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May 26, 2016 10:53:56 AM (6 years ago)
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vondreele
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cleanup

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Tutorials/MerohedralTwins
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  • Tutorials/MerohedralTwins/Merohedral twin refinement in GSAS.htm

    r2293 r2294  
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     25  <o:Author>Von Dreele</o:Author>
     26  <o:LastAuthor>Von Dreele</o:LastAuthor>
     27  <o:Revision>2</o:Revision>
     28  <o:TotalTime>0</o:TotalTime>
     29  <o:Created>2016-05-26T15:52:00Z</o:Created>
     30  <o:LastSaved>2016-05-26T15:52:00Z</o:LastSaved>
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    2440<link rel=themeData
    2541href="Merohedral%20twin%20refinement%20in%20GSAS_files/themedata.thmx">
     
    795811
    796812<p class=MsoNormal>In these exercises you will use GSAS-II to refine the
    797 structure of a few single crystal structures where there is merohedral
    798 twinning. There is a general discussion of twinning in International Tables for
     813structure of a few single crystal structures where there is merohedral twinning.
     814There is a general discussion of twinning in International Tables for
    799815Crystallography (2006), Vol. C, Chapter 1.3, pp 10-14. Merohedral twinning
    800816occurs when the twin operation (2, m or <span style='position:relative;
    801 top:3pt'><span style='mso-no-proof:yes'><img width=9 height=19 id="_x0000_i1064"
     817top:3pt'><span style='mso-no-proof:yes'><img width=9 height=19 id="_x0000_i1058"
    802818src="Merohedral%20twin%20refinement%20in%20GSAS_files/image015.png"></span>)
    803819belongs to the point group of the vector lattice but not of the crystal
     
    892908<b><span
    893909style='font-family:"Calibri",sans-serif'>1b.cif</span></b>) and select it. A
    894 popup window will appear</p>
    895 
    896 <p class=MsoNormal><span style='mso-no-proof:yes'><img width=445 height=249
    897 id="_x0000_i1063"
    898 src="Merohedral%20twin%20refinement%20in%20GSAS_files/image005.png"></span></p>
    899 
    900 <p class=MsoNormal>Press <b><span style='font-family:"Calibri",sans-serif'>Yes</span></b>;
     910popup window will appear. Press <b><span style='font-family:"Calibri",sans-serif'>Yes</span></b>;
    901911a new popup will appear</p>
    902912
    903913<p class=MsoNormal><span style='mso-no-proof:yes'><img width=336 height=137
    904 id="_x0000_i1062"
     914id="_x0000_i1057"
    905915src="Merohedral%20twin%20refinement%20in%20GSAS_files/image006.png"></span></p>
    906916
     
    909919
    910920<p class=MsoNormal><span style='mso-no-proof:yes'><img width=650 height=500
    911 id="_x0000_i1061"
     921id="_x0000_i1056"
    912922src="Merohedral%20twin%20refinement%20in%20GSAS_files/image007.png"></span></p>
    913923
    914924<p class=MsoNormal>Next do <b><span style='font-family:"Calibri",sans-serif'>Import/Structure
    915925Factor/from CIF file</span></b> (that is the format of a “<span class=SpellE>fcf</span>”
    916 file); a file selection dialog box will appear with the previously selected
    917 directory chosen. Select <b><span style='font-family:"Calibri",sans-serif'>1b.fcf</span></b>;
     926file); a file selection dialog box will appear with the previously selected directory
     927chosen. Select <b><span style='font-family:"Calibri",sans-serif'>1b.fcf</span></b>;
    918928a popup window will appear</p>
    919929
    920930<p class=MsoNormal><span style='mso-no-proof:yes'><img width=447 height=249
    921 id="_x0000_i1060"
     931id="_x0000_i1055"
    922932src="Merohedral%20twin%20refinement%20in%20GSAS_files/image008.png"></span></p>
    923933
     
