Dewar parameters

MINDO/3, MNDO, and AM 1 wxrc developed by the Dervar group at the University of i exasat Austin. This group ehose many parameters, such as heats of formation and geometries of sample molecules, to reproduce experimental quantities. The Dewar methods yield results that are closer to experiment than the CN DO and IN DO methods. 

L sirif Lhc above asymptotic forms of the two-ceiiLer Lwo-dccLron integrals, the paramelers A. and can he derived. C. ertainly, parameter A. is different for different orbitals even though they reside on the same atom, Dewar used AM to represent the parameter A obtained via AD to represent the parameter A obtained via and AQ to represent the parameter A obtained from Hpp-... 

Dewar s treatment of transition state structure, using reactivity numbers, has the logical defect that in the intermediate kinds of transition states for which it provides evidence the electron localisation is only partial. However, in obtaining the values of the reactivity numbers (which are approximate localization energies), the process of localization is considered to be complete thus, values of parameters which strictly are relevant only to the Wheland type of transition state are incorporated into a different model. ... 

These are the only differences between the MNDO and AMI functional form. Dewar s group regenerated AMI parameters for the elements H, B, C, N, 0, F, Al, Si, P, S, Cl, Zn, Ge, Br, and Sn and found that the main gains in AMI over MNDO were the ability to reproduce hydrogen bonds and the promise of better activation energies for reactions. AMI does not significantly change the computation time compared with MNDO. 

The group centred around M. J. S. Dewar has used a combination of (2) and (3) for assigning parameter values, resulting in a class of commonly used methods. The molecular data used for parameterization are geometries, heats of formation, dipole moments and ionization potentials. These methods are denoted modified as their parameters have been obtained by fitting. 

The utihty stream gets started at operating temperature and flow rate. In the following experiments, the utihty stream is heated so as to initiate the reaction. The main and secondary process tines are fed with water at room temperature and with the same flow rate as one of the experiments. Once steady state is reached, operating parameters are recorded. Process tines are then fed with the reactants, hydrogen peroxide and sodium thiosulfate. At steady state, operating parameters are recorded, and a sample of a known mass of reactor products is introduced in the Dewar vessel. Temperature in the Dewar vessel is recorded until equilibrium is reached, that is, until the reaction ends. This calorimetric method is aimed at calculating the conversion rate at the product outlet and thus the conversion rate in the reactor. The latter is also determined by thermal balances between process inlet and outlet of the reactor. Finally, the reactor is rinsed with water. This procedure is repeated for each experiment... 

The ESR measurements were made at RT or 77 K on a Varian E-9 spectrometer (X-band), equipped with an on-line computer for data analysis. Spin-Hamiltonian parameters (g and A values) were obtained from calculated spectra using the program SIM14 A [26]. The absolute concentration of the paramagnetic species was determined from the integrated area of the spectra. Values of g were determined using as reference the sharp peak at g = 2.0008 of the E i center (marked with an asterisk in Fig. 3) the center was formed by UV irradiation of the silica dewar used as sample holder. 

One example showing a serious discrepancy of the frontier electron method was reported by Dewar H8,ii9). This is 10,9-borazaphenanthrene, and the value of / -B) was reported to have been calculated by the Pople method, but the parameters usyd were not indicated. Fujimoto s calculation by the Pariser-Parr-Pople method 120>, in perfect disagreement with Dewar s, gives the most reactive position as 8, which parallels experiment. The ambiguity involved in the integral values adopted seems to be serious, so that the establishment of parametrization for boron heterocycles is desirable. 

The second philosophy holds that one should seek agreement with experiment. Therefore the parameters in this case implicitly include some correlation effects. Examples of this approach are MINDO/3 (Bingham et al., 1975) and MNDO (Dewar and Thiel, 1977). [In all of these techniques, C stands for complete, I for intermediate, N for neglect, and DO for differential (diatomic differential in MNDO) overlap differential overlap is the overlap between different orbitals on the same or different atoms.]... 

