Point-Group Symmetry
Previous  Top  Next

Dialog Box: Point-Group Symmetry [Symmetry dialog, Input menu]

This dialog allows direct selection of common point groups through the radio buttons, and also input of any other point group through special files.

VIBRATZ uses two methods to supply the data for point group symmetry, one for crystallographic point groups and one for non-crystallographic groups. The 32 crystallographic groups are those with rotation axes of order 6 or less, excluding 5-fold axes, and excluding linear groups. The operators for these groups are generated internally, in the same way as space groups are generated. These groups may be identified in this dialog by the presence of both Schoenflies (chemical) and Hermann-Mauguin (crystallographic) symbols.

The non-crystallographic groups shown in this dialog include the 5-fold, 7-fold and 8-fold axial groups, the icosahedral groups, the linear groups, and the group D4d (which is actually of order 8). The information for the listed non-crystallographic groups is read from files. These files have the extension.SYM and should be in the subdirectory \SYM in the directory containing the VIBRATZ application (VIBRATZ.EXE).

The methods used in symmetry analysis require that the symmetry elements be fixed in orientation, and the atomic coordinates must conform to these orientations - see the Orientation topic in the Operation - Special Topics section. Usually to make the atoms conform to the required orientation is a matter of rotation about the z-axis (or very rarely other axes), which can be done in the Input Atoms dialog in the Input menu.

In all cases, VIBRATZ uses polynomial basis functions to generate the representations that are used in symmetry analysis. These are taken up to 8th order if necessary, which is adequate for all but one species among the listed groups - xxx of D8h. However, the non-optically-active species of the icosahedral groups have not yet been worked out.

The Read Symmetry from file option reads a file in the same format as the non-crystallographic groups represented by the other radio buttons. See Making Symmetry Files for how to prepare a file for use in VIBRATZ. It is possible to generate a file including the optically active species for any point group for which a character table is available, since the first- and second order infrared and Raman active components are normally given in such tables and these components are the first- and second-order basis functions for the active species.