Averaging Named Selections


I o averageu torsion, select the fouratoms of the torsion, name the torsion tor, for exam pie, and then select tor as Lh e quan tity Lo be averaged from the Molecular Dynamics. Averages dialog box.  [c.321]

The electron distribution in a molecule is one of the most important factors influencing its physical, chemical, or biological properties. Detailed information on the electron distribution of a molecule can be obtained from quantum mechanical calculations of various degrees of sophistication (see Section 7.4 and Chapter 7, Sections 2 and 4 in the Handbook). However, chemists have always liked the idea of dissecting the electron distribution of a molecule and assigning the pieces thus obtained to the individual atoms. Thus, the picture of a molecule consisting of atoms carrying partial charges has emerged and was used to explain many phenomena. Intuitively appeahng as this picture is, there is no unequivocal criterion to decide how to assign the electron distribution to the various atoms. The most widely used method is a Mulhken population analysis [26]. However, the values obtained with this method are heavily dependent on the level of quantum mechanical approach being taken and the basis set chosen. Several other methods have been proposed but none has yet found general acceptance. Thus, it has to be concluded that no unanimously accepted method for the definition of partial atomic charges is available, making it diflScult, when using any other method for the calculation of charges, to select widely accepted data for comparison. In the end, any method for the calculation of partial atomic charges can only be judged by how valuable the resulting charges are in reproducing experimental data.  [c.329]


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