Parameters atomic

Atomic lines result from the transitions between energy levels, and fines can form between substates that differ in the projected angular momentum m by Am = 0, 1. The maximum separation of the components thus depends on the combined separation of the individual states involved with the line formation  

Increased resolution of specfrographs equipped with CCD detectors now permit resolution of A/AA, 10 to 10 , with attendant reductions in the limits on global fields. 

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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. 

Waals radius rj for each atom type and a hardness parameter ej that determines the depth of the attractive well and how easy (or difficult) it is to push atoms close together. There are interactions for each nonbonded ij pair, including all pairs. The parameters for a pair are obtained from individual atom parameters as follows ... 

The techniques for determining and refining the atomic parameters obtained from a crystal stmcture are not described in detail in this article. For more details see the book by Glusker and Tmeblood. The steps that the crystaHographer takes to determine and refine the stmcture are outlined below. 

Step 11. At this point a computer program refines the atomic parameters of the atoms that were assigned labels. The atomic parameters consist of the three position parameters x,j, and for each atom. Also one or six atomic displacement parameters that describe how the atom is "smeared" (due to thermal motion or disorder) are refined for each atom. The atomic parameters are varied so that the calculated reflection intensities are made to be as nearly equal as possible to the observed intensities. During this process, estimated phase angles are obtained for all of the reflections whose intensities were measured. A new three-dimensional electron density map is calculated using these calculated phase angles and the observed intensities. There is less false detail in this map than in the first map. 

Zinc dust is smaller in particle size and spherical in shape, whereas zinc powder is coarser in size and irregular in shape. The particle size of zinc dust, important in some appHcations, is controUed by adjusting the rate of condensation. Rapid cooling produces fine dust, slower condensation coarse dust. In the case of zinc powder, changes in the atomization parameters can be employed to change particle size to some degree. The particle size distributions for commercial zinc powders range from 44 to 841 p.m (325—20 mesh). The purity of zinc powders is 98—99.6%. 

The van der Waals distance, Rq, and softness parameters, depend on both atom types. These parameters are in all force fields written in terms of parameters for the individual atom types. There are several ways of combining atomic parameters to diatomic parameters, some of them being quite complicated. A commonly used method is to take the van der Waals minimum distance as the sum of two van der Waals radii, and the interaction parameter as the geometrical mean of atomic softness constants. 

One way of reducing the number of parameters is to reduce the dependence on atom types. Torsional parameters, for example, can be taken to depend only on the types of the two central atoms. All C-C single bonds would then have the same set of torsional parameters. This does not mean that the rotational barriers for all C-C bonds are identical, since van der Waals and/or electrostatic tenns also contribute. Such a reduction replaces all tetra-atomic parameters with diatomic constants, i.e. 

The percentage of the spin-density on the 3s and 3p orbitals of the sulfur has been recalculated using the atomic parameters given by J. R. Morton and K. F. Preston, J. Magn. Reson., 30, 577 (1978). 

A set of improved atomic parameters was obtained from three successive Fourier projections on the xy plane, the first based on 34, the second on 56, and the third on 98 of the 126 observed hkO reflections, From... 

The phase transiton from a paraelectric to a ferroelectric state, most characteristic for the SbSI type compounds, has been extensively studied for SbSI, because of its importance with respect to the physical properties of this compound (e.g., J53, 173-177, 184, 257). The first-order transition is accompanied by a small shift of the atomic parameters and loss of the center of symmetry, and is most probably of a displacement nature. The true structure of Sb4S5Cl2 106), Bi4S5Cl2 194), and SbTel 108,403) is still unknown. In contrast to the sulfides and selenides of bismuth, BiTeBr 108) and BiTel (JOS, 390) exhibit a layer structure similar to that of the Cdl2 structure, if the difference between Te, Br, and I (see Fig. 36) is ignored. 

As it stands, this expression is awkward and inconvenient to test because it contains the concentrations of bromine and hydrogen atoms, parameters that are not easily measured. In principle these quantities could be eliminated by solving this equation simultaneously with the differential equations for each of these species. 

Another approach to relating the hardness to atomic parameters is that of Grimvall and Thiessen (1986) in which hardness is related to vibrational energies. Their theory is slightly modified here by using vibrational energy densities instead of the energies themselves. Specific heat data measure the excitation... 

Biltz-Zen trend have been discussed and represented as a function of a charge transfer atomic parameter which correlates with Pauling s electronegativity. This approach has been successfully employed for groups of binary alloys formed by the alkaline earths and the divalent rare earth elements. 

As in the case of DREIDING, the underlying philosophy of UFF does not allow for parameters to be developed for specific molecular systems (e.g. amino, nitro or nitroso). Rather, these are extracted from atomic parameters using a set of combination rales. Several calculations on amino and nitro compounds were performed as part of the validation of UFF58,59. In the following we refer to the structural results and save the discussion of the energetic ones to Section II.D. 

The final refinement was done with Ce, Cu and P on the positions as discussed above. All atom positions and B s, including the crystal orientation and thickness of each data set were refined. The final atomic parameters are given in Table 3. The refined parameters of each diffraction set and the individual R values are given in Table 4. 

The isomorphous replacement method requires attachment of heavy atoms to protein molecules in the crystal. In this method, atoms of high atomic number are attached to the protein, and the coordinates of these heavy atoms in the unit cell are determined. The X-ray diffraction pattern of both the native protein and its heavy atom derivative(s) are determined. Application of the so-called Patterson function determines the heavy atom coordinates. Following the refinement of heavy atom parameters, the calculation of protein phase angles proceeds. In the final step the electron density of the protein is calculated. 

Strategic Limitations. While the actual application of mm calculations to carbohydrate molecules is in most cases straightforward, there are a number of pitfalls that may trap the unwary. The first concerns the choice of an appropriate potential energy function to be used for a particular problem. The adjustable parameters that appear in the energy function must be carefully chosen to give the closest match possible to relevant experimental data. Unfortunately, because observable atomic characteristics vary as a function of environment, atomic parameters developed for the... 

This new model f6), called MNDO for Modified Neglect of Diatomic Overlap, was published oy Dewar and Thiel in 1977. With MNDO the average errors (5) for the same survey of C, H, N and O molecules decreased to 6.3 kcal/mol for AHf, 0.014 A for bond lengths and 0.48 eV for ionization potentials. Since MNDO used only atomic parameters, parameterization of MNDO to include additional elements was much easier than with MINDO/3, and, over the next eight years, parameters were optimized for 16 elements in addition to C, H, N and O. 

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