    926936
    927937<p class=MsoNormal><span style='mso-no-proof:yes'><img width=336 height=137
    928 id="_x0000_i1059"
     938id="_x0000_i1054"
    929939src="Merohedral%20twin%20refinement%20in%20GSAS_files/image009.png"></span></p>
    930940
     
    933943
    934944<p class=MsoNormal><span style='mso-no-proof:yes'><img width=320 height=310
    935 id="_x0000_i1058"
     945id="_x0000_i1053"
    936946src="Merohedral%20twin%20refinement%20in%20GSAS_files/image010.png"></span></p>
    937947
     
    955965
    956966<p class=MsoNormal><span style='mso-no-proof:yes'><img width=522 height=301
    957 id="_x0000_i1057"
     967id="_x0000_i1052"
    958968src="Merohedral%20twin%20refinement%20in%20GSAS_files/image016.png"></span></p>
    959969
     
    966976
    967977<p class=MsoNormal><span style='mso-no-proof:yes'><img width=434 height=444
    968 id="_x0000_i1056"
     978id="_x0000_i1051"
    969979src="Merohedral%20twin%20refinement%20in%20GSAS_files/image011.png"></span></p>
    970980
     
    9911001
    9921002<p class=MsoNormal><span style='mso-no-proof:yes'><img width=434 height=444
    993 id="_x0000_i1055"
     1003id="_x0000_i1050"
    9941004src="Merohedral%20twin%20refinement%20in%20GSAS_files/image013.png"></span></p>
    9951005
     
    10031013
    10041014<p class=MsoNormal><span style='mso-no-proof:yes'><img width=449 height=500
    1005 id="_x0000_i1054"
     1015id="_x0000_i1049"
    10061016src="Merohedral%20twin%20refinement%20in%20GSAS_files/image014.png"></span></p>
    10071017
     
    10181028
    10191029<p class=MsoNormal>In this case the data collection Laue statistics (very
    1020 nearly <span style='position:relative;top:3pt'><span style='mso-no-proof:yes'><img
    1021 width=9 height=19 id="_x0000_i1053"
    1022 src="Merohedral%20twin%20refinement%20in%20GSAS_files/image026.png"></span>m)
     1030nearly <span style='position:relative;top:3pt'><span style='mso-no-proof:yes'>-3</span>m)
    10231031and systematic extinctions indicated that the space group was either one of the
    10241032<span class=SpellE>enantiomorphic</span> pair P3<sub>1</sub>21 or P3<sub>2</sub>21
    1025 or possibly one of the pair P3<sub>1</sub> or P3<sub>2</sub>. If the true structure
    1026 was of the lower symmetry P3<sub>1</sub> or P3<sub>2</sub> then there must be
    1027 Type 2 merohedral twinning to make the diffraction pattern appear to have
    1028 higher symmetry (<span style='position:relative;top:3pt'><span
    1029 style='mso-no-proof:yes'><img width=9 height=19 id="_x0000_i1052"
    1030 src="Merohedral%20twin%20refinement%20in%20GSAS_files/image026.png"></span>m
    1031 instead of <span style='position:relative;top:3pt'><span style='mso-no-proof:
    1032 yes'><img width=9 height=19 id="_x0000_i1051"
    1033 src="Merohedral%20twin%20refinement%20in%20GSAS_files/image026.png"></span>);
    1034 this exercise will explore this possibility. The data were collected with <span
    1035 class=SpellE>MoK</span><span style='font-family:Symbol'>a</span> radiation so
    1036 there is little resonant scattering and thus the choice of hand is
    1037 indeterminate.&nbsp; Solving the structure in P3<sub>2</sub>21 yielded a poorly
    1038 fitting structure with disorder particularly in the cation. Resolving the
    1039 structure in P3<sub>2</sub> gave a structure with no disorder and a better fit
    1040 to the data (but still not great). As this work was done with <span
    1041 class=SpellE>Shelx</span> this result is in the form of a <span class=SpellE>Shelx</span>
    1042 input file (2b.ins – thanks to V. Young for this example). If you have not done
    1043 so already, start GSAS-II and start with a fresh project.</p>
     1033or possibly one of the pair P3<sub>1</sub> or P3<sub>2</sub>. If the true
     1034structure was of the lower symmetry P3<sub>1</sub> or P3<sub>2</sub> then there
     1035must be Type 2 merohedral twinning to make the diffraction pattern appear to
     1036have higher symmetry (<span style='position:relative;top:3pt'><span
     1037style='mso-no-proof:yes'>-3</span>m instead of <span style='position:relative;
     1038top:3pt'><span style='mso-no-proof:yes'>-3</span>); this exercise will explore
     1039this possibility. The data were collected with <span class=SpellE>MoK</span><span
     1040style='font-family:Symbol'>a</span> radiation so there is little resonant
     1041scattering and thus the choice of hand is indeterminate.&nbsp; Solving the
     1042structure in P3<sub>2</sub>21 yielded a poorly fitting structure with disorder
     1043particularly in the cation. Resolving the structure in P3<sub>2</sub> gave a
     1044structure with no disorder and a better fit to the data (but still not great).
     1045As this work was done with <span class=SpellE>Shelx</span> this result is in
     1046the form of a <span class=SpellE>Shelx</span> input file (2b.ins – thanks to V.
     1047Young for this example). If you have not done so already, start GSAS-II and
     1048start with a fresh project.</p>
    10441049
    10451050<h2><span style='mso-fareast-font-family:"Times New Roman"'>Step 1. Setup for
     
