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<div>Dear Respected <g class="gr_ gr_52 gr-alert gr_spell gr_inline_cards gr_run_anim ContextualSpelling ins-del multiReplace" id="52" data-gr-id="52">Xavaier</g> Sir</div><div>Thank you very much for detailed explanation.</div><div>My system is not having any d-d interaction (no TM element) so I do not think I should go for DFT+U, Onsite Hybrid functional and Full hybrid calculations!</div><div><br></div><div>With TB-mBJ it is showing a band gap. The VBM is widely dispersed, flat for a wide range, near the VBM. So this is a semiconductor.<br></div><div><br></div><div>As we can not relax the structure using mBJ so there is no way to use it for the phonopy purpose (phonon bands).</div><div><br></div><div>Due to lack of good computational facilities, I want to avoid hybrid calculation which will never finish if I go for Phonopy.</div><div><br></div><div>What I see it, I use the combination of <g class="gr_ gr_94 gr-alert gr_spell gr_inline_cards gr_run_anim ContextualSpelling ins-del multiReplace" id="94" data-gr-id="94">tolf</g> and -<g class="gr_ gr_98 gr-alert gr_spell gr_inline_cards gr_run_anim ContextualSpelling ins-del multiReplace" id="98" