Electrodynamic bonding

The four rather distinct forms of chemical bonding between atoms are metallic, ionic, covalent, and dispersive (Van der Waals). All of them are sub-topics of quantum electrodynamics. That is, they are all mediated by electronic and electromagnetic forces. There are also mixed cases, as in carbides and other compounds, where both metallic and covalent bonding occur. 

Chemical bonding consists of electrostatic and electrodynamic interactions between valence electrons and positive ions. However, there is more than one category. In small groups of atoms (molecules), pairs of electrons may reside between pairs of ions and electrostatically attract the ions to form bonds (covalent bonding). 

A fourth possibility is electrodynamic bonding. This arises because atoms and molecules are not static, but are dynamically polarizable into dipoles. Each dipole oscillates, sending out an electromagnetic field which interacts with other nearby dipoles causing them to oscillate. As the dipoles exchange electro-magnetic energy (photons), they attract one another (London, 1937). 

The electrodynamic forces proposed for stabilizing jellium provide the principal type of bonding in molecular crystals such as solid methane, rare gas crystals, solid anthracene, and the like. These forces also form the inter-chain bonding of long-chain molecules in polymeric materials (the intra-molecular bonding within the chains is usually covalent). 

To obtain a clear understanding of electrodynamic bonding, start with the field of a static electric dipole. Then, let the dipole oscillate so it emits electromagnetic waves (photons). Consider what happens when the emitted field envelopes another dipole (London, 1937). Finally, determine the factors that convert neutral molecules into dipoles (that is, their polarizabilities). 

These speculations about the ionic, polar, or electronic nature of chemical bonding, which arose largely from solution theory, resulted mostly in static models of the chemical bond or atom structure. In contrast is another tradition, which is more closely identified with ether theory and electrodynamics. This tradition, too, may be associated with Helmholtz, especially by way of his contributions to nineteenth-century theories of a "vortex atom" that would explain chemical affinities as well as the origin of electromagnetism, radiation, and spectral lines. 

One result of studying nonlinear optical phenomena is, for instance, the determination of this susceptibility tensor, which supplies information about the anharmonicity of the potential between atoms in a crystal lattice. A simple electrodynamic model which relates the anharmonic motion of the bond charge to the higher-order nonlinear susceptibilities has been proposed by Levine The application of his theory to calculations of the nonlinearities in a-quarz yields excellent agreement with experimental data. 

Tapia, O. Quantum mechanics and the theory of hydrogen bond and proton transfer. Beyond a Bom-Oppenheimer description of chemical interconversions, J. Mol. Struct. (Theochem), 433 (1998), 95-105. Feynman, R.P. Quantum electrodynamics, Benjamin, Inc., New York, 1961. 

Walter Heitler (1904—1981). German physicist. Heitler left Germany in 1933 to escape the Nazi regime and spent the war years at the University of Bristol. He joined the faculty at the University of Zurich in 1949. In addition to his work on valence bond theory, he made important contributions to quantum electrodynamics and quantum field theory. 

Finally, the whole system (molecule + metal nanoparticle) can be treated atomistically via TD-DFT or other quantum chemical methods. The interaction between the metal nanoparticle and the molecule are treated on the same foot as the intra-molecule and intra-nanoparticle ones. This method is therefore able to include much more than just the electrodynamics coupling, as it can include mutual polarization, chemical bonding, charge transfers (also in excited states). On the down side, at present this approach is limited to very small metal particles (a few tens of atoms, a few nm in size). Moreover, electrodynamics coupling is limited to the quasi-static limit, as standard molecular Hamiltonian includes only non-retarded Coulombic potential. Nevertheless, this method represents a fully ab initio approach to molecular plasmonics. 

Levine, B.F. (1969) Electrodynamical bond-charge calculation of nonlinear optical susceptibilities. Physical Review Letters, 22, 787. 

Besides the computational savings, ECPs have the advantage that they allow for the implicit inclusion of relativistic effects, even of the Breit interaction or quantum electrodynamic (QED) corrections, by simple parametrizations to relativistic AE data. Furthermore, ECPs permit the usage of smaller basis sets and thus the basis set superposition error is less significant compared to AE calculations. Even the difficulties due to open shells may be avoided by applying ECPs, if these open shells are included in the core system as it is the case for the 4f-in-core [7-9] and 5f-in-core [10-12] pseudopotentials (PP) for lanthanides and actinides, respectively. However, these PPs can only be applied, if the f orbitals do not participate significantly in chemical bonding (see Section 6.3.1). 

Microactuators often require reverse elements as in the case of an electrodynamic microlinear actuator [144]. This can be achieved, for instance, by using a microstructured glass/copper composite coil, which can be produced by microelectroforming (see Sect. 10.8). This coil was joined to the reverse element by adhesive bonding (sticking). Figure 11.10 shows two different designs... 

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