    10561061
    10571062<p class=MsoNormal><span style='mso-no-proof:yes'><img width=410 height=262
    1058 id="_x0000_i1050"
     1063id="_x0000_i1048"
    10591064src="Merohedral%20twin%20refinement%20in%20GSAS_files/image012.png"></span></p>
    10601065
     
    10631068
    10641069<p class=MsoNormal><span style='mso-no-proof:yes'><!--[if gte vml 1]><v:shape
    1065  id="Picture_x0020_39" o:spid="_x0000_i1049" type="#_x0000_t75" style='width:371.25pt;
     1070 id="Picture_x0020_39" o:spid="_x0000_i1047" type="#_x0000_t75" style='width:371.25pt;
    10661071 height:176.25pt;visibility:visible;mso-wrap-style:square'>
    10671072 <v:imagedata src="Merohedral%20twin%20refinement%20in%20GSAS_files/image001.png"
     
    10861091
    10871092<p class=MsoNormal><span style='mso-no-proof:yes'><img width=624 height=336
    1088 id="_x0000_i1048"
     1093id="_x0000_i1046"
    10891094src="Merohedral%20twin%20refinement%20in%20GSAS_files/image027.png"></span></p>
    10901095
     
    10951100
    10961101<p class=MsoNormal><span style='mso-no-proof:yes'><img width=478 height=358
    1097 id="_x0000_i1047"
     1102id="_x0000_i1045"
    10981103src="Merohedral%20twin%20refinement%20in%20GSAS_files/image028.jpg"></span></p>
    10991104
     
    11091114
    11101115<p class=MsoNormal><span style='mso-no-proof:yes'><img width=366 height=262
    1111 id="_x0000_i1046"
     1116id="_x0000_i1044"
    11121117src="Merohedral%20twin%20refinement%20in%20GSAS_files/image022.png"></span></p>
    11131118
     
    11221127
    11231128<p class=MsoNormal><span style='mso-no-proof:yes'><img width=624 height=535
    1124 id="_x0000_i1045"
     1129id="_x0000_i1043"
    11251130src="Merohedral%20twin%20refinement%20in%20GSAS_files/image029.png"></span></p>
    11261131
     
    11461151
    11471152<p class=MsoNormal><span style='mso-no-proof:yes'><img width=522 height=301
    1148 id="_x0000_i1044"
     1153id="_x0000_i1042"
    11491154src="Merohedral%20twin%20refinement%20in%20GSAS_files/image016.png"></span></p>
    11501155
     
