Polarizability models potential parameters

Results of parameter optimization and MD simulations of small model compounds have been published, including alcohols [63], alkanes [63], aromatic [64] and heteroaromatic [209] compounds and liquid amides [65], Studies of ions in aqueous solution were also performed [61, 88] and results from an MD simulation on a DPPC lipid monolayer have been reported (Harder, MacKerell, Roux, submitted). Notable from the monolayer study was the reproduction of the dipole potential across the monolayer, a value that cannot be reproduced using non-polarizable models. This exciting, unforeseen observation points to the types of results that may be obtained from polarizable macromolecular force fields that are not accessible to the present additive models. 

Its important to note that in developing a polarizable model of a liquid, the permanent charges and the Lennard-Jones parameters appropriate to the effective pair potential must be modified. 

Veldhuizen and de Leeuw (1996) used the OPLS parameters for methanol and both a nonpolarizable and a polarizable model for carbon tetrachloride for MD simulations over a wide range of compositions. The polarization contribution was found to be very important for the proper description of mixture properties, such as the heat of mixing. A recent study by Gonzalez et at (1999) of ethanol with MD simulations using the OPLS potential concluded that a nonpolarizable model for ethanol is sufficient to describe most static and dynamic properties of liquid ethanol. They also suggested that polarizabilities be introduced as atomic properties instead of the commonly approach of using a single molecular polarizability. 

Table 1 Potential parameters for the non-polarizable and polarizable surfactant models...

Table 3 Potential parameters for the fully polarizable surfactant model...

The potential parameters of the hydroxide ions are the same as those used successfully by de Leeuw (1995), which were modified by Baram and Parker (1996) to include a polarizable oxygen ion and were applied in their woik on hydroxide formation at quartz and zeolite surfaces. Methanoic add and methylamine molecules were modeled using the Consistent Valence Force Held (CVFF) from the Insight 11 package (Molecular Simulations Inc, San Diego, CA). The interaction between methanoic acid and methylamine with a quartz surface was adopted according to the partial charges on the atoms. 

It should be stressed that the potential parameters are the same for all properties, and that the values of quadrupole moment and the polarizability were taken from independent experiments and were not treated as adjustable parameters. It should also be stressed that the fits depend on a proper choice of the spherical part of the potential, ifig. Models for (frg simpler than Equation (5) are inadequate. 

Table 1 Potential Parameters of the Non-polarizable Water Models...

This expression reproduces the experimentally measured ionization efficiency curves surprisingly well, considering the simplicity of the model on which it is based. There is a discontinuity in the function at the maximum (when X = Xmax) but this affects only a small region of the ionization efficiency curve, and satisfactory values of the cross section are still obtained over this region. A great advantage of this method is that it is very simple to apply, depending on only three parameters the molecular polarizability volume, the ionization potential, and the maximum electron impact ionization cross section. These can be measured or calculated values (from the ab initio EM method described above, for example). 

A common feature of the various methods that we have developed for the calculation of electronic effects in organic molecules is that they start from fundamental atomic data such as atomic ionization potentials and electron affinities, or atomic polarizability parameters. These atomic data are combined according to specific physical models, to calculate molecular descriptors which take account of the network of bonds. In other words, the constitution of a molecule (the topology) determines the way the procedures (algorithms) walk through the molecule. Again, as previously mentioned, the calculations are performed on the entire molecule. 

In addition to these external electric or magnetic field as a perturbation parameter, solvents can be another option. Solvents having different dielectric constants would mimic different field strengths. In the recent past, several solvent models have been used to understand the reactivity of chemical species [55,56]. The well-acclaimed review article on solvent effects can be exploited in this regard [57]. Different solvent models such as conductor-like screening model (COSMO), polarizable continuum model (PCM), effective fragment potential (EFP) model with mostly water as a solvent have been used in the above studies. 

Molecular dynamic studies used in the interpretation of experiments, such as collision processes, require reliable potential energy surfaces (PES) of polyatomic molecules. Ab initio calculations are often not able to provide such PES, at least not for the whole range of nuclear configurations. On the other hand, these surfaces can be constructed to sufficiently good accuracy with semi-empirical models built from carefully chosen diatomic quantities. The electric dipole polarizability tensor is one of the crucial parameters for the construction of such potential energy curves (PEC) or surfaces [23-25]. The dependence of static dipole properties on the internuclear distance in diatomic molecules can be predicted from semi-empirical models [25,26]. However, the results of ab initio calculations for selected values of the internuclear distance are still needed in order to test and justify the reliability of the models. Actually, this work was initiated by F. Pirani, who pointed out the need for ab initio curves of the static dipole polarizability of diatomic molecules for a wide range of internuclear distances. 

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