Imposing Symmetry

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

• Enable point group symmetry: Enable GaussView's symmetry features.

• Constrain to subgroup: Select a point group to which to constrain the structure.

• For: Select All changes to impose symmetry on future structural changes. None disables symmetry constrains.

• Approximate higher-order point groups: Higher symmetry subgroups which might apply.

• Tolerance: Cutoff level below which to consider structural parameters equal.

• Symmetrize button: Impose the selected higher-order point group on the molecular structure immediately.

• Always track point group symmetry: Tells GaussView to continuously compute point group assignments as the geometry changes. Using 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.

Figure 15. Imposing Symmetry on a Molecular Structure
In this 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 Symmetry button (on the toolbar) and the Edit=>Symmetry 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).