    11581163
    11591164<p class=MsoNormal><span style='mso-no-proof:yes'><img width=507 height=444
    1160 id="_x0000_i1043"
     1165id="_x0000_i1041"
    11611166src="Merohedral%20twin%20refinement%20in%20GSAS_files/image030.png"></span></p>
    11621167
     
    11681173structure would not be very satisfactory. However, we have suspected from the
    11691174equivalence of <span style='position:relative;top:3pt'><span style='mso-no-proof:
    1170 yes'><img width=9 height=19 id="_x0000_i1042"
     1175yes'><img width=9 height=19 id="_x0000_i1040"
    11711176src="Merohedral%20twin%20refinement%20in%20GSAS_files/image031.png"></span>&nbsp;and
    11721177<span style='position:relative;top:3pt'><span style='mso-no-proof:yes'><img
    1173 width=9 height=19 id="_x0000_i1041"
     1178width=9 height=19 id="_x0000_i1039"
    11741179src="Merohedral%20twin%20refinement%20in%20GSAS_files/image026.png"></span>m1
    1175 Laue data symmetries that there is twinning so don’t bother trying to make it
    1176 better by refining the structure. To determine the possible twin law we refer
    1177 to Table 1.3.4.2 of the International Tables for Crystallography Vol C. and see
    1178 that indeed Laue <span style='position:relative;top:3pt'><span
    1179 style='mso-no-proof:yes'><img width=9 height=19 id="_x0000_i1040"
     1180Laue data symmetries that there is twinning so don’t bother trying to make it better
     1181by refining the structure. To determine the possible twin law we refer to Table
     11821.3.4.2 of the International Tables for Crystallography Vol C. and see that
     1183indeed Laue <span style='position:relative;top:3pt'><span style='mso-no-proof:
     1184yes'><img width=9 height=19 id="_x0000_i1038"
    11801185src="Merohedral%20twin%20refinement%20in%20GSAS_files/image026.png"></span>m1
    11811186as P3<sub>2</sub>21 can be simulated by twinned Laue <span style='position:
    11821187relative;top:3pt'><span style='mso-no-proof:yes'><img width=9 height=19
    1183 id="_x0000_i1039"
     1188id="_x0000_i1037"
    11841189src="Merohedral%20twin%20refinement%20in%20GSAS_files/image026.png"></span>&nbsp;crystals
    11851190in P3<sub>2</sub>; the possible operator is shown in Table 1.3.4.1 (reproduced
     
    11881193specifics. See P3m1 and P3<sub>2</sub>21 space groups in International Tables
    11891194for Crystallography, Vol A for the operations m as <span style='position:relative;
    1190 top:3pt'><span style='mso-no-proof:yes'><img width=26 height=19
    1191 id="_x0000_i1038"
    1192 src="Merohedral%20twin%20refinement%20in%20GSAS_files/image032.png"></span>&nbsp;or
    1193 2 as <span style='position:relative;top:3pt'><span style='mso-no-proof:yes'><img
    1194 width=26 height=19 id="_x0000_i1037"
    1195 src="Merohedral%20twin%20refinement%20in%20GSAS_files/image033.png"></span>; we
    1196 will use the 2-fold since the m inverts the structure which we are not
     1195top:3pt'><span style='mso-no-proof:yes'>-y-xz</span>&nbsp;or 2 as <span
     1196style='position:relative;top:3pt'><span style='mso-no-proof:yes'>yx-z</span>;
     1197we will use the 2-fold since the m inverts the structure which we are not
    11971198sensitive to in this experiment.</p>
    11981199
     
