Vibrational frequencies semiempirical

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

AMI Divide and Conquer for Semiempirical MO Methods Green s Function Ionization Potentials in Semiempirical MO Theory Hydrogen Bonds Semiempirical Methods Localized MO SCF Methods MNDO/d Parameterization of Semiempirical MO Methods PM3 Polymers Semiempirical Calculations Population Analyses for Semiempirical Methods Semiempirical Vibrational Frequencies (Including Scaling). 

Vibrational frequencies from semiempirical calculations tend to be qualitative in that they reproduce the general trend mentioned in the introduction here. However, the actual values are erratic. Some values will be close, whereas others are often too high. SAMI is generally the most accurate semiempirical... 

Molecular descriptors must then be computed. Any numerical value that describes the molecule could be used. Many descriptors are obtained from molecular mechanics or semiempirical calculations. Energies, population analysis, and vibrational frequency analysis with its associated thermodynamic quantities are often obtained this way. Ah initio results can be used reliably, but are often avoided due to the large amount of computation necessary. The largest percentage of descriptors are easily determined values, such as molecular weights, topological indexes, moments of inertia, and so on. Table 30.1 lists some of the descriptors that have been found to be useful in previous studies. These are discussed in more detail in the review articles listed in the bibliography. 

Scott, A. P., Radom, L., 1996, Harmonic Vibrational Frequencies An Evaluation of Hartree-Fock, Moller-Plesset, Quadratic Configuration Interaction, Density Functional Theory, and Semiempirical Scale Factors , J. Phys. Chem., 100, 16502. 

The correlation of the recorded IR spectrum with semiempirical calculations and the reversible cleavage into oxohexapentaenylidene (97) and carbon monoxide give a consistent picture for both, dione 96 and carbene 97. The spectrum of 97 correlates with scaled ab initio vibrational frequencies.123 C60 97 should be a triplet molecule. Indeed, Weltner et al.124 observed during the experiment concerning C4O T-88 another triplet ESR signal which they ascribed to T-97. 

One of the main aims of such computations is the prediction and rationalization of the optoelectronic spectra in various steric and electronic environments by either semiempirical or ab initio methods or a combination of these, considering equilibrium structures, rotation barriers, vibrational frequencies, and polarizabilities. The accuracy of the results from these calculations can be evaluated by comparison of the predicted ionization potentials (which are related to the orbital energies by Koopman s theorem) with experimental values. 

Scott AP, Radom L(1996) Harmonic vibrational frequencies An evaluation of Hartree-Fock, Moller-Plesset, quadratic configuration interaction, density functional theory, and semiempirical scale factors. J Phys Chem 100 16502-16513... 

The analogies in semiempirical (SE) methods to MM procedures for developing a forcefield arise from the need to fit experimental values to parameters in equations. In SE parameterization heats of formation, geometric parameters, etc. are used to adjust the values of integrals in the Hamiltonian of quantum-mechanical equations. In MM vibrational frequencies, geometric parameters, etc. are used to... 

The recent progress of computational quantum chemistry has made it possible to get realistic descriptions of vibrational frequencies for polyatomic molecules in solution. The first attempt in this direction was made by Rivail el al. [1] by exploiting a semiempirical QM molecular model coupled with a continuum description of the medium to compute vibrational frequency shifts for molecular solutes. An extension to ab initio QM methods, including the treatment of electron correlation effects and electrical and mechanical anharmonicities, was then proposed [2 1] in the framework of the Polarizable Continuum Model (PCM). 

Scott, A. R Radom, L. Harmonic vibrational frequencies an evaluation of Hartree-Fock, Moller-Plesset, quadratic configuration interaction, density functional theory, and semiempirical scale factors, J. Phys. Chem. 1996,100, 16502-16513. Halls, M. D. VeUcovski, J. Schlegel, H. B. Harmonic frequency scaling factors for Hartree-Fock, S-VWN, B-LYP, B3-LYP, B3-PW91 and MP2 with the Sadlej pVTZ electric property basis set, Theor. Chem. Acc. 2001,105, 413-421. 

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