Semi-empirical electronic structure method performance

The performance of several semi-empirical (modified neglect of diatomic overlap (MNDO), AMI, PM3, and SAMI) and ab initio (Hartree-Fock (FIF) and MP2/6-31G ) methods for determining structural and electronic factors of a series of isothiazolo[5,4-b]pyridines was compared by Martinez-Merino et al. <1996T8947>. They found that most of the semi-empirical methods calculated reasonable molecular structures when compared to the actual X-ray structures (compounds 3-5) (see, for example. Table 1 for selected bond lengths of compound 3). Flowever, the dipole moments were not reproducible using these methods. 

In contrast to the pseudopotential methods where the Hartree-Fock method is used to construct the subset of orbitals spanning the core and valence carrier subspaces, whereas the calculation in the valence subspace can be performed at any level of correlation accounting, for the overwhelming majority of the semi-empirical methods, the electronic structure of the valence shell is described by a single determinant (HFR) wave function eq. (1.142). 

In fact, none of these points is really so important in a semi-empirical method, because such methods are not designed for performing absolute calculations on single molecules, but rather for studying the trends of physical properties in a series of related compounds. Computational experience shows that the general picture of the electronic structure is not significantly altered whatever method may be chosen, as long as the parameters or the coordinate axes of orbitals are varied within reasonable limits. 

This paper will discuss the state of the art in 3D structure refinement using empirical, semi-empirical and ab initio methods. We believe that the success story of liquid state NMR in protein structure elucidation is going to continue within the solid state (or membrane environment) if chemical shifts can be successfully exploited. Neutron and X-ray diffraction methods owe their success to a simple formula that connects the measured reflex intensities with the nuclear positions or the electron density, respectively. The better we understand how chemical shifts change with the three-dimensional arrangement of atoms, the more reliably we can construct molecular models from our NMR experiments. As we can in principle determine up to six numbers per nucleus if we perform a full chemical shift tensor analysis, we need to address the question whether whole CS tensor or at least its principal values can be used in structure calculations. 

Several studies have focused on extensive MD simulations of Pt nanoparticles adsorbed on carbon in the presence or absence of ionomers [109-113]. Lamas and Balbuena performed classical molecular dynamics simulations on a simple model for the interface between graphite-supported Pt nanoparticles and hydrated Nation [113]. In MD studies of CLs, the equilibrium shape and structure of Pt clusters are usually simulated using the embedded atom method (EAM). Semi-empirical potentials such as the many-body Sutton-Chen potential (SC) [114] are popular choices for the close-packed metal clusters. Such potential models include the effect of the local electron density to account for many-body terms. The SC potential for Pt-Pt and Pt-C interactions provides a reasonable description of the properties of small Pt clusters. The potential energy in the SC potential is expressed by... 

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