    12211222for <span class=SpellE>MoKa</span> radiation allowing detection of Type 1 <span
    12221223class=SpellE>merohedry</span>. The data set has apparent 6/m Laue symmetry but
    1223 may be 6/mmm for the space groups P61/P65 or P6122/P6522. The combination of
    1224 Type 1 and Type 2 twins requires 4 twin laws to be explored for this <span
    1225 class=SpellE>pentamethylcyclopentadienyl</span> Cs complex. As this work was
    1226 done with <span class=SpellE>Shelx</span> this result is in the form of a <span
    1227 class=SpellE>Shelx</span> input file (3a.ins – thanks to V. Young for this
    1228 example). If you have not done so already, start GSAS-II and start with a fresh
    1229 project.</p>
     1224may be 6/mmm for the space groups P6<sub>1</sub>/P6<sub>5</sub> or P6<sub>1</sub>22/P6<sub>5</sub>22.
     1225The combination of Type 1 and Type 2 twins requires 4 twin laws to be explored
     1226for this <span class=SpellE>pentamethylcyclopentadienyl</span> Cs complex. As
     1227this work was done with <span class=SpellE>Shelx</span> this result is in the
     1228form of a <span class=SpellE>Shelx</span> input file (3a.ins – thanks to V.
     1229Young for this example). If you have not done so already, start GSAS-II and
     1230start with a fresh project.</p>
    12301231
    12311232<h2><span style='mso-fareast-font-family:"Times New Roman"'>Step 1. Setup for
     
    12471248<p class=MsoNormal>It is; press the <b><span style='font-family:"Calibri",sans-serif'>Yes</span></b>
    12481249button. You will then see the SHELX Read Warning; you will have to change the
    1249 space group to P61 in a bit. Press <b><span style='font-family:"Calibri",sans-serif'>OK</span></b>
     1250space group to P6<sub>1</sub> in a bit. Press <b><span style='font-family:"Calibri",sans-serif'>OK</span></b>
    12501251and leave the name alone (press <b><span style='font-family:"Calibri",sans-serif'>OK</span></b>
    12511252again). Change the Space group to <b><span style='font-family:"Calibri",sans-serif'>P
     
    13131314To determine the possible twin <span class=GramE>laws</span> we again refer to
    13141315Table 1.3.4.2 of the International Tables for Crystallography Vol C. and see
    1315 that indeed Laue 6/mmm as P6122 can be simulated by twinned Laue 6/m crystals
    1316 in P61; the possible operator is shown in Table 1.3.4.1 (reproduced above) as
    1317 either a mirror or a 2-fold. The detailed symbolism in the table (‘m..,
    1318 ..2/.2.’) indicates (perhaps obscurely) the operator specifics; they are m as <span
    1319 style='position:relative;top:3pt'><!--[if gte msEquation 12]><m:oMath><m:acc><m:accPr><m:chr
     1316that indeed Laue 6/mmm as P6<sub>1</sub>22 can be simulated by twinned Laue 6/m
     1317crystals in P6<sub>1</sub>; the possible operator is shown in Table 1.3.4.1
     1318(reproduced above) as either a mirror or a 2-fold. The detailed symbolism in
     1319the table (‘m.., ..2/.2.’) indicates (perhaps obscurely) the operator
     1320specifics; they are m as <span style='position:relative;top:3pt'><!--[if gte msEquation 12]><m:oMath><m:acc><m:accPr><m:chr
    13201321    m:val="&#773;"/><span style='font-family:"Cambria Math",serif;mso-ascii-font-family:
    13211322   "Cambria Math";mso-hansi-font-family:"Cambria Math";font-style:italic;
     
