Point charge model, calculations

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). 

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... 

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... 

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]. 

To test our suspicion, we used a point charge model and ab initio calculated atomic charges in order to evaluate the electrostatic energy of 1,2-ds and 1,1-difluoro-ethylene. 

It is quite remarkable that electrostatic calculations based on a simple model of integral point charges at the nuclear positions of ionic crystals have produced good agreement with values of the cohesive energy as determined experimentally with use of the Born-Haber cycle. The point-charge model is a purely electrostatic model, which expresses the energy of a crystal relative to the assembly of isolated ions in terms of the Coulombic interactions between the ions. 

The cohesive energy of ionic crystals is mainly due to electrostatic interaction and can be calculated on the basis of a point-charge model. Following Born, the cohesive energy (U) of a crystal containing oppositely charged ions with charges Zj and Zj is written as the sum of two terms, one due to attraction and the other due to repulsion ... 

In this work the use of molecular electrostatic potential (MEP) maps for similarity studies is reviewed in light of the latest results. First, methods of obtaining MEP maps is overviewed. The methodology, reliability and the efficiency of calculations based on semi-empirical as well as ab initio methods are discussed in detail. Point-charge models and multipole expansion methods which provide MEP maps of satisfactory quality are evaluated critically. Later on, similarity indices of various kinds are analyzed, compared and examples of their use are shown. Finally, the last section lists and summarizes software packages capable of calculating MEP map based similarity indices. 

The crystal field splitting estimated for Fe2+ in the enstatite M2 site is in reasonable agreement with the A0 value of 6,522 cm-1 calculated by a theoretical point charge model (Goldman and Rossman, 1977a Rossman, 1988). 

Dependent on the type and quality of QA, the point charge model can be used as a more or less crude approximation of the full quantum chemical calculation of (i according to Equation 6.48). As outlined below, semiempirical mapping procedures have been developed to derive QA such that Equation 6.49 yields a best fit (within the model parameters employed) to exact (experimental) dipole moments. 

Afzal and Frost476 have recently made a detailed analysis of the energy terms which contribute to the total energy in FSGO calculations. A point-charge model of HaO was also found to be capable of yielding accurate values of various one-electron properties.477... 

Brill et ah IS21 investigated the 3SC1(NQR) spectrum and used the point charge model to calculate the EFG.Semiempirical LCAO-MO calculations have also been used. An empirical Stemheimer factor of -10 for chlorine was used to explain the crystal field effects which amount to 10 % for y(35Cl) in [SnCl6]2. ... 

Fig. 13 The change in calculated optical rotation of glycine as a function of inclusion of water molecules (QM or in form of a point-charge model) up to a cut-off distance from the solute. MD data to prepare plot taken from Kundrat and Autschbach [162]...

Fig. 22. Calculated -electron charge densities at C-14 for 11-cis-retinal 77, its Schiff base 78, the protonated Schiff base 79, and rhodopsin 80, as indicated by the external point-charge model. From Honig et al. [82],...

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