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Typus
KomplettPlagiat
Bearbeiter
Klgn
Gesichtet
Yes.png
Untersuchte Arbeit:
Seite: 44, Zeilen: 1 ff. (entire page)
Quelle: Accelrys Inc. - Forcefield-Based Simulations 1998
Seite(n): 40, 41, Zeilen: 40:18ff; 41
Metals and their immediate ligands are treated with the above prescription, summing their formal charges to get a net fragment charge. Delocalized π−systems are treated in an analogous fashion. σ−systems are treated using a localized approach in which the charges of an atom depend simply on its neighbors. Note that this approach, unlike the straightforward implementations based on the equalization of electronegativity, does include some resonance effects in the π−system.

The electronegativity and hardness in the above equations must be determined. In earlier force fields they were often determined from experimental ionization potentials and electron affinities; however, these spectroscopic states do not correspond to the valence states involved in molecules. For this reason, ESFF is based on electronegativities and hardnesses, calculated using density functional theory. The orbitals are (fractionally) occupied in ratios appropriate for the desired hybridization state, and calculations are performed on the neutral atom as well as on positive and negative ions.

ESFF uses the 6-9 potential for the van der Waals interactions. Since the van der Waals parameters must be consistent with the charges, they are derived using rules that are consistent with the charges.

Starting with the London formula:

\left ( B_i \sim \alpha_i^2.\mathit{IP} \right ) (2.10)

where α is the polarizability and IP the ionization potential of the atoms, the polarizability, in a simple harmonic approximation, is proportional to n/IP where n is the number of electrons. Across any one row of the periodic table, the core electrons remain unchanged, so that the following form is reasonable:

\alpha=\frac{a'}{\mathit{IP}}+\frac{b' n_{\mathit{eff}}}{\mathit{IP}} (2.11)

[page 40]

Metals and their immediate ligands are treated with the above prescription, summing their formal charges to get a net fragment charge. Delocalized π systems are treated in an analogous fashion. And σ systems are treated using a localized approach in which the charges of an atom depend simply on its neighbors. Note that this approach, unlike the straightforward implementations based on the equalization of electronegativity, does include some resonance effects in the π system.

Electronegativity and hardness obtained by DFT

The electronegativity and hardness in the above equations must be determined. In earlier forcefields they were often determined from experimental ionization potentials and electron affinities; however, these spectroscopic states do not correspond to the valence states involved in molecules. For this reason, ESFF is based on electronegativities and hardnesses, calculated using density functional theory as implemented in DMol. The orbitals are (fraction-

[page 41]

ally) occupied in ratios appropriate for the desired hybridization state, and calculations are performed on the neutral atom as well as on positive and negative ions.

van der Waals interactions

ESFF uses the 6–9 potential for the van der Waals interactions. Since the van der Waals parameters must be consistent with the charges, they are derived using rules that are consistent with the charges.

Derivations

Starting with the London formula:

( B_i \sim \alpha_i^2 \cdot \mathit{IP} ) Eq. 14

where α is the polarizability and IP the ionization potential of the atoms, the polarizability, in a simple harmonic approximation, is proportional to n / IP where n is the number of electrons. Across any one row of the periodic table, the core electrons remain unchanged, so that the following form is reasonable:

\alpha=\frac{a'}{\mathit{IP}}+\frac{b' n_{\mathrm{eff}}}{\mathit{IP}} Eq. 15

Anmerkungen

The source is not given.

Sichter
(Klgn), WiseWoman

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