Molecular orbital methods electron correlation

Ionization potentials calculated for cyclopentadienide ions (1 -8 ) by a semi-empirical molecular-orbital method (AMI) correlate with the observed electron... 

In subsequent studies attempting to find a correlation of physicochemical properties and antimicrobial activity, other parameters have been employed, such as Hammett O values, electronic distribution calculated by molecular orbital methods, spectral characteristics, and hydrophobicity constants. No new insight on the role of physiochemical properties of the sulfonamides has resulted. Acid dissociation appears to play a predominant role, since it affects aqueous solubiUty, partition coefficient and transport across membranes, protein binding, tubular secretion, and reabsorption in the kidneys. An exhaustive discussion of these studies has been provided (10). 

A semi-empirical molecular orbital method for the correlation of charge distributions with 13C shifts in amino acids was described [95]. Plotting of chemical shift parameters versus charge density changes of a-carbon atoms relative to the corresponding atoms in the parent hydrocarbons permits prediction of the chemical shifts of the a-carbons with an accuracy of about 10%. However, the slope (280 ppm per electron) in Fig. 5.12 is... 

Semiempirical molecular orbital methods23-25 incorporate parameters derived from experimental data into molecular orbital theory to reduce the time-consuming calculation of two-electron integrals and correlation effects. Examples of semiempirical molecular orbital methods include Dewar s AMI, MNDO, and MINDO/3. Of the three quantum chemical types, the semiempirical molecular orbital methods are the least sophisticated and thus require the least amount of computational resources. However, these methods can be reasonably accurate for molecules with standard bond types. 

Schrodinger equation have been performed using molecular orbital methods. The zero-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 Cl, MP or CC methods. There are two characteristics of such approaches (1) the one-electron functions, the -MQs, are delocalized over the whole molecule, and (2) an accurate treatment of the... 

The DV-Xa cluster method is a molecular orbital method, assuming a Hartree-Fock-Slater (HFS) approximation. With this method, local electronic structures around hydrogen can be calculated accurately. In this method, the exchange-correlation between electrons, given by the following Slater s... 

Looking at the history of correlation from the fifties to the seventies, one may be led to ask whether correlation has been a scientific fashion or a real problem. Twenty years ago, almost everybody seemed to accept the idea that the simple molecular orbital method (MO) must be completed by configuration interaction (Cl), in order to obtain reliable prediction for the physical properties of atoms and molecules. Ten years ago, electron correlation was considered as the central problem of Quantum Chemistry (7). Nowadays, about 90% of the quantum-mechanical calculations on molecules are performed by the self-consistent-field method (SCF) using more or less extended sets of basis functions, without any consideration of the possible effects of correlation. 

When most quantum ohemists were pursuing improvements in the molecular orbital method, he returned to the valence bond theory and developed the so-called GVB methods that allow electron correlation to be included within a valence bond framework... 

Finally, we can hardly close this review without referring to the work carried out on the correlations between the electronic structure and the carcinogenic activity of molecules. Besides its proper value, the work illustrates the possibilities of utilizing the molecular-orbital method for the understanding of the mode of action of drugs, whether harmful or chemotherapeutic. 

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