Model suspicion

Starting with Bohr s version of 1913, the evolution of this model was examined in an attempt to highlight the assumptions and approximations that were made at each stage. As in the case of many other papers in this volume, there is an educational motivation for raising these questions, especially in view of the central role of the atomic orbital model at all levels of chemical education. My suspicion is that many chemical educators do not appreciate the extent to which this model is an approximation and the conditions under which it ceases to be applicable. 

The model proposed by Bowden and Tabor has been regarded as the most successful one for presenting a simple and logical theory capable of explaining the Amontons friction law. However, suspicions concerning the two fundamental assumptions in the model were gradually aroused over past years. Friction has been attributed, in Bowden and Tabor s model, to the adhesion between asperities in contact and torn-off of the adhesive junctions when the shear stress exceeds a critical value. This implies that plastic flow and surface destruction may occur at the moment of slip, and that friction is dominated by the shear strength of the adhesive conjunctions, which is material dependent. 

Such an analysis indicates that the zero-sink assumption must be used with extreme caution if accurate flux calculations are required at the local root level. Potassium, for example, is close to the limiting value of A, for the zero sink assumption to be fulfilled, and simulations with larger roots or larger buffer powers could well lead to inaccurate simulation results. Any zero-sink model involving nitrate should be treated with some suspicion. The zero-sink assumption is also widely used in root architecture models (see later). 

To test our suspicion, we used a point charge model and ab initio calculated atomic charges in order to evaluate the electrostatic energy of 1,2-ds and 1,1-difluoro-ethylene. 

A. The trend to increase more than f- oo))ocTis expected from harmonic vibrations with increasing T. The latter fact leaves the suspicion that the exposed DDH model still contains some oversimplification or wrong assumptions. 

The highly empirical nature of chemometric tools can lead to customer suspicion regarding the efficacy of chemometric models and... 

The input parameters can be taken from measurements on model systems. If the structure of the catalyst is known and one has a suspicion which is the active crystal surface and do experiments on this model with all the chemical phase, then one can isolate this phase, usually in the form of a single and structural characterization tools available in surface science. 

So, while there is growing interest in hybrid models of all sorts (as discussed in more detail in the next chapter), the choice of a mixed solvent model is not necessarily intrinsically better than a pure explicit or pure implicit model. In general, unless there is a strong suspicion that first-solvation-shell effects are drastically different from those more typically encountered, there is no particularly compelling reason to pursue a mixed modeling strategy. An example... 

In process analytics, chemometrics can undergo a staling process during the early phases of implementation, the benefits of the technology are most apparent (e.g. interference rejection, multi-component analysis capabilities, and fault monitoring), but over time, as conditions change and new interferences appear, the models can lose their accuracy. This can lead to suspicions of bait and switch. ... 

The chief merit of the ionic theory is the ease with which quantitative calculations can be performed within a simple electrostatic approach. These will be discussed in Chapter 5. Even if it could be conclusively proven that crystals such as NaCl do not contain ions, chemists would be reluctant to discard the ionic description. The model even has successes-and this should arouse suspicion - in situations where few chemists would seriously entertain the real existence of ions. For example, the 4d and 5d elements tend to exhibit high oxidation states more readily than their counterparts in the 3d series. This can be explained if we note (see Section 4.3) that the energies required to attain highly ionised states are much smaller for the heavier atoms, provided that we are prepared to believe that compounds like WF6, Re03 and 0s04 are ionic ... 

Using this formula, FQ is expected to accumulate 50-times more than CQ in the parasitic DV. However, both mathematical models could be subject to suspicion of bias as special transport mechanism(s) could not be excluded and are even strongly suspected. Indeed, the binding to free heme should contribute to uptake [130, 131]. 

These are the biological memories that we are familiar with, and if it were not for the mathematical model probably we wouldn t feel any need to look for others. According to that model, however, a multicellular system can have a collective memory , and this does raise the suspicion that a more general memory could exist. More precisely it makes us think about a supracellular memory to which all the body s apparatuses contribute, a true body memory. 

Why should one go to all this trouble and do all these integrations if there are other, less complex methods available to theorize about ionic solutions The reason is that the correlation function method is open-ended. The equations by which one goes from the gs to properties are not under suspicion. There are no model assumptions in the experimental determination of the g s. This contrasts with the Debye-Htickel theory (limited by the absence of repulsive forces), with Mayer s theory (no misty closure procedures), and even with MD (with its pair potential used as approximations to reality). The correlation function approach can be also used to test any theory in the future because all theories can be made to give g(r) and thereafter, as shown, the properties of ionic solutions. 

The fact that the O2+ measurements exceed the Langevin-Giomousis-Stevens (LGS) capture cross section raise a suspicion that these GIB cross sections may be too high. On the other hand, the simple LGS capture model may not be applicable in a case where rapidly dissociating states are produced, as is the case in the 02" " + Na system. Inspection of the O2/O2+ vibrational coordinates on a charge transfer energy scale... 

The sensitivity tests in the model qualification studies confirmed some of the suspicions that we expressed earlier (87)—Le, that the quasi-equilibrium relationship between ozone and the oxides of nitrogen does not seem to be recovered in the data. The largest departures are for the highest ozone levels. Attempting to represent the physical setting consistently, we find it difficult to use the measured ultraviolet intensity to account simultaneously for the observed ozone buildup and NO-conversion simultaneously. The inconsistency even appears in the initial behavior of a modeling run as a transient induction process that rapidly... 

---