Semiempirical method, for transition

Semiempirical method for transition metal complexes with open d-shells... 

There are a few semiempirical methods for modeling transition metals. These tend to have limited applicability. None has yet become extremely far-ranging in the type of system it can model accurately. 

Semiempirical and ah initio methods for transition metals are compared in... 

Andres, J., Moliner, V., Krechl, J. and Silla, E. Comparison of several semiempirical and ab initio methods for transition state structure characterization. Addition of C02 to CH3NHCONH2, J. Phys. Chem., 98 (1994), 3664-3668... 

Thompson has also studied the effects of solvation on spectra. He has calculated the spectral shifts for the v -> n transition for some simple carbonyl compounds.This work uses an AMI semiempirical method for the atoms in the QM region but is unusual in that it includes polarization on the... 

As with any potential energy surface calculation, it is important that a sufficient level of theory is used in order to get reliable information about reaction mechanisms in heterocyclic chemistry. Molecular mechanics has been used extensively to calculate the endpoints of these reactions and to afford information about thermodynamic control and relief of strain, but the technique is not suited to the general study of transition structures. Semiempirical approaches have found wide application but a cautious user would compare the results with higher-level calculations or experimental observations. Although often successful, at least one study has found limitations in the semiempirical methods for heterocycles containing N-N bonds. ... 

Currently, semiempirical methods for TM compounds are at the development stage, because the lack of accurate experimental data particularly for energies makes it mandatory that accurate ab initio calculations are used for the derivation of the necessaiy parameters. The recent progress in ab initio and DFT studies of TM compounds has given an impetus for further work in the field of semiempirical methods (see MNDO/d PRDDO Semiempirical Methods Transition Metals and SINDOl Parameterization and Application). A similar situation exists for empirical methods, where accurate parameters for MM calculations can only be derived for a well-defined subclass of TM compounds. This is one reason why the combination of quantum mechanical and MM methods has been proposed as an appropriate method for calculating large TM compounds. ... 

PM3/TM is an extension of the PM3 method to include d orbitals for use with transition metals. Unlike the case with many other semiempirical methods, PM3/TM s parameterization is based solely on reproducing geometries from X-ray diffraction results. Results with PM3/TM can be either reasonable or not depending on the coordination of the metal center. Certain transition metals tend to prefer a specific hybridization for which it works well. 

Another related issue is the computation of the intensities of the peaks in the spectrum. Peak intensities depend on the probability that a particular wavelength photon will be absorbed or Raman-scattered. These probabilities can be computed from the wave function by computing the transition dipole moments. This gives relative peak intensities since the calculation does not include the density of the substance. Some types of transitions turn out to have a zero probability due to the molecules symmetry or the spin of the electrons. This is where spectroscopic selection rules come from. Ah initio methods are the preferred way of computing intensities. Although intensities can be computed using semiempirical methods, they tend to give rather poor accuracy results for many chemical systems. 

The optimization of a transition structure will be much faster using methods for which the Hessian can be analytically calculated. For methods that incrementally compute the Hessian (i.e., the Berny algorithm), it is fastest to start with a Hessian from some simpler calculation, such as a semiempirical calculation. Occasionally, dilficulties are encountered due to these simpler methods giving a poor description of the Hessian. An option to compute the initial Hessian at the desired level of theory is often available to circumvent this problem at the expense of additional CPU time. 

Free-radical reactivity of thiazole has been calculated by semiempirical methods, and results free valence and localization energy) have been compared with experimental data. For mono- and dimethylthiazoles the radical localization energy of the unsubstituted position may be correlated with the logarithm of experimental reactivity (180, 200). The value of the slope shows that a Wheland-type complex is involved in the transition state. 

Normally, you would expects all 2p orbitals in a given first row atom to be identical, regardless of their occupancy. This is only true when you perform calculations using Extended Hiickel. The orbitals derived from SCE calculations depend sensitively on their occupation. Eor example, the 2px, 2py, and 2pz orbitals are not degenerate for a CNDO calculation of atomic oxygen. This is especially important when you look at d orbital splittings in transition metals. To see a clear delineation between t2u and eg levels you must use EHT, rather than other semiempirical methods. 

Molecular orbital calculations, whether by ab initio or semiempirical methods, can be used to obtain structures (bond distances and angles), energies (such as heats of formation), dipole moments, ionization energies, and other properties of molecules, ions, and radicals—not only of stable ones, but also of those so unstable that these properties cannot be obtained from experimental measurements." Many of these calculations have been performed on transition states (p. 279) this is the only way to get this information, since transition states are not, in general, directly observable. Of course, it is not possible to check data obtained for unstable molecules and transition states against any experimental values, so that the reliability of the various MO methods for these cases is always a question. However, our confidence in them does increase when (1) different MO methods give similar results, and (2) a particular MO method works well for cases that can be checked against experimental methods. ... 

Smooth COSMO solvation model. We have recently extended our smooth COSMO solvation model with analytical gradients [71] to work with semiempirical QM and QM/MM methods within the CHARMM and MNDO programs [72, 73], The method is a considerably more stable implementation of the conventional COSMO method for geometry optimizations, transition state searches and potential energy surfaces [72], The method was applied to study dissociative phosphoryl transfer reactions [40], and native and thio-substituted transphosphorylation reactions [73] and compared with density-functional and hybrid QM/MM calculation results. The smooth COSMO method can be formulated as a linear-scaling Green s function approach [72] and was applied to ascertain the contribution of phosphate-phosphate repulsions in linear and bent-form DNA models based on the crystallographic structure of a full turn of DNA in a nucleosome core particle [74],... 

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