The term "semi-empirical" has been reserved commonly for electronic-based calculations which also starts with the Schrodinger equation.9-31 Due to the mathematical complexity, which involve the calculation of many integrals, certain families of integrals have been eliminated or approximated. Unlike ab initio methods, the semi-empirical approach adds terms and parameters to fit experimental data (e.g., heats of formation). The level of approximations define the different semi-empirical methods. The original semi-empirical methods can be traced back to the CNDO,12 13 NDDO, and INDO.15 The success of the MINDO,16 MINDO/3,17-21 and MNDO22-27 level of theory ultimately led to the development of AMI28 and a reparameterized variant known as PM3.29 30 In 1993, Dewar et al. introduced SAMI.31 Semi-empirical calculations have provided a wealth of information for practical applications. 

Demiralp E, Cagin T, Goddard WA(1999) Morse stretch potential charge equilibrium force field for ceramics Application to the quartz-stishovite phase transition and to silica glass. Phys Rev l tt 82 1708-1711 Dewar MJS (1977) Ground states of molecules. The MNDO method. Approximations and parameters. J Am Chem Soc 99 4899-4907... 

Third, with recent advances made in theoretical and computational quantum mechanics, it is possible to estimate thermochemical information via electronic structure calculations (Dewar, 1975 Dunning et al., 1988). Such a capability, together with the transition state theory (TST) (Eyring, 1935), also allows the determination of the rate parameters of elementary reactions from first principles. Our ability to estimate activation energy barriers is... 

MNDO. Despite its success, Dewar recognized certain weaknesses (6) in MINDO/3 due to the INDO approximation, such as the inability to model lone pair - lone pair interactions. Additionally, due to the use of diatomic parameters in MINDO/3, it was increasingly difficult to extend MINDO/3 to additional elements. Because of this, Dewar began working on a new model based on the better NDDO approximation. 

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. 

Screening The carousel containing the crystals to be screened is removed from the storage dewar and placed into the dewar of the MarCSC samplechanging robot. A protocol file that contains the screening parameters for each crystal is generated automatically by extracting the necessary... 

Some functionals, in particular B3LYP, incorporate parameters that are optimized by comparisons between DFT and experimental results. Thus, B3LYP calculations are, like the MINDO, MNDO, and AMI methods that were developed by the late Michael Dewar in the 1970s and 1980s, semiempirical in nature. In this sense, B3LYP could be called the AMI of the twenty-first century . 

Table 27. Reactivity Parameters of the Ring Atoms in Hexahelicene Nr (Dewar number), Fr (free valence number), pr (Mulliken overlap population) and Lr (localization energy), Rates of Protiodetritiation Relative to 9-T-Phenanthrene (kre[) and Corresponding Partial Rate Factors (f)...

The first general parameterization to be reported by Dewar and co-workers was a third-generation modified INDO model (MINDO/3 Bingham, Dewar, and Lo, 1975). Some of the specific modifications to the INDO framework included the use of different t exponents in s and p type STOs on the same atom, the definition of pair parameters /Iab between two atoms A and B that were not averages of atomic parameters (actually, four such parameters... 

J. P. Stewart, subsequently left Dewar s labs to work as an independent researcher. Stewart felt that the development of AMI had been potentially non-optimal, from a statistical point of view, because (i) the optimization of parameters had been accomplished in a stepwise fashion (thereby potentially accumulating errors), (ii) the search of parameter space had been less exhaustive than might be desired (in part because of limited computational resources at the time), and (iii) human intervention based on the perceived reasonableness of parameters had occurred in many instances. Stewart had a somewhat more mathematical philosophy, and felt that a sophisticated search of parameter space using complex optimization algorithms might be more successful in producing a best possible parameter set within the Dewar-specific NDDO framework. 

There is a possibility that the PM3 parameter set may actually be the global minimum in parameter space for the Dewar-NDDO functional form. However, it must be kept in mind that even if it is the global minimum, it is a minimum for a particular penalty function, which is itself influenced by the choice of molecules in the data set, and the human weighting of the errors in the various observables included therein (see Section 2.2.7). Thus, PM3 will not necessarily outperform MNDO or AMI for any particular problem or set of problems, although it is likely to be optimal for systems closely resembling molecules found in the training set. As noted in the next section, some features of the PM3 parameter set can lead to very unphysical behaviors that were not assessed by the penalty function, and thus were not avoided. Nevertheless, it is a very robust NDDO model, and continues to be used at least as widely as AMI. 

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