    13371338src="Merohedral%20twin%20refinement%20in%20GSAS_files/image004.gif" v:shapes="_x0000_i1025"><![endif]></span><![endif]>&nbsp;or
    133813392 as <!--[if gte msEquation 12]><m:oMath><i style='mso-bidi-font-style:normal'><span
    1339  style='font-family:"Cambria Math",serif'><m:r><span style='position:relative;
    1340   top:3pt'>yx</m:r></span></i><m:acc><m:accPr><m:chr m:val="&#773;"/><span
    1341    style='font-family:"Cambria Math",serif;mso-ascii-font-family:"Cambria Math";
    1342    mso-hansi-font-family:"Cambria Math";font-style:italic;mso-bidi-font-style:
    1343    normal'><m:ctrlPr></m:ctrlPr></span></m:accPr><m:e><i style='mso-bidi-font-style:
    1344    normal'><span style='font-family:"Cambria Math",serif'><m:r>z</m:r></span></i></m:e></m:acc></m:oMath><![endif]--><![if !msEquation]><span
     1340 style='font-family:"Cambria Math",serif'><span style='position:relative;
     1341 top:3pt'><m:r>y</m:r><m:r>x</span></m:r></span></i><m:acc><m:accPr><m:chr
     1342    m:val="&#773;"/><span style='font-family:"Cambria Math",serif;mso-ascii-font-family:
     1343   "Cambria Math";mso-hansi-font-family:"Cambria Math";font-style:italic;
     1344   mso-bidi-font-style:normal'><m:ctrlPr></m:ctrlPr></span></m:accPr><m:e><i
     1345   style='mso-bidi-font-style:normal'><span style='font-family:"Cambria Math",serif'><m:r>z</m:r></span></i></m:e></m:acc></m:oMath><![endif]--><![if !msEquation]><span
    13451346style='font-size:12.0pt;font-family:"Times New Roman",serif;mso-fareast-font-family:
    13461347"Times New Roman";mso-fareast-theme-font:minor-fareast;position:relative;
     
    13511352  o:title="" chromakey="white"/>
    13521353</v:shape><![endif]--><![if !vml]><img width=26 height=19
    1353 src="Merohedral%20twin%20refinement%20in%20GSAS_files/image006.gif" v:shapes="_x0000_i1025"><![endif]></span><![endif]>;
    1354 we will use the 2-fold, m and inversion operators to fully test for presence of
    1355 the 4 possible twin laws. Press the <b><span style='font-family:"Calibri",sans-serif'>Add
    1356 Twin Law</span></b> button <b><span style='font-family:"Calibri",sans-serif'>3
    1357 times</span></b>; each time the window will be redrawn. When done you should
    1358 see.</p>
     1354src="Merohedral%20twin%20refinement%20in%20GSAS_files/image006.gif" v:shapes="_x0000_i1025"><![endif]></span><![endif]><span
     1355style='font-family:"Cambria Math",serif'>; we will use the 2-fold, m and
     1356inversion operators to fully test for presence of the 4 possible twin laws.
     1357Press the </span><b><span style='font-family:"Calibri",sans-serif'>Add Twin Law</span></b><span
     1358style='font-family:"Cambria Math",serif'> button </span><b><span
     1359style='font-family:"Calibri",sans-serif'>3 times</span></b><span
     1360style='font-family:"Cambria Math",serif'>; each time the window will be
     1361redrawn. When done you should see.<o:p></o:p></span></p>
    13591362
    13601363<p class=MsoNormal><span style='mso-no-proof:yes'><img width=434 height=444
     
    14191422style='mso-bidi-font-weight:normal'><span style='font-family:"Calibri",sans-serif;
    14201423mso-ascii-theme-font:minor-latin;mso-hansi-theme-font:minor-latin'>Edit/Insert
    1421 H atoms</span></b>; a Distance Angle Controls will appear. Occasionally these values
    1422 may need to be changed but not this time; press <b style='mso-bidi-font-weight:
     1424H atoms</span></b>; a Distance Angle Controls will appear. Occasionally these
     1425values may need to be changed but not this time; press <b style='mso-bidi-font-weight:
    14231426normal'><span style='font-family:"Calibri",sans-serif;mso-ascii-theme-font:
    14241427minor-latin;mso-hansi-theme-font:minor-latin'>Ok</span></b>. The next popup is
     
