Point-charge models

Specific solute-solvent interactions involving the first solvation shell only can be treated in detail by discrete solvent models. The various approaches like point charge models, siipennoleciilar calculations, quantum theories of reactions in solution, and their implementations in Monte Carlo methods and molecular dynamics simulations like the Car-Parrinello method are discussed elsewhere in this encyclopedia. Here only some points will be briefly mentioned that seem of relevance for later sections. 

In contrast to the point charge model, which needs atom-centered charges from an external source (because of the geometry dependence of the charge distribution they cannot be parameterized and are often pre-calculated by quantum mechanics), the relatively few different bond dipoles are parameterized. An elegant way to calculate charges is by the use of so-called bond increments (Eq. (26)), which are defined as the charge contribution of each atom j bound to atom i. 

VVe therefore return to the point-charge model for calculating electrostatic interactions. If sufficient point charges are used then all of the electric moments can be reproduced and the multipole interaction energy. Equation (4.30), is exactly equal to that calculated from the Coulomb summation. Equation (4.19). 

On the basis of the point-charge model formalism, applied on the experimental nuclear quadrupole splitting rationalization, I Agxp I, the results obtained were interpreted in terms of strong complex formation by either Me2Sn(OH)2 or Me3Sn(0H)(H20) with (n = 1 or 2, obtained in phosphate buffer) and... 

Pempointner, M., Seth, M. and Schwerdtfeger, P. (1998) A Point-Charge Model for the Nuclear Quadmpole Moment. Accurate Coupled-Cluster, Dirac-Fock, Douglas-KroD and Nomelativistic Hartree-Fock Calculations... 

Kelld, V. and Sadlej, A.J. (2000) The point charge model of nuclear quadrupoles How and why does it work. Journal of Chemical Physics, 112, 522—526. 

La2 xSrxCu04 EFG tensor, disagrees with point charge model, charge state, holes from substitution of La by Sr are localized mainly at the oxygen sites in the CU-O2 plane... 

RBa2Cu307 (R = rare earth element or Y), La2 (5r,.Cu04 (0 < X < 0.3) Eu-155(Gd-155) emission Mossbauer spectroscopy, EFG tensor at R sites, in good agreement with point charge model when holes are supposed to be mainly in sublattices of the chain and at oxygen in Cu-O plane... 

RBa2Cu40g (R = Sm, Y, Er) Nuclear-quadrupole coupling parameters at the rare-earth metal and copper sites from Cu ( Zn) and Ga( Zn) Mossbauer emission spectroscopy, EEG tensor in comparison with point charge model, shows that holes in lattices are localized primarily at chain-oxygen sites... 

Y2Ba4Cu7025 Nuclear quadrupole interaction at copper sites, EFG tensor at all sites is calculated using the point charge model, conclusion that holes in the Y2Ba4Cu70i5 lattice are localized predominantly at positions of chain oxygen... 

Banks JL, Kaminski GA, Zhou RH, Mainz DT, Berne BJ, Friesner RA (1999) Parametrizing a polarizable force field from ab initio data. I. The fluemating point charge model. J Chem Phys 110(2) 741—754... 

Svishchev IM, Kusalik PG, Wang J, Boyd RJ (1996) Polarizable point-charge model for water results under normal and extreme conditions. J Chem Phys 105(11) 4742 1750... 

Williams, D. E., and J.-M. Yan. 1988. Point-Charge Models for Molecules Derived from Least-Squares Fitting of the Electric Potential. Adv. Atomic Mol. Phys. 23, 87. 

How to Determine the Crystal-Field Parameters 3. The Effective Point Charge Model... 

We have shown in this chapter that the major electronic features that determine the spin dynamics of SIMs based on lanthanides can be directly correlated with the local coordination environment around the 4f metal ions. By using an effective point-charge model that accounts for covalent effects, we have shown that the splitting of the ground state,/, of the lanthanide into Mj sublevels, caused by the influence of the CF created by the surrounding ligands, is consistent with... 

The EFGs for both 69,7 Ga and 14N have been calculated using the WIEN-97 program, and the results compare more favorably with the experimental results than do those from a simple point charge model including Stemheimer antishielding factors (note that the experimental 14N NQCC value used in the comparison was somewhat larger than the accurate value reported above) [52]. 

The actual natural charge in the cyclic pentamer is qnat = 0.570, which makes the Coulombic point-charge estimate entirely unrealistic.) Thus, no matter how q is chosen, a simple Coulombic point-charge model will give >10% errors for one or the other of these clusters. 

If separate D-RESP charge sets are fitted for every single one of the 36 frames, the standard deviation of the electrostatic field generated varies between 3.5 and 5% with respect to the full quantum reference. This accuracy is the best (in the least-squares sense) that can be obtained if the system is modeled with time-dependent atomic point-charges and represents the accuracy limit for a fluctuating point charge model of the dipeptide. 

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