Experimental reference data atomization energies

The different emission products which are possible after photoionization with free atoms lead to different experimental methods being used for example, electron spectrometry, fluorescence spectrometry, ion spectrometry and combinations of these methods are used in coincidence measurements. Here only electron spectrometry will be considered. (See Section 6.2 for some reference data relevant to electron spectrometry.) Its importance stems from the rich structure of electron spectra observed for photoprocesses in the outermost shells of atoms which is due to strong electron correlation effects, including the dominance of non-radiative decay paths. (For deep inner-shell ionizations, radiative decay dominates (see Section 2.3).) In addition, the kinetic energy of the emitted electrons allows the selection of a specific photoprocess or subsequent Auger or autoionizing transition for study. 

X-ray photoelectron spectroscopy of atomic core levels (XPS or ESCA) is a very powerful tool for characterization of the chemical surrounding of atoms in molecules. In particular, since the method is very surface sensitive, it is possible to monitor the first stages of the interface formation, i.e., in our case the interaction between individual metal atoms and the polymer. Standard core level bonding energies are well known for common materials. However, in our case, we are studying new combinations of atoms and new types of structures for which there are no reference data available. In order to interpret the experimental chemical shifts it is useful to compare with theoretical estimates of the shifts. 

The quantum mechanical methods described so lar all properly treat the electrons as quantum particles. A vastly simpler approach toward obtaining molecular structures and energies is to treat atoms as classical particles. The potential energy is then just a function of the nuclear coordinates. MM, also referred to as force field methods, involves specifying the various functions used to relate nuclear positions to energy and fitting these functions to experimental data. It is a highly empirical approach, dependent on the choice of reference data, the functional form, and selection of parameters. 

Equation 2 has been applied to more than 110 solid binary compounds, with calculated atomization energies agreeing within an average of about 3% with the experimental values. Before discussing the problem of evaluating n and a, let us consider some typical examples. Basic data for all calculations reported here are taken chiefly from standard references 3, 4, 13, U, 16),... 

The latter free energy can be represented as a surface integral over the solvent accessible surface of the molecule on the basis of a local free energy surface density (FESD) p. This surface density function is represented in terms of a three-dimensional scalar field which is comprised of a sum of atomic increment functions to describe lipophilicity in the molecular environment.The empirical model parameters are obtained by a least squares procedure with experimental log P values as reference data. It is found that the procedure works not only for the prediction of unknown partition coefficients but also for the localization and quantification of the contribution of arbitrary fragments to this quantity. In addition, the... 

Eor atomization energies, we used the standard reference set G2-148 compiled by Curtiss et al. [34], listing experimental data of 148 molecules, compounded of first- and second-row elements. 

As suggested by Kohn, Becke and Parr [ 11J, the use of DFT is preferable (over the traditional methods) for systems with more than 5-10 atoms for which a lower accuracy is acceptable. This does not mean that for system with a less number of atoms the DFT is not reliable. With regard to this fact we mention the recent work of Seminario [6], in which, the DF atomization energy of water, obtained with different exchange-correlation functionals, is well reproduced the results are closer to the experiment than that obtained at CCDS(T) high level of theory which is often chosen as reference when the experimental data are not available. 

Figure 7 The correlation between the F12 contributions obtained at the CCSD(F12) and MP2-F12 levels of theory. The MP2-F12 values were scaled with the interference factor /int = 0.78 determined by minimizing the deviations between the calculated and experimental atomization energies. The CCSD(F12) values contain the correction associated with the improvements made in the (T) contribution. The dashed line refers to the linear function fitted to the presented data and the dotted line reflects the hypothetical perfect correlation.

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