    14331436src="Merohedral%20twin%20refinement%20in%20GSAS_files/image012.gif" v:shapes="Picture_x0020_42"><![endif]></span></p>
    14341437
    1435 <p class=MsoNormal>It specifies the expected geometry around each C-atom for
    1436 H-atom placement. As expected we want 3 H-atoms on the terminal CH<sub>3</sub>
    1437 groups on the <span class=SpellE>Cp</span>* ring and one for each C-atom for
    1438 the NC<sub>5</sub>H<sub>5</sub> molecule. Press <b style='mso-bidi-font-weight:
     1438<p class=MsoNormal>It specifies the expected geometry around each C-atom for H-atom
     1439placement. As expected we want 3 H-atoms on the terminal CH<sub>3</sub> groups
     1440on the <span class=SpellE>Cp</span>* ring and one for each C-atom for the NC<sub>5</sub>H<sub>5</sub>
     1441molecule. Press <b style='mso-bidi-font-weight:normal'><span style='font-family:
     1442"Calibri",sans-serif;mso-ascii-theme-font:minor-latin;mso-hansi-theme-font:
     1443minor-latin'>Ok</span></b>; you will see the H-atoms added in turn to the
     1444structure and they will be inserted immediately after their respective C-atom.
     1445Their positions and thermal parameters are tied to the C-atoms so that after
     1446refinement of the structure they can be shifted to the <span class=SpellE>stereochemically</span>
     1447best positions. For the refinement, double click the <b style='mso-bidi-font-weight:
    14391448normal'><span style='font-family:"Calibri",sans-serif;mso-ascii-theme-font:
    1440 minor-latin;mso-hansi-theme-font:minor-latin'>Ok</span></b>; you will see the
    1441 H-atoms added in turn to the structure and they will be inserted immediately
    1442 after their respective C-atom. Their positions and thermal parameters are tied
    1443 to the C-atoms so that after refinement of the structure they can be shifted to
    1444 the <span class=SpellE>stereochemically</span> best positions. For the
    1445 refinement, double click the <b style='mso-bidi-font-weight:normal'><span
     1449minor-latin;mso-hansi-theme-font:minor-latin'>Refine</span></b> column heading
     1450and select <b style='mso-bidi-font-weight:normal'><span style='font-family:
     1451"Calibri",sans-serif;mso-ascii-theme-font:minor-latin;mso-hansi-theme-font:
     1452minor-latin'>X</span></b> and <b style='mso-bidi-font-weight:normal'><span
    14461453style='font-family:"Calibri",sans-serif;mso-ascii-theme-font:minor-latin;
    1447 mso-hansi-theme-font:minor-latin'>Refine</span></b> column heading and select <b
     1454mso-hansi-theme-font:minor-latin'>U</span></b> from the popup; press <b
    14481455style='mso-bidi-font-weight:normal'><span style='font-family:"Calibri",sans-serif;
    1449 mso-ascii-theme-font:minor-latin;mso-hansi-theme-font:minor-latin'>X</span></b>
    1450 and <b style='mso-bidi-font-weight:normal'><span style='font-family:"Calibri",sans-serif;
    1451 mso-ascii-theme-font:minor-latin;mso-hansi-theme-font:minor-latin'>U</span></b>
    1452 from the popup; press <b style='mso-bidi-font-weight:normal'><span
     1456mso-ascii-theme-font:minor-latin;mso-hansi-theme-font:minor-latin'>OK</span></b>.
     1457The Atom table will be redrawn with XU in every entry under Refine. Now do <b
     1458style='mso-bidi-font-weight:normal'><span style='font-family:"Calibri",sans-serif;
     1459mso-ascii-theme-font:minor-latin;mso-hansi-theme-font:minor-latin'>Calculate/Refine</span></b>
     1460from the main GSASII data tree window; the <span class=SpellE>Rw</span> will
     1461drop to ~1.92%. If you look at the <b style='mso-bidi-font-weight:normal'><span
    14531462style='font-family:"Calibri",sans-serif;mso-ascii-theme-font:minor-latin;
    1454 mso-hansi-theme-font:minor-latin'>OK</span></b>. The Atom table will be redrawn
    1455 with XU in every entry under Refine. Now do <b style='mso-bidi-font-weight:
    1456 normal'><span style='font-family:"Calibri",sans-serif;mso-ascii-theme-font:
    1457 minor-latin;mso-hansi-theme-font:minor-latin'>Calculate/Refine</span></b> from the
    1458 main GSASII data tree window; the <span class=SpellE>Rw</span> will drop to
    1459 ~1.92%. If you look at the <b style='mso-bidi-font-weight:normal'><span
    1460 style='font-family:"Calibri",sans-serif;mso-ascii-theme-font:minor-latin;
    1461 mso-hansi-theme-font:minor-latin'>Data</span></b> tab the Twin element fractions
    1462 are much clearer.</p>
     1463mso-hansi-theme-font:minor-latin'>Data</span></b> tab the Twin element
     1464fractions are much clearer.</p>
    14631465
    14641466<p class=MsoNormal><span style='mso-no-proof:yes'><!--[if gte vml 1]><v:shape
     
