Semiempirical basic description

Today we know that the HF method gives a very precise description of the electronic structure for most closed-shell molecules in their ground electronic state. The molecular structure and physical properties can be computed with only small errors. The electron density is well described. The HF wave function is also used as a reference in treatments of electron correlation, such as perturbation theory (MP2), configuration interaction (Cl), coupled-cluster (CC) theory, etc. Many semi-empirical procedures, such as CNDO, INDO, the Pariser-Parr-Pople method for rr-eleetron systems, ete. are based on the HF method. Density functional theory (DFT) can be considered as HF theory that includes a semiempirical estimate of the correlation error. The HF theory is the basie building block in modern quantum chemistry, and the basic entity in HF theory is the moleeular orbital. 

Discussing and comparing the basic adsorption theories presented so far, it is difficult to say which one is fundamentally right. The Polanyi thermodynamic theory neither determines a definite adsorption isotherm equation nor gives a detailed mechanism of the process. Development of this theory for description of adsorption on microporous substances leads to the analytical DR equation but its character is semiempirical. The Langmuir and BET theories introduce the concept of localized mono- and multilayer formed on the energetically homogeneous solid surface. 

The focus in this chapter is on quantum chemical methods. These can be classified as semiempirical, ab initio, and density functional theory (DFT) methods. The latter ones usually involve empirical parameterization and, hence, sometimes are also considered as semiempirical methods. Alternatively, a distinction on the basic quantity - wave function (WF) or electronic density - can be made as wave-function-based methods (WFT) and DFT. In this classification scheme, wave-function-based methods include semiempirical as well as ah initio procedures. Although the impact of semiempirical methods on the progress of quantum chemistry can hardly be overestimated [13], their use now is mainly restricted to very large systems [14]. Thus, in the following the description of wave-function-based procedures will be restricted to ah initio methods. 

---