Flexible charge model

Other flexible molecular models of nitromethane were developed by Politzer et al. [131,132]. In these, parameters for classical force fields that describe intramolecular and intermolecular motion are adjusted at intervals during a condensed phase molecular dynamics simulation until experimental properties are reproduced. In their first study, these authors used quantum-mechanically calculated force constants for an isolated nitromethane molecule for the intramolecular interaction terms. Coulombic interactions were treated using partial charges centered on the nuclei of the atoms, and determined from fitting to the quantum mechanical electrostatic potential surrounding the molecule. After an equilibration trajectory in which the final temperature had been scaled to the desired value (300 K), a cluster of nine molecules was selected for a density function calculation from which... 

Within the model represented by equations (1) and (2), the intermolecular potential energy function is fully determined by the set of -1 charges and n n +1) Lennard-Jones parameters, where n is the number of different types of atoms in the system. For example, the water intermolecular potential in this approach requires 5 different parameters. In practice, this is modified in two ways First, one may wish to add additional point charges to provide more flexibility in modeling the molecular charge distribution. In this case, the locations of the point charges are not necessarily identified with the equilibrium positions of the atoms. Second, a major simplification can be achieved if one uses the following approximation[13] ... 

Keywords All-atom. Basis sets Continuum models Coulombic interaction Dispersion. Dispersion effects Downscaling of charges Electronic structure methods. Fully-flexible Polarizable models Reduced charges Simulation time. Structure. Transferability United atom... 

Shown in Fig. 21 are the quantum and classical solvent curves calculated for the same set of potential parameters. The water model was a flexible point-charge model, the specifics of which are described in... 

Electrokinetic Motion of Heterogeneous Particles, Fig. 2 (a) Linear electrophoresis of a flexible, charged chain connected to a neutral bead in regimes of small, moderate, and large velocity as a model of end-labeled... 

Figure 11.1. Conolly surface plot of the oxygen atoms obtained from molecular dynamic simulation of the water/l,2-dichloroethane (DCE) junction (a). A flexible SPC model was employed for the 343 water molecules, while the 108 DCE molecules were described by a four-centre, simple charge, flexible model. A spherical probe was used with a radius of SA and the Gibbs dividing surface is located at z = 4A. Density variations of the water/vapour, water/DCE and DCE/vapour junction as obtained from the average of trajectories over 200 ps at 300 K (b). Reprinted with permission from ref.[6]. Copyright (1996) American Chemical Society.

However, it must be emphasized that these results are only mathematical artifacts, as the exact solutions for the potential and counterion distribution, for the line charge model with a finite radius, do not show any such phase-transition-like discontinuities (Alfrey et al. 1951). The three major objections to the line-charge model of flexible polyelectrolyte are zero thickness, infinite length, and no-chain flexibility. 

Extra radial flexibility has been proved necessary in order to model the valence charge density of metal atoms, in minerals [6,11], and coordination complexes [5], and similar evidence of the inability of single-exponential deformation functions to account for all the information present in the observations have also been found in studies of organic [12, 13] and inorganic [14] molecular crystals. 

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