Imposing Symmetry

The Point Group Symmetry dialog is used to specify the desired symmetry for a molecular structure. It is reached via the Tools=>Point Group menu path.

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Imposing Symmetry on a Molecular Structure

The controls in the upper section of the dialog have the following meanings:

The Approximate higher-order point groups area displays and allows you to apply higher symmetry subgroups to the current molecular structure. It contains the following controls:

The Always track point group symmetry: Tells GaussView to continuously compute point group assignments as the geometry changes. Use this item in conjunction with a symmetry constraint setting of None in order to view point group changes continuously as you adjust structural parameters (e.g., use the bond length slider). Normally, the point group is identified only after the change is completed. For very large molecules on slower systems, enabling this item may cause noticeable delays in response time.

In the preceding example, GaussView has identified the current point group of this distorted benzene molecule as C1. It also recognizes that the C2v point group might also be appropriate. Clicking the Symmetrize button will apply the specified point group to the structure. The Tolerance field can be used to tighten or relax the symmetry identification process. In this case, loosening the tolerance allows GaussView to identify the highest applicable point group as D6h.

The Symmetrize button (on the toolbar) and the Edit=>Symmetrize menu item both immediately impose the maximum identifiable point group on the current structure, using the most recent Tolerance setting from the Point Group Symmetry dialog for this molecule (or the default of Normal). They also have the side effect of enabling point group symmetry for the molecule (again, using the previous settings, if any).

The View window status bar will indicate any identified and constrained symmetry for the molecule:

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Symmetry-Related Status Bar Displays

Four views of the symmetry information section of the status bar for the triazine molecule: (a) symmetry has not been turned on; (b) molecule symmetry (D3h) but no symmetry constraint (C1); (c) molecule symmetry (D3h) and structure constrained to D3 symmetry; and (d) molecule symmetry and constraint symmetry are both D3h.