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*To*:*Multiple recipients of list <coredmg@iucr.org>***Subject**:**Re: proposal for valence items****From**:**Gotzon Madariaga <wmpmameg@lg.ehu.es>***Date*:*Wed, 29 Nov 2000 17:55:18 GMT**Reply-To*:*coredmg@iucr.org*

Given that I promoted the inclusion of twin data items in the Core here is a proposal of what I consider the essential definitions. The approach is very practical and probably it will start a new dicussion. I would not want to interfere with the approval of other more mature items like those concerning bond valence. I have followed the treatment given to the twinning problem by two programs Shelx (Sheldrick, 19xx) and Jana (Petricek,19xx-2000). Since I am more familiar with the effects twinning causes in reciprocal space my proposal includes twin matrices operating on Miller indices and I have not considered antiphase domains. Criticisms are welcome. 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 detected in the sample. ; 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 matrices that generate the Miller indices of each domain from the Miller indices of the first one: (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 given by _cell_length_ and _cell_angle_. All the twin matrices including the identity should be entered. ; data_refine_ls_twin_id _name '_refine_ls_twin_id' _category refine _type char _list yes _list_link_parent '_diffrn_reflns_twin_id' _definition ; An identifier for the refined twin fractions. It must match a corresponding _diffrn_reflns_twin_id. ; data_refine_ls_twin_fraction _category refine _type numb _type_conditions esd _list yes _list_reference 'refine_ls_twin_id' _definition ; The refined values of the twin fractions. Twin fractions, k(i), are defined through the following expression: F^2^~c~=s^2^ Sumi k(i)F^2^~c,i~ (i=1,n) where F~c~ is the calculated structure factor, F~c,i~ the calculated structure factor of domain i, s the overall scale factor and n the number of domains. The sum of twin fractions must be unity and therefore only k(2)...k(n) are refined. However all the twin fractions including k(1) should be entered. ; ###very long name... 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 "observed" structure factor of the first domain used to evaluate the difference Fourier map. ; =============================================================================== Best regards Gotzon ----------------------------------------------------------------------------- Gotzon Madariaga | E-mail: wmpmameg@lg.ehu.es Dpto. Fisica de la Materia Condensada | Facultad de Ciencias | Phone: 34 946012467 Universidad del Pais Vasco | FAX : 34 944648500 Apdo. 644 | 48080 Bilbao (SPAIN) | -----------------------------------------------------------------------------

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