There are three modes of operation:

Select Mode: program takes a SHELX HKLF file and selects a specified number of reflections at random and creates a file for input into Duisenberg's DIRAX program. Each reflection is only selected once per list created. There are several criteria to evaluate the suitability of reflections.

Reindex Mode: This mode is used when the data was collected on a composite cell. The user is prompted for the orientation matrix with which the data was collected, the number of twin components, and a twin law (or the orientation matrices for the single domain fragments). The program reads a SHELX HKLF 4 file and outputs a SHELX HKLF 5 file.

Overlap Mode: This mode is used when the data was collected on a single fragment of the twin and the orientation matrix(cies) (or twin law) is (are) known for the other twin fragment(s). The program takes a SHELX HKLF 4 file and produces a SHELX HKLF 5 file.

For a phase transition between a high symmetry phase of non-magnetic point group symmetry G and low symmetry phase of non-magnetic point group F, this program calculates:

1. The coset and double coset decomposition of the point group G with respect to the point group subgroup F.
2. Indexes the domain states S(i) which arise in such a transition.
3. Calculates the point group symmetry F(i) of each domain state.
4. Calculates the permutations of the domain states S(i) under the action of elements g of the point group G.
5. Calculates all domain pair {S(i),S(j)} in each class of domain pairs.
6. Calculates a representative domain pair {S(1),S(j)} from each class of domain pairs.
7. Calculates for each representative domain pair its symmetry group J(1,j) and its twinning group K(1,j).
8. Calculates the domain pair {S(1), gS(1)} and its symmetry and twinning groups.
9. Calculates the symmetry and twinning groups of an arbitrary domain pair {S(i),S(j)}.

1. Single crystal data sets can be tested if they are subject to twinning by merohedry.
2. The volume fractions of the different twin domains can be estimated.

1. G. DONNAY and J.D.H. DONNAY, CANADIAN MINERALOGIST, 12, 422-425 (1974)
2. T. ARAKI, Z. KRISTALLOGR. 194, 161-191 (1991)

1. A. LE BAIL, Y. GAO and C. JACOBONI, EUR. J. SOLID STATE INORG. CHEM. 26, 281-288 (1989)
2. A. HEMON, A. LE BAIL and G. COURBION, J. SOLID STATE CHEM. 81, 299-304 (1989)
3. A. LE BAIL, Y. GAO, J.L. FOURQUET and C. JACOBONI, MAT. RES. BULL. 25, 831-839 (1990)
4. M.C. MORON, A. LE BAIL and J. PONS, J. SOLID STATE CHEM. 88, 498-504 (1990)
5. J.J. ROUSSEAU, B. BOULARD, H. DUROY and J.L. FOURQUET, EUR. J. SOLID STATE INORG. CHEM. 27, 913-924 (1990)
6. Y. GAO, J. GUERY, A. LE BAIL and C. JACOBONI, J. SOLID STATE CHEM. 98, 11-24 (1992)
7. M. LEBLANC, G. FEREY and R. DE PAPE, ACTA CRYST. C41, 657-660 (1985)
8. A. LE BAIL, C. JACOBONI, M. LEBLANC, R. DE PAPE, H. DUROY and J.L. FOURQUET, J. SOLID STATE CHEM. 77, 96-101 (1988)
9. J. OSTORERO and M. LEBLANC, ACTA CRYST. C46, 1376-1378 (1990)

• Definition of a twin
• Different kinds of twins
• Twin refinement in SHELXL
• Warning signs of twinning
• Example of a merohedral twinned structure
• Racemic twinning
• Example for a pseudo-merohedral twinned structure
• Example for twinning by reticular merohedry