<div dir="ltr">This is not a simple question to answer. In both DFT and XRD the symmetry of the structure is an input parameter, and in both cases it is sometimes wrong. The most obvious question to ask is whether you used the same Space Group for your refinement in Wien2k that was used in the XRD refinement. I have a suspicion that you did not, as the presence of (x,x,x) positions implies a Space Group constraint and if you are getting (x,y,z) then you probably have a different symmetry in Wien2k.<div><br><div>Check first that the symmetry is completely the same.</div><div><br></div><div>If they are the same, the question then would be why one gets (x,x,x) in the XRD refinement? It could be disorder or something else, but without more information it is hard to guess.</div><div><br></div></div></div><div class="gmail_extra"><br><div class="gmail_quote">On Mon, Sep 8, 2014 at 11:37 AM, shamik chakrabarti <span dir="ltr"><<a href="mailto:shamikphy@gmail.com" target="_blank">shamikphy@gmail.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
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<div dir="ltr">Dear wien2k users,
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<div> We have optimized atomic coordinates of a spinel cubic compound containing 56 atoms/cell. The most important variation we have seen between experimental structural coordinates & DFT (GGA+U) optimized coordinates is that.</div>
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<div>the oxygen coordinate changes from (x,x,x) configuration to (x,y,z) configuration as we have not constrained the oxygen coordinates during optimization.</div>
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<div>Although, this change has generated a structure having very small forces (< 1 mRy/Bohr) per atoms, the change in oxygen coordinates are significant (difference between x & z is 0.021) enough to change the relative intensities of different peaks of XRD
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<div>Now my query is (before starting the experimental simulation by myself again!!) whether there is a way to constrain atomic coordinates such that it remain constrained to (x,x,x) configuration while changing the value of x?</div>
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<div>& also whether such a constrained way should be adopted or should we accept the DFT optimized structure (a bit different from experimental one!!) as it is for further calculation?</div>
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<div>Looking forward to your opinions & suggestions.</div>
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<div>Thanks in advance,</div>
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<div>with regards, <span class="HOEnZb"><font color="#888888"><br clear="all">
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-- <br>
Shamik Chakrabarti<br>
Senior Research Fellow<br>
Dept. of Physics & Meteorology<br>
Material Processing & Solid State Ionics Lab<br>
IIT Kharagpur<br>
Kharagpur 721302<br>
INDIA </font></span></div>
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</blockquote></div><br><br clear="all"><div><br></div>-- <br><div dir="ltr">Professor Laurence Marks<br>Department of Materials Science and Engineering<br>Northwestern University<br><a href="http://www.numis.northwestern.edu" target="_blank">www.numis.northwestern.edu</a><div>Corrosion in 4D: <a href="http://MURI4D.numis.northwestern.edu" target="_blank">MURI4D.numis.northwestern.edu</a><br>Co-Editor, Acta Cryst A<br>"Research is to see what everybody else has seen, and to think what nobody else has thought"<br>Albert Szent-Gyorgi</div></div>
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