Extended Huckel method strengths

Very small differences in bond strength between different silica polymorphs were found. Since the Extended Huckel Method is too approximate to calculate reliably the small diffferences in energy between low-density material, containing micropores, and high-density material without micropores, work was initiated to study the same problem but now with two rigorous techniques that are currently considered to be state of the art. 

According to q.(2.31) only the correction due to double counting of the electron- electron interactions on the same atom has to be explicitly accounted for. Within the Extended-Huckel method this correction is usually ignored. As we will sec later, depending on orbital occupation the contribution to the chemical bond strength... 

Thermodynamic arguments cannot be used to derive the reaction path of dissociating CO. We will show below that the minimum-energy reaction path is that path which leads to maximum electron population of the bond weakening CO 2t orbitals. Dissociation can be studied by means of the AS ED method, introduced by Anderson I and discussed in section (2.2). In order to predict equilibrium distances and dissociation paths, the total bond strength has to be considered to be the sum of a repulsive and an attractive part. In the ASED method the attractive path of the chemical bond strength is computed with the aid of the Extended-Huckel method. Anderson has developed empirical expressions for the repulsive part of the chemical band. If used with care, they yield remarkably useful results. 

The input of the problem requires total analytically measured concentrations of the selected components. Total concentrations of elements (components) from chemical analysis such as ICP and atomic absorption are preferable to methods that only measure some fraction of the total such as selective colorimetric or electrochemical methods. The user defines how the activity coefficients are to be computed (Davis equation or the extended Debye-Huckel), the temperature of the system and whether pH, Eh and ionic strength are to be imposed or calculated. Once the total concentrations of the selected components are defined, all possible soluble complexes are automatically selected from the database. At this stage the thermodynamic equilibrium constants supplied with the model may be edited or certain species excluded from the calculation (e.g. species that have slow reaction kinetics). In addition, it is possible for the user to supply constants for specific reactions not included in the database, but care must be taken to make sure the formation equation for the newly defined species is written in such a way as to be compatible with the chemical components used by the rest of the program, e.g. if the species A1H2PC>4+ were to be added using the following reaction ... 

Spectrophotometric methods of obtaining thermodynamic parameters of ionic equilibria often require comparatively high ionic strengths. This makes desirable an extended Debye-Huckel equation, or experimental values of the relevant ionic activities. In non-aqueous solvents such information is frequently still unavailable (see Chapter 2). 

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