Screw Axes

To begin with, we need to add some more symmetry operations to our list. These are ones which include translational components as well as rotations or mirrors.

If you add translations to rotation axes, you form what are call screw axes. For an nm screw axis, the rotational component is 360/n degrees, and the translations is m/n of the unit translation along the axis.

So, what happens for a screw axis? The rotation is 360/2=180 degrees, and the translation is 1/2 along the axis. Here's the symbol and a schematic showing the operation.

Here's what happens when the two-fold screw axis lies in the plane of the page.

There are also three-fold, four-fold and six-fold screw axes. The symbols for some of them are as follows:

Note that and are enantiomorphs of each other. One of the screw axes is right handed while the other is left handed.

The other screw axes are , , , and . and are enantiomorphs, as are and .

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copyright © Ron Stenkamp stenkamp@u.washington.edu Most recent update 4/1/03