian chemistry. In Quantum Frontiers of Atoms and Molecules, Putz, M. V. (Ed.), Nova Publishers Inc., New York,... 

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