Universal force field value

The Universal Force Field, UFF, is one of the so-called whole periodic table force fields. It was developed by A. Rappe, W Goddard III, and others. It is a set of simple functional forms and parameters used to model the structure, movement, and interaction of molecules containing any combination of elements in the periodic table. The parameters are defined empirically or by combining atomic parameters based on certain rules. Force constants and geometry parameters depend on hybridization considerations rather than individual values for every combination of atoms in a bond, angle, or dihedral. The equilibrium bond lengths were derived from a combination of atomic radii. The parameters [22, 23], including metal ions [24], were published in several papers. 

All resulting e and r) parameters should be checked through their consistency with inorganic periodic principles. A universal force field (UFF) has been presented for atom types throughout the entire periodic table [62], and includes values for xj( = 2rf) and D ( = e ). The UFF values were derived from atomic properties, observed and calculated, with considerable assumption and uncertainty. Nevertheless there is general agreement between the UFF values for uncharged elements and other literature values, and these UFF... 

The parameters utilized for this approach are from Breck [57] for the gases He, H2, CO2, O2, N2 and CH4, Poling [58] for the gases CO, Ar, C2Huniversal force field (UFF) values [59] are used for the surface atoms, as summarized in Table 5.2. This potential difference has been termed the suction energy since a positive W translates to a suction force of the molecule from the outside to the inside of the pore, while a negative W translates to a repulsive force directing the molecule away from the pore [23]. From this, a new transport mechanism is proposed in [23] as suction diffusion , where enhanced velocities are predicted as the gas molecules are sucked into the pore. 

Here, Nq refers to the preferred coordination number of the central ion and the empirically determined term in square brackets accounts for the effect of polarization (oa, Ox refer to the absolute softness values of the cation and anion, respectively) as well as the influence of higher coordination shells. Practically, the coefficients /i = 0.9185 and /2 = 0.2285 eV were derived by comparison with the parameters used in other empirical force fields such as the universal force field (UFF) [27]. The level of agreement for the resulting values with the established UFF forcefield can be judged from Fig. 7 (/ 0.95). 

In computational chemistry it can be very useful to have a generic model that you can apply to any situation. Even if less accurate, such a computational tool is very useful for comparing results between molecules and certainly lowers the level of pain in using a model from one that almost always fails. The MM+ force field is meant to apply to general organic chemistry more than the other force fields of HyperChem, which really focus on proteins and nucleic acids. HyperChem includes a default scheme such that when MM+ fails to find a force constant (more generally, force field parameter), HyperChem substitutes a default value. This occurs universally with the periodic table so all conceivable molecules will allow computations. Whether or not the results of such a calculation are realistic can only be determined by close examination of the default parameters and the particular molecular situation. ... 

The structure of the EDL is neither simple nor universal it depends to a great extent on the physico-chemical properties of particles and dispersion medium. In general, it is assumed that some ions from the solvent adhere on the particle surface and partially neutralise the surface charge. This layer of immobile ions is called Stem layer. The other ions spread in the solvent by thermal motion yet are subject to the electric field generated from the charged surface. With growing surface distance the concentrations of the ionic species tend to their equilibrium values of the free solvent. The region adjacent to the Stem layer with excess of counter-ions is called the diffuse layer. In this part of the EDL, the ion distribution results from the balance of electrostatic and osmotic forces. 

The higher the value of /, the smaller the value of A. Here is Faraday s constant, R is the universal gtis constant, T is the absolute temperature and e is the electrical permittivity of the fluid. This quantity e is the product of the relative dielectric constant of the medium (for water = 78.54 at 25 °C) and the electrical permittivity So of vacuum (= 8.8542 x 10 farad/cm or coulomb/volt-cm = 8.854 x 10 cou-lomh /newton-m, where recall that 1 newton-m = 1 volt-coulomh = 1 joule). In a uni-univalent electrolyte solution of 0.1 M strength (of, say, NaCl) the value of A at 25 °C is 9.6 X 10 cm, i.e. 0.96 nm (Newman, 1973). The ions of opposite charge shield the charge of the ion of interest, and the effect of the ion of interest decays very rapidly with distance. So the description of the electrical force on an ion in an applied field E by definition (3.1.8) is generally satisfactory. 

From observations of distant galaxies by the Hubble Space Telescope and other big telescopes evidence for an accelerated expansion was found. This can be explained by a repulsive force and is taken into account by A non zero value of means that the universe is not controlled by gravity alone. In fact A was the cosmological constant that was introduced by Einstein in order to keep the solutions of his field equations stable—at the time of propagating his general relativity theory (1916) it was not known that the universe expands. 

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