Semi-empirical method, selection

To limit a semi-empirical calculation to part of a molecule, select the atoms of interest. Then choose Bttend to sp3 on the Select menu to terminate correctly the selected group of atoms. Only the selected atoms move, but the frozen atoms influence the calculation. This is now possible with all the semi-empirical methods in HyperChem. 

The algorithms of the mixed classical-quantum model used in HyperChem are different for semi-empirical and ab mi/io methods. The semi-empirical methods in HyperChem treat boundary atoms (atoms that are used to terminate a subset quantum mechanical region inside a single molecule) as specially parameterized pseudofluorine atoms. However, HyperChem will not carry on mixed model calculations, using ab initio quantum mechanical methods, if there are any boundary atoms in the molecular system. Thus, if you would like to compute a wavefunction for only a portion of a molecular system using ab initio methods, you must select single or multiple isolated molecules as your selected quantum mechanical region, without any boundary atoms. 

Semi-empirical methods are zero-dimensional, just as force field mefhods are. There is no way of assessing the reliability of a given result within the method. This is due to the selection of a fixed (minimum) basis set. The only way of judging results is by comparing the accuracy of other calculations on similar systems with experimental data. 

Regarding current ab initio calculations it is probably fair to say that they are not really ab initio in every respect since they incorporate many empirical parameters. For example, a standard HF/6-31G calculation would generally be called "ab initio", but all the exponents and contraction coefficients in the basis set are selected by fitting to experimental data. Some say that this feature is one of the main reasons for the success of the Pople basis sets. Because they have been fit to real data these basis sets, not surprisingly, are good at reproducing real data. This is said to occur because the basis set incorporates systematical errors that to a large extent cancel the systematical errors in the Hartree-Fock approach. These features are of course not limited to the Pople sets. Any basis set with fixed exponent and/or contraction coefficients have at some point been adjusted to fit some data. Clearly it becomes rather difficult to demarcate sharply between so-called ab initio and semi-empirical methods.4... 

Pericas and coworkers173 studied the endo selective reactions of 1-alkoxy-l,3-butadienes and 1-alkoxy-l,3-octadienes with maleic anhydride. They found that the trans-2-phenyl-cyclohexan-l-ol and 3-exo-(neopentyloxy)isobornan-l-ol based chiral dienes induced the highest facial selectivities. The relative transition state energies for the formation of the different diastereomers were calculated using semi-empirical methods (AMI). 

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. 

XPS and UPS for polyethylene have been reported while there are some discrepancies among them [60,61,65]. Semi-empirical methods provided some agreement with experiment in selected energy ranges, but failed in others [65]. HF offered a better description of the general features [92-94], but the difference between theory and experiment varied from 2 eV to 5 eV in the energy range of the valence bands. Recently, we applied MBPT(2) to interpret the measured XPS and UPS data and help resolve the discrepancies [66]. 

Martin dt at. (70) have published a detailed study of phenylethylamine, utilizing EHT, CNDO, INDO, PCILO and AI. These authors used semi-empirical methods to generate a series of conformational energy surfaces and then used an AI procedure with a large basis set for a few selected points. EHT, CNDO and INDO all indicated a minimum for x at 90° EHT predicts a rotational barrier of 7 kcal/mol, while CNDO and INDO show a barrier of about 2 kcal/mol. PCILO gives a rotational barrier of about 2 kcal/mol, but it shows a very broad minimum for Xi, ranging from 45° to 135°. With respect to rotation about X2 these authors found the extended... 

In semi-empirical methods for the determination of electronic structure, the Schrodinger equation is written in terms of parameters chosen to agree with selected experimental quantities. 

He also pioneered the use of quantum-mechanical calculations by developing very reliable semi-empirical procedures which for the first time produced molecular resonance energies for numerous ben-zenoid hydrocarbons. We have seen in this review the remarkably high degree of internal consistency of his RE calculations on a selection of benzenoids. Despite these successful applications of semi-empirical methods, his contributions apparently were not appreciated in some theoretical chemistry circles. At the same time, M. J. S. Dewar showed at best limited interest in the currently popular ab initio calculations and was disappointed, if not bitter, about difficulties he encountered in attempts to publish his work because f was not an official quantum chemist, f often had major problems getting my work published, even when it was really important, and fam sure that many others have found themselves in the same boat 929.930... 

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