Facilities for handling twins inCRYSTALS
S. Parsons, Richard Ian Cooper · Acta Crystallographica Section A Foundations of Crystallography · 2012
A twinned crystal is an aggregate in which different domains are joined together according to a specific symmetry operation, the twin law.The diffraction patterns derived from different domains are rotated, reflected or inverted with respect to one another, depending on the nature of the relationship between the different domains, and weighted according to the quantity of a particular domain present in the crystal. 1 The diffraction pattern measured during data collection is a superposition of all of these.Reflections from different domains may overlap and different refinement programs differ in the way this is specified.The aim of this talk is to describe how twins are handled in the refinement program CRYSTALS. 2 In CRYSTALS the user provides up to nine twin law matrices, and each reflection carries a flag indicating which domains contribute to an intensity observation.This format can handle situations where all observations contain contributions from all domains (e.g.merohedral twins) or where only some reflections are affected by twinning (e.g. in non-merohedral twinning).If twinning has been taken into account during integration an HKLF5 format reflection file can be imported into the program.Once a set of twin laws has been defined domain scale factors can be refined with a constraint that their sum is equal to unity.CRYSTALS also has a facility called ROTAX 3 which is able to assist in identification of twin laws.It is most useful for non-merohedral twins where only certain zones of data contain contributions from more than one domain.If the crystal structure is refined without taking twinning into account F calc these reflections is systematically lower than F obs .Matrices which transform the indices of these reflections to integers are possible twin laws.