    14911493normal'><span style='font-family:"Calibri",sans-serif;mso-ascii-theme-font:
    14921494minor-latin;mso-hansi-theme-font:minor-latin'>Edit/Update H atoms</span></b> to
    1493 put them in their best positions. <span style='mso-spacerun:yes'> </span>Next
     1495put them in their best positions.<span style='mso-spacerun:yes'>  </span>Next
    14941496select all the atoms by double clicking the empty box in upper left corner of
    14951497the table; all atoms will be highlighted &amp; plotted green. Do <b
     
    15081510<p class=MsoNormal>Check the <b style='mso-bidi-font-weight:normal'><span
    15091511style='font-family:"Calibri",sans-serif;mso-ascii-theme-font:minor-latin;
    1510 mso-hansi-theme-font:minor-latin'>Choose inversion</span></b> box and
    1511 (important) set <b style='mso-bidi-font-weight:normal'><span style='font-family:
    1512 "Calibri",sans-serif;mso-ascii-theme-font:minor-latin;mso-hansi-theme-font:
    1513 minor-latin'>Choose unit cell</span></b> to <b style='mso-bidi-font-weight:
    1514 normal'><span style='font-family:"Calibri",sans-serif;mso-ascii-theme-font:
    1515 minor-latin;mso-hansi-theme-font:minor-latin'>1 1 1</span></b>; this will put
    1516 the inverted structure back inside the unit cell. Press <b style='mso-bidi-font-weight:
    1517 normal'><span style='font-family:"Calibri",sans-serif;mso-ascii-theme-font:
    1518 minor-latin;mso-hansi-theme-font:minor-latin'>Ok</span></b>; the table and plot
    1519 will both be redrawn. Now do <b style='mso-bidi-font-weight:normal'><span
     1512mso-hansi-theme-font:minor-latin'>Choose inversion</span></b> box and (important)
     1513set <b style='mso-bidi-font-weight:normal'><span style='font-family:"Calibri",sans-serif;
     1514mso-ascii-theme-font:minor-latin;mso-hansi-theme-font:minor-latin'>Choose unit
     1515cell</span></b> to <b style='mso-bidi-font-weight:normal'><span
     1516style='font-family:"Calibri",sans-serif;mso-ascii-theme-font:minor-latin;
     1517mso-hansi-theme-font:minor-latin'>1 1 1</span></b>; this will put the inverted
     1518structure back inside the unit cell. Press <b style='mso-bidi-font-weight:normal'><span
     1519style='font-family:"Calibri",sans-serif;mso-ascii-theme-font:minor-latin;
     1520mso-hansi-theme-font:minor-latin'>Ok</span></b>; the table and plot will both
     1521be redrawn. Now do <b style='mso-bidi-font-weight:normal'><span
    15201522style='font-family:"Calibri",sans-serif;mso-ascii-theme-font:minor-latin;
    15211523mso-hansi-theme-font:minor-latin'>Calculate/Refine</span></b>; the <span
     
    15361538the other ~1/3 while the other two are ~zero. You can delete them and repeat
    15371539the refinement; <span class=SpellE>Rw</span> ~1.7%. This completed the
    1538 merohedral twin exercises. You can save this project if desired.<o:p></o:p></p>
     1540merohedral twin exercises. You can save this project if desired.</p>
    15391541
    15401542<p class=MsoNormal><o:p>&nbsp;</o:p></p>
    15411543
    1542 </div>
     1544</span></span></span></span></span></span></span></span></span></span></span></div>
    15431545
    15441546</body>
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