Boundary - Unit Cell
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Dialog Box - called from: Boundary Dialog

This option (not intended for molecules) locates the faces of the unit cell, at various choices of central distance.

Each of the sub-options may be chosen as the Default, which is the boundary option used when importing a file (Import File command in the File menu).

You can choose either the primitive or Bravais cell if the lattice is non-primitive. If the Bravais lattice is primitive, or if you elect not to use the primitive cell, the faces will be (100), (010) and (001) and their negatives. If you choose to show the primitive unit cell for non-primitive lattices, the faces will be {110} and {001} for a C-cell, etc.

The central distances for the faces depend on which Sub option is chosen. The central distance is the perpendicular distance from the face to the center of the crystal.

If you choose the -0.5 to 0.5 cell contents sub-option, the faces are at central distances of 0.5009 of the dimensions of the unit cell if the h index is positive, or 0.499 if negative; if h is zero, k is used; and if k is zero, l is used. Thus the volume enclosed is essentially that of the unit cell, with the origin at the center. This is the only option which gives the correct number of atoms for one unit cell.

The 0.0 to 1.0 inclusive sub-option gives you faces at the full d-spacing for positive indices, and zero for negative indices, plus a small increment in both cases. Thus the "unit cell" in this case includes atoms with fractional coordinates 0.0 to 1.0 inclusive.

The 0.0 to 1.0 plus bonds sub-option adds to the central distances of the preceeding sub-option the largest bond distance or coordination distance entered in bond and/or polyhedron input. This allows for completion of the coordination of all atoms in the unit cell. Note that atoms may be included which are not considered to be coordinated to the atoms in the cell, and that the distances used are the limits entered in the bond and polyhedron dialogs, not the actual bond distances.

The -1 to 1 inclusive sub-option gives faces at the full d-spacing, plus a small increment, for both positive and negative indices. While all the previous options give one unit cell or somewhat more, this option gives eight unit cells and somewhat more. Using this option insures that at least one complete molecule, or any other non-polymerized unit, will be included.

The Central Distance sub-option sets all the faces of the unit cell at the given central distance. Atoms "on" the faces are included.

The Get Faces button will calculate d-spacings and list the faces.

A single unit cell will not necessarily include a complete molecule or polymer; see the Locate Groups command in the Transform menu for information on isolating groups.

If you select this boundary option, you can later switch to the Enter Forms option; this preserves the forms and the symmetry (i.e. no symmetry for the faces).

Note that the Translation Limits option, with all limits set to zero, will include atoms with fractional coordinates 0.0 to 0.99999, or in other words the same volume as sub-option -0.5 to 0.5 cell contents, but displaced by half a unit cell on all axes.