Property based potential model calculations

V. CALCULATIONAL ASPECTS OF PROPERTY-BASED POTENTIAL MODELS... 

CA 55, 24011(1961) (Equation of state of the products in. RDX detonation) Mj) W. Fickett, "Detonation Properties of Condensed Explosives Calculated with an Equation of State Based on Intermo-lecular Potentials , Los Alamos Scientific Laboratory Report LA-2712(1962), Los Alamos, New Mexico, pp 9-10 (Model of von Neumann-Zel dovich), pp 153-66 [Comparison of KW (Kistiakowsky-Wilson) equation of state with those of LJD (Lennard-Jones-Devonshire) and Constant-/ ] M2) C.L. 

There are several properties of a chemical that are related to exposure potential or overall reactivity for which structure-based predictive models are available. The relevant properties discussed here are bioaccumulation, oral, dermal, and inhalation bioavailability and reactivity. These prediction methods are based on a combination of in vitro assays and quantitative structure-activity relationships (QSARs) [3]. QSARs are simple, usually linear, mathematical models that use chemical structure descriptors to predict first-order physicochemical properties, such as water solubility. Other, similar models can then be constructed that use the first-order physicochemical properties to predict more complex properties, including those of interest here. Chemical descriptors are properties that can be calculated directly from a chemical structure graph and can include abstract quantities, such as connectivity indices, or more intuitive properties, such as dipole moment or total surface area. QSAR models are parameterized using training data from sets of chemicals for which both structure and chemical properties are known, and are validated against other (independent) sets of chemicals. 

The knowledge of the surface potential for the dispersed systems, such as metal oxide-aqueous electrolyte solution, is based on the model calculations or approximations derived from zeta potential measurements. The direct measurement of this potential with application of field-effect transistor (MOSFET) was proposed by Schenk [108]. These measurements showed that potential is changing far less, then the potential calculated from the Nernst equation with changes of the pH by unit. On the other hand, the pHpzc value obtained for this system, happened to be unexpectedly high for Si02. These experiments ought to be treated cautiously, as the flat structure of the transistor surface differs much from the structure of the surface of dispersed particle. The next problem may be caused by possible contaminants and the surface property changes made by their presence. 

A key question about the use of any molecular theory or computer simulation is whether the intermolecular potential model is sufficiently accurate for the particular application of interest. For such simple fluids as argon or methane, we have accurate pair potentials with which we can calculate a wide variety of physical properties with good accuracy. For more complex polyatomic molecules, two approaches exist. The first is a full ab initio molecular orbital calculation based on a solution to the Schrddinger equation, and the second is the semiempirical method, in which a combination of approximate quantum mechanical results and experimental data (second virial coefficients, scattering, transport coefficients, solid properties, etc.) is used to arrive at an approximate and simple expression. 

Based on DFT calculations on chlorophylls and, additionally, on ubiquinone and the RC main detergent, lauryl dimethylamine oxide or LDAO, we have then developed a force field for their classical modelization. Our approach to this undertaking was straightforward. We initially use the DFT optimized structures and the vibrational analysis to determine the bonded part of the potential parameters described by the AMBER potential function. Then, atomic ab initio partial charges on the chromophore are used to account for electrostatic effects. At a later stage, experimental data from X-ray crystallography are used to check the structural properties of the molecule in the condensed state and to refine the intermolecular Lennard-Jones parameters. 

The Niesar-Corongiu-Clementi (NCC) potentiaP is a polarizable extension of the quantum chemistry based MCY model discussed earlier. It consists of site-site two-body terms and polarizable sites located on the intramolecular OH bonds. It contains a total of six interaction sites the oxygen, hydrogens, M site, and two polarization sites. Because of this, it is one of the computationally more expensive models to use in simulations. The parameters were determined by fitting the potential to ab initio results of water-water interactions. In this way, it differs from most potentials that are fit to macroscopic properties. Hence, the agreement between the experimental and calculated pair functions is good, but the pressure is negative. 

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