tailieunhanh - Ebook Introduction to computational chemistry (2nd edition): Part 2

(BQ) Part 2 book "Introduction to computational chemistry" has contents: Valence bond methods, relativistic methods, wave function analysis, molecular properties, illustrating the concepts, optimization techniques, statistical mechanics and transition state theory, simulation techniques, qualitative theories,.and other contents. | 7 Valence Bond Methods Essentially all practical calculations for generating solutions to the electronic Schrödinger equation have been performed with molecular orbital methods. The zeroth-order wave function is constructed as a single Slater determinant and the MOs are expanded in a set of atomic orbitals, the basis set. In a subsequent step the wave function may be improved by adding electron correlation with either CI, MP or CC methods. There are two characteristics of such approaches: (1) the one-electron functions, the MOs, are delocalized over the whole molecule, and (2) an accurate treatment of the electron correlation requires many (millions or billions) “excited” Slater determinants. The delocalized nature of the MOs is partly a consequence of choosing the Lagrange multiplier matrix to be diagonal (canonical orbitals, eq. ()), they may in a subsequent step be mixed to form localized orbitals (see Section ) without affecting the total wave function. Such a localization, however, is not unique. Furthermore, delocalized MOs are at variance with the basic concept in chemistry that molecules are composed of structural units (functional groups) which to a very good approximation are constant from molecule to molecule. The MOs for propane and butane, for example, are quite different, although “common” knowledge is that they contain CH3 and CH2 units that in terms of structure and reactivity are very similar for the two molecules. A description of the electronic wave function as having electrons in orbitals formed as linear combinations of all (in principle) atomic orbitals is also at variance with the chemical language of molecules being composed of atoms held together by bonds, where the bonds are formed by pairing unpaired electrons contained in atomic orbitals. Finally, when electron correlation is important (as is usually the case), the need to include many Slater determinants obscures the picture of electrons residing in orbitals. There is an .