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*To*:*Multiple recipients of list <coredmg@iucr.org>***Subject**:**CIF description of twinning****From**:**Brian McMahon <bm@iucr.org>***Date*:*Wed, 9 May 2001 10:44:00 +0100 (BST)**Reply-To*:*coredmg@iucr.org*

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

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