Boundary - Enter Forms
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Dialog Box - called from: Boundary Dialog

This dialog may apply either to individual atoms, or to molecules (in which case the dialog title will be Enter Forms - Molecules). In the latter case, bonded groups in the unit cell will be identified and the center of each taken as the non-weighted average of atomic positions. Then the boundaries will be determined with respect to the molecular centers.

With this option, the most powerful of the boundary options, you can set the boundaries of a crystal structure to be any rational faces consistent with the axial system defined in the Title/Axes dialog. You do not need to enter every individual face; you can enter any one of the faces of a form (symmetry-equivalent set), and the remainder will be supplied by symmetry. You can use the full symmetry of the crystal class (point group), but you may also elect to use a lower symmetry (center of inversion only, or no symmetry at all) to have more flexibility in the volume defined.

The Revise or Add Forms buttons call up the Add/Revise Form dialog for actual entry of indices and central distance.

The Use individual atom distances option allows the boundary surfaces to be non-planar with respect to the centers of the atoms. If you use this option, the actual central distances for each boundary form are then entered (actually revised) during input for each atom (Revise Atom: Central Distance Tab). However, the distance entered for each form in this dialog is still necessary - it will be used: 1) As the default distance for the boundary faces for each atom. You will need to revise the actual boundary face distances for each form during atom input. 2) As the default for display of the crystal edges. The display forms can be given new distances or changed completely with the Crystal Forms for Display dialog in the Input1 menu. 3) For the initial boundary search limits while locating atoms. The distance entered here in the boundary option should therefore not be too different from the distances for individual atoms to be entered in the Revise Atom: Central Distance Tab in the Input1 dialog.

Individual atom distances are not used in the Enter Forms - Molecules option.

There is a maximum of six forms (not faces) if you choose individual atom boundary distances.

You can disable lattice translation on any of the structure axes, if you wish to use this option to draw a molecule or polymer. Although it is generally simpler to use the Translation Limits option for polymers, you can use the Enter Forms option to show more diverse shapes for a two-dimensional polymer. For example with appropriate choice of faces you can give the sheet a hexagonal shape, while the Translation Limits option will always give a parallelogram shape.

You can use this option for molecules to isolate a section, slab, or any convex polyhedral volume. To prevent the repetition of the structure by translation, you must disable all the lattice translations. It is not necessary to specify enough faces to define a closed polyhedron, although you will get warning messages during calculation if you do not. For example, to effectively slice through the middle of a molecule and just view the back half, give only the "face" (100) at central distance 0.0 (set the face symmetry to No symmetry).

Hint : specifying a center of symmetry for boundary faces, and the simple forms (100), (010) and (001) at central distances of about 10 Angstroms will usually give 500-700 atoms for typical inorganic crystal structures. Central distances of about 7 Angstroms usually gives about 300 atoms. Numbers are usually smaller for organic crystals.