The are basically three groups of information that must be supplied for each epitaxial crystals. At the top of the dialog is the type of contact geometry.
Contact adds a contact plane, with the same indices as the guest alignment face (see below), similar to the composition plane of a twin, to the guest crystal at a specified central distance; if you select this option the distance should be entered in the box below the host and guest indices. When the guest crystal is first generated, there may or may not be a crystal face with these indices; if there is, the central distance for the epitaxial plane should be less than that of the normal crystal face. That is, the epitaxial contact plane should truncate the guest crystal, not sit off in space. After the guest crystal is generated, it is rotated and translated so as to contact the host crystal on the specified faces. The specified alignment face of the host crystal should be one which is actually present; otherwise you will get an error message.
Interpenetration does not add a contact plane or try to match faces by translation. It simply establishes the mutual orientation and sets the crystals at a specified center-to-center distance. This distance is in central distance units and should be entered in the box below the host and guest indices. The host crystal remains at the center and the center of the guest is displaced to the specified distance along the perpendicular to the epitaxial face.
In the middle of the dialog, you must specify the orientations of the host and guest crystals, by giving the indices of a face (khl) and a vector [uvw] parallel to this face for the host and for the guest. The face and the vector of the guest will be brought parallel to the corresponding elements of the host. If the vector is not parallel to the face, you will get an error message, and be asked to try again. A vector parallel to a face satisfies the relation uh + vk + wl = 0 .
Note that if the host and guest crystals are the same, and you enter the same face and vector for each, the two crystals are not in parallel orientation. Rather they are in twin orientation, related by rotation of 180 degrees around the alignment vector. Parallel orientation is attained by giving for the face on the guest crystal, the negatives of the indices for the face on the host crystal.
You have the option of specifying translations in addition to that parallel to the intercrystal vector. Movement can be made in either of two mutually perpendicular directions, specified with respect to the epitaxial plane on the host crystal: 1) parallel to the vector in this face which was used to orient the crystal; and 2) perpendicular to this vector, still in the plane of the face. Displacements are specified in central distance units. A right-handed system is formed by 1) the host-crystal vector; 2) the alignment vector in the host crystal face; and 3) the vector perpendicular to the alignment vector in the crystal face.
In the lower part of the dialog, enclosed in two boxes, are the data for the guest crystal. If the current epitaxial crystal is a duplicate of another crystal or has been read in from a file, these data may already be present; if not, you will have to click on the Revise Title/Axes,Revise Symmetry and Revise Forms buttons to enter these data. The dialogs called up are the same as those for entry of the main crystal, i.e. Title/Axes,Symmetry and Forms List. You must always use these buttons to modify epitaxial crystals - when called from the Input1 menu, the Title/Axes,Symmetry and Forms List only modify the main or host crystal.
Clicking on the Rescale button will change all the central distances of the guest crystal by the factor entered in the Rescale factor box. This does not affect the distance entered for a contact plane.