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*To*:*Multiple recipients of list <coredmg@iucr.org>***Subject**:**Re: CIF description of twinning****From**:**"George M. Sheldrick" <gsheldr@shelx.uni-ac.gwdg.de>***Date*:*Wed, 9 May 2001 11:58:45 +0100 (BST)**Reply-To*:*coredmg@iucr.org*

Dear Brian, Many thanks for your CIF proposals. I will have a good look at them, but I doubt if I will be able to get back to you for about three weeks because I am going to two meetings in the US and must deal with a lot of administrative chores before I go. I am very grateful that you are giving me early warning, you have to bear in mind that even when I release a new version of my programs (which I do at about 5 year intervals) it takes two or three years to persuade most of the the 2000+ groups using them to upgrade. Best wishes, George ========================================================================= Brian McMahon wrote: > > Dear Colleagues > > I am sending this mailing to two discussion lists: the CIF Core dictionary > management group, and the structural chemistry list recently established > by Judy Flippen-Anderson. There has been some discussion recently among > structural chemists that it is rather difficult to extract information about > (or even identification of) twinned crystal structures, for example from the > Cambridge Structural database. One way of addressing this is of course to > have community standard ways of describing twinning in database entries. > Judy plans to canvass the strchem members for their views on what is needed. > > I recalled our earlier discussions on twinning in the coreDMG which prompted > the proposal of the following definitions, and so I offer them (a) as a > catalyst for the strchem discussion and (b) as a reminder of unfinished > coreDMG business (this is the third of my promised topics for extending the > Core dictionary to version 2.3). > > The listing here incorporates Gotzon's initial proposals and Howard's > response. Readers new to the discussion will find the complete discussion > with carefully argued rationale at > http://www.iucr.org/iucr-top/lists/coredmg/msg00123.html > http://www.iucr.org/iucr-top/lists/coredmg/msg00143.html > http://www.iucr.org/iucr-top/lists/coredmg/msg00146.html > > I would suggest we proceed by inviting the strchem list to discuss their > perceived requirements for a while (in general terms rather than proposing > CIF definitions as such). I shall monitor the strchem discussion and bring > back any discussion to the coredmg list for consideration in a subsequent > review of these CIF names. > > The proposed definitions are: > > =============================================================================== > data_diffrn_reflns_twin_id > _name '_diffrn_reflns_twin_id' > _category diffrn_reflns > _type char > _list yes > _list_link_child '_refine_ls_twin_id' > _definition > ; A unique identifier for each domain state whose presence > is assumed to be permissible. > ; > > data_diffrn_reflns_twin_matrix_ > loop_ _name '_diffrn_reflns_twin_matrix_11' > '_diffrn_reflns_twin_matrix_12' > '_diffrn_reflns_twin_matrix_13' > '_diffrn_reflns_twin_matrix_21' > '_diffrn_reflns_twin_matrix_22' > '_diffrn_reflns_twin_matrix_23' > '_diffrn_reflns_twin_matrix_31' > '_diffrn_reflns_twin_matrix_32' > '_diffrn_reflns_twin_matrix_33' > _category diffrn_reflns > _type numb > _list yes > _list_reference '_diffrn_reflns_twin_id' > _definition > ; The set of twin-law matrices in which the ith matrix transforms > the Miller indices of a reflection in the first domain state > into those of the twin-equivalent reflection resulting from > the ith domain state. A twin law transformation of Miller > indices is written in matrix form as: > > (h~i~) (m~11~ m~12~ m~13~) (h~1~) > (k~i~) = (m~21~ m~22~ m~23~) (h~2~) > (l~i~) (m~31~ m~32~ m~33~) (h~3~) > > The twin matrices are referred to the basis reciprocal to > that derived from _cell_length_ and _cell_angle_. It is > mandatory to specify all twin-law matrices, including that > of the identity. > ; > > data_refine_ls_twin_id > _name '_refine_ls_twin_id' > _category refine > _type char > _list yes > _list_link_parent '_diffrn_reflns_twin_id' > _definition > ; The identifier of one domain state. This must be > identical to a _diffrn_reflns_twin_id. > ; > > data_refine_ls_twin_fraction > _category refine > _type numb > _type_conditions esd > _list yes > _list_reference 'refine_ls_twin_id' > _enumeration_range 0.0:1.0 > _definition > ; The refined values of the twin fractions. Twin fractions, k(i), > are defined through the following expression: > > G~c~=s^2^ Sumi k(i)|F|^2^~c,i~ (i=1,n) > > Sumi k(i) = 1 (i=1,n) > > where G~c~ is the calculated twinned intensity for reflection > h1 h2 h3, F~c,i~ the calculated structure factor of domain > state i, s the overall scale factor and n the number of domain > states. It is mandatory to specify the refined twin fractions > of all domain states. > > The _enumeration_range of 0.0:1.0 is correctly interpreted as > meaning (0.0 - 3u) =< x =< (1.0 + 3u). > ; > > data_refine_diff_twin_F_obs_deconvolution_description > _name '_refine_diff_twin_F_obs_deconvolution_description' > _category refine > _type char > _definition > ; A description of the method used to calculate the > Fourier coefficients used in electron density calculations. > ; > > =============================================================================== > > Regards > Brian -- Prof. George M. Sheldrick Dept. Structural Chemistry Tammannstr. 4 D37077 Goettingen, Germany Tel. +49-551-393021 Fax. +49-551-392582

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