Artifact models

Every method of development must be good at building its artifacts—models, designs, plans, and so on—from smaller pieces. We can get our heads around only a small chunk at a time and can build big things only by sticking small ones together. Moreover, parts are more likely to be reusable if they can be put together in various ways with predictable results. 

Therefore, the following chapter discusses and specializes the connection between EEA artifacts modeled in PREEvision and safety requirements. A simplified excerpt of the PREEvision meta-model, extended by safety aspects encircling the super class Safety Related Element, is applied for this discussion. The class diagram of the applied meta-model is depicted in Figure 10. 

The subchain of this model is an artifact about which we have no information. After developing expressions for the behavior of the subchains, we must describe the latter in terms of the actual polymer chains. 

Optimization should be viewed as a tool to aid in decision making. Its purpose is to aid in the selection of better values for the decisions that can be made by a person in solving a problem. To formulate an optimization problem, one must resolve three issues. First, one must have a representation of the artifact that can be used to determine how the artifac t performs in response to the decisions one makes. This representation may be a mathematical model or the artifact itself. Second, one must have a way to evaluate the performance—an objective function—which is used to compare alternative solutions. Third, one must have a method to search for the improvement. This section concentrates on the third issue, the methods one might use. The first two items are difficult ones, but discussing them at length is outside the scope of this sec tion. 

For thin-film samples, abrupt changes in refractive indices at interfrees give rise to several complicated multiple reflection effects. Baselines become distorted into complex, sinusoidal, fringing patterns, and the intensities of absorption bands can be distorted by multiple reflections of the probe beam. These artifacts are difficult to model realistically and at present are probably the greatest limiters for quantitative work in thin films. Note, however, that these interferences are functions of the complex refractive index, thickness, and morphology of the layers. Thus, properly analyzed, useful information beyond that of chemical bonding potentially may be extracted from the FTIR speara. 

Both space-filling and electron density models yield similar molecular volumes, and both show the obvious differences in overall size. Because the electron density surfaces provide no discernible boundaries between atoms (and employ no colors to highlight these boundaries), the surfaces may appear to be less informative than space-filling models in helping to decide to what extent a particular atom is exposed . This weakness raises an important point, however. Electrons are associated with a molecule as a whole and not with individual atoms. The space-filling representation of a molecule in terms of discernible atoms does not reflect reality, but rather is an artifact of the model. The electron density surface is more accurate in that it shows a single electron cloud for the entire molecule. 

Artifact removal and/or linearization. A common form of artifact removal is baseline correction of a spectrum or chromatogram. Common linearizations are the conversion of spectral transmittance into spectral absorbance and the multiplicative scatter correction for diffuse reflectance spectra. We must be very careful when attempting to remove artifacts. If we do not remove them correctly, we can actually introduce other artifacts that are worse than the ones we are trying to remove. But, for every artifact that we can correctly remove from the data, we make available additional degrees-of-freedom that the model can use to fit the relationship between the concentrations and the absorbances. This translates into greater precision and robustness of the calibration. Thus, if we can do it properly, it is always better to remove an artifact than to rely on the calibration to fit it. Similar reasoning applies to data linearization. 

There is indeed some disagreement as to whether values of AC carry chemical meaning. Although many workers5 believe they do, others6 would argue that these small effects are instead more likely the artifacts of an incomplete theoretical model or a chemical path that intrudes to a minor extent near one of the temperature extremes. It seems that the matter cannot be settled in any general way. 

One could view the occurrence of the metric terms in the equations of motion as an annoying complication, but we hold a more positive view. First they assure that whatever the choice of parameters to be used as dynamical variables, that choice will not introduce unphysical artifacts. Second, the metric terms are another component of the theory with potential for providing guiding principles for development of XC models. Those terms also allow the mathematical origin of physical affects to be assigned. 

Mathematically,/(l) can be determined from F t) or W t) by differentiation according to Equation (15.7). This is the easiest method when working in the time domain. It can also be determined as the response of a dynamic model to a unit impulse or Dirac delta function. The delta function is a convenient mathematical artifact that is usually defined as... 

More realistic treatment of the electrostatic interactions of the solvent can be made. The dipolar hard-sphere model is a simple representation of the polar nature of the solvent and has been adopted in studies of bulk electrolyte and electrolyte interfaces [35-39], Recently, it was found that this model gives rise to phase behavior that does not exist in experiments [40,41] and that the Stockmeyer potential [41,42] with soft cores should be better to avoid artifacts. Representation of higher-order multipoles are given in several popular models of water, namely, the simple point charge (SPC) model [43] and its extension (SPC/E) [44], the transferable interaction potential (T1PS)[45], and other central force models [46-48], Models have also been proposed to treat the polarizability of water [49],... 

Unfortunately the development of models is hindered by a lack of reliable experimental data. For example, the rates of ion-transfer reactions measured at different times and by different groups vary widely. Also, it has been suggested that the high interfacial capacities that are measured in certain systems are an experimental artifact [13]. While this is frustrating for the researcher who wants to decide between competing models, it can also be viewed as a sign that the electrochemistry of liquid-liquid interfaces is an active field, where fundamental issues are just being explored. 

Capacitance and surface tension measurements have provided a wealth of data about the adsorption of ions and molecules at electrified liquid-liquid interfaces. In order to reach an understanding on the molecular level, suitable microscopic models have had to be considered. Interpretation of the capacitance measurements has been often complicated by various instrumental artifacts. Nevertheless, the results of both experimental approaches represent the reference basis for the application of other techniques of surface analysis. 

The lattice gas model of Bell et al. [33] neither gave any detailed mechanism of the orientational ordering nor separated the contributions of the headgroup and the acyl chain. Lavis et al. [34] discussed Ref. 33 critically and concluded that the sharp kink point in the isotherm at transition was an artifact of the mean field approximation used. An improved correspondence to experimental data was claimed by the use of the real-space renormalization group method [35]. The same authors returned to the problem [35] and concluded that in addition to the orientation of the molecules, chain melting had to be included in a model which could interpret the phase transitions. 

Are these off-centering real or due to an artifact of the reconstruction The fact that they occur the same way on two unequivalent molecules is already an indication. The best way to completely answer the question is to reconstruct the spin density with a MaxEnt method and a non-uniform prior, a prior in which the density is centered on the nuclei. We have done this reconstruction, taking as a prior for the two molecules the averaged parametrized spin density refined above. The result is shown in Figure 7. The off-centering of the Nl-Ol density and of the negative Cl density is still there, even at the price of a loss of entropy, as it departs from the model. 

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