Configuration interaction semiempirical calculations

Configuration Interaction Semiempirical Calculations MNDO MNDO/d Parameterization of Semiempirical MO Methods PM3 Semiempirical Vibrational Frequencies (Including Scaling). 

AMI Configuration Interaction Semiempirical Calculations Green s Functions and Propagators for Chemistry MNDO MNDO/d PM3. 

Combined Quantum Mechanical and Molecular Mechanical Potentials Combined Quantum Mechanics and Molecular Mechanics Approaches to Chemical and Biochemical Reactivity Configuration Interaction Configuration Interaction Semiempirical Calculations Density Functional Theory (DFT), Hartree-Fock (HF), and the Self-consistent Field Force Fields A General Discussion Hybrid Methods MNDO Quantum Mechanical/Molecular Mechanical (QM/MM) Coupled Potentials Quantum Mecha-nics/Molecular Mechanics (QM/MM). 

Circular Dichroism Electronic Configuration Interaction Semiempirical Calculations Molecular Mechanics Conjugated Systems Spectroscopy Computational Methods Structure Determination by Computer-based Spectrum Interpretation Symmetry in Chemistry. 

Indazoles have been subjected to certain theoretical calculations. Kamiya (70BCJ3344) has used the semiempirical Pariser-Parr-Pople method with configuration interaction for calculation of the electronic spectrum, ionization energy, tt-electron distribution and total 7T-energy of indazole (36) and isoindazole (37). The tt-densities and bond orders are collected in Figure 5 the molecular diagrams for the lowest (77,77 ) singlet and (77,77 ) triplet states have also been calculated they show that the isomerization (36) -> (37) is easier in the excited state. 

Since the ab initio determination of the structural dependence of these quantities is feasable only for small systems, we present here in Section 2 a formalism for a straightforward determination of SOC within the context of configuration interaction (Cl) calculations [9]. This formalism has been implemented for the semiempirical MNDOC-CI method [10] and allows for routine determinations of PE surfaces and SOC surfaces at the same level of theory. Some results for 1,1-, 1,2-, 1,3- and 1,4-biradicals will be given in Section 3 and compared to ab initio results available from the literature. In Section 4 some simple models will be discussed, which allow for a rationalization of the structural dependence of SOC in the biradicals discussed in the previous sections, and finally, in Section 5 some general conclusions are presented concerning the mechanisms of organic triplet photoreactions. 

Two other types of basis set that have been used successfully in hfs calculations are Chipman s contracted [3s,2p] bases, and basis sets based on Slater type orbitals (STOs). The former of these is mainly used in single excitation configuration interaction (CIS) calculations, and are based on a very fortuitous cancellation of errors between method and basis set. The performance of the CIS/[3s,2p] approach lies within 20-25% of experiment. One should recall, though, that once we go to larger molecular systems, the CIS method becomes computationally very demanding, STOs have mainly been used in semiempirical INDO hfcc calculations (STO-SG) and in the density functional theory (DFT) studies of Ishii and Shimitzu (STO-6G). The number of hfcc studies using these basis sets at the ab initio or DFT levels is however to date very limited. 

The primary piece of information obtained from most theoretical calculations is the molecular structure. If homoaromatic interactions are important in a molecule, they may cause the molecule to adopt an unusual geometry. In suitable radicals, ESR evidence has been taken to indicate systems of high symmetry which in turn has been interpreted in terms of homoaromatic interactions (Dai et al., 1990). A computational example of this effect is shown in the semiempirical calculations of Williams and Kurtz (1988) on the bisannelated semibullvalene [108]. Here simple configuration interaction... 

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