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Dialog Box - called from: Crystal Forms for Display Dialog, Boundary - Enter Forms Dialog

The central distance is the perpendicular distance, in Angstroms, from the center of the structure to the faces of the form in question: the greater the distance, the greater the number of atoms which will be included. Also, if there are many forms, the greater the distance the less prominent the form (the smaller the area of the faces). Decimal values may be used for the central distance, but not for the indices. Use the Next Form button to add a form.

Keep in mind that there may be alterate orientations of symmetry elements with respect to structure axes, and the indices of faces in a standard morphological setting may be different from those in the setting of a particular space group. See section IV-5 for further details of variant orientations.

You can use negative central distances to define a region which does not include the origin. For example, if you want to display the part of the structure from y = 0.25 to y = 0.75 in an orthorhombic crystal with b = 10.0, enter the face (010) at distance 7.5 Angstroms and the face (0-10) at distance -2.5 Angstroms. If the crystal axes are not orthogonal, you should use fractions of the d-spacing for central distances, not axis lengths. The Enter Forms and Default Unit Cell boundary options list d-spacings of the forms, uncorrected for systematic absences; the Slice boundary option will give a d-spacing corrected for systematic absences.

For hexagonal and trigonal crystals with hexagonal axes, four indices, (hkil), are commmonly used, but one of the first three is redundant. In ATOMS, you must omit the third, or i index.