From a9850c0e4cce33bd18da455bb3ecf9c5c07c8222 Mon Sep 17 00:00:00 2001 From: James Hester Date: Fri, 1 Dec 2023 12:40:20 +1100 Subject: [PATCH] Add some basic information about MSG construct types. (#51) Co-authored-by: James.Hester --- cif_mag.dic | 37 ++++++++++++++++++++++++++++++++++--- 1 file changed, 34 insertions(+), 3 deletions(-) diff --git a/cif_mag.dic b/cif_mag.dic index d37e134..e111c9e 100644 --- a/cif_mag.dic +++ b/cif_mag.dic @@ -2616,12 +2616,43 @@ save_SPACE_GROUP_MAGN _definition.id SPACE_GROUP_MAGN _definition.scope Category _definition.class Set - _definition.update 2016-10-10 + _definition.update 2023-09-13 _description.text ; The data items in this category provide identifying and/or - descriptive information about the relevant magnetic symmetry - group and setting. + descriptive information about the relevant space group (MSG) or + magnetic superspace group (MSSG) and its setting. + + There are 1651 distinct equivalence classes of MSGs, each of + which can be referred to as an MSG "type" following the usage of + this word in the International Tables of Crystallography. Similarly, + there are over 300000 distinct equivalence classes of MSSGs with + up to 3 independent modulations, each of which can be referred to + as an MSSG "type". + + However, it is important to appreciate that the word “type” is commonly + used in an entirely different way in the context of MSGs and MSSGs. + Any magnetic group can be constructed by starting with + a non-magnetic space group F, and then by adding the time-reversal + operation to half or all or none of its elements. The + four ways of doing this give rise to four distinct "construct types", + which we refer to simply as type-1, type-2, type-3, and type-4, + though some refer to type-3 as type-3a and type-4 as and type3b. + + For a type-1 MSG/MSSG, M = F, so that there are no time-reversed elements. + For a type-2 MSG/MSSG, M = F + F1', so that there is both a time-reversed + and a non-time-reversed copy of each element in F. + For a type-3 or type-4 MSG/MSSG, M = D + (F - D)1', where D is an index-2 + subgroup of F whose elements are not time reversed, whereas every + element in F – D (the complement of D in F) is time reversed. + For a type-3 MSG/MSSG, F and D have the same translation subgroup (lattice) + but different point groups. + For a type-4 MSG/MSSG, F and D and the same point groups but different + translation subgroups. + + Ref: 'Magnetic Group Tables' by D.B. Litvin at + http://www.iucr.org/publ/978-0-9553602-2-0. + ISO-MAG tables of H.T. Stokes and B.J. Campbell at http://iso.byu.edu. ; _name.category_id MAGNETIC _name.object_id SPACE_GROUP_MAGN