Exchange-repulsion energy

Figure Al.5.2 First-order Coulomb (O) and exchange-repulsion ( ) energies for Fle-FIe. Based on data from Komasa and Thakkar [70].

The exchange-repulsion energy is approximately proportional to the overlap of the charge densities of the interacting molecules [71, 72 and 73]... 

To complete the first order terms, the exchange-repulsion energy can be evaluated through an overlap model [14, 59] as ... 

The exchange repulsion energy in EFP2 is derived as an expansion in the intermolecular overlap. When this overlap expansion is expressed in terms of frozen LMOs on each fragment, the expansion can reliably be truncated at the quadratic term [44], This term does require that each EFP carries a basis set, and the smallest recommended basis set is 6-31-1— -G(d,p) [45] for acceptable results. Since the basis set is used only to calculate overlap integrals, the computation is very fast and quite large basis sets are realistic. 

The exchange-repulsion energy has been fitted to the following functional form [51,52]... 

E. Fraschini and A. J. Stone, /. Comput. Chem., 19, 847 (1998). H—H Model Potential for Exchange Repulsion Energy of Methane Dimer. 

Xo-type interactions and analyzed their energy components (electrostatic, induction, dispersion, and exchange repulsion energies). In addition, by studying the corresponding water-Y complexes, we compared them with phenol-Y complexes. 

M. Gutowski, G. Chalasinski, and J. van Duijneveldt-van de Rijdt, Int. J. Quantum Chem., 26, 971 (1984). Effective Basis Sets for Calculations of Exchange-Repulsion Energy. 

The exchange repulsion energy (2) is an energy component which is always repulsive, i.e., which is responsible for the finite density of molecules. It arises from the Pauli principle repulsion between the electron clouds of the molecules. It should be emphasized that this component is fundamentally quantum mechanical, i.e., unlike the electrostatic component, there is no classical model for this phenomenon. Although classically, the repulsion between the positively charged nuclei would be expected to keep atoms and molecules apart, such a classical repulsion would occur at much shorter distances than that due to the Pauli principle repulsion. The Pauli principle repulsion is much shorter range than typical electrostatic components and is represented in analytical models with functions that depend on the inverse 9th, inverse 12th or exponential power of the atom-atom distances [3]. 

Table 5.3 shows a decomposition of the EFP interaction energy in water-TBA dimers. As expected. Coulomb and exchange-repulsion energies contribute the most to the total hydrogen bonding energy for all of the dimers. The Coulomb energy fraction decreases in... 

In the gas phase, where there are energy contributions from solvation or polarization, Ap may be measured by means of ion cyclotron spectroscopy.Proton affinities are often given in kJ/mole 1 kJ/mole equals 0.0104 eV 1 kcal/mole equals 0.0436 eV. Exchange repulsion energies do not enter into proton affinities.The relative protonicity of solvents, akin to the donicity concept for electrons, is discussed by Bayless et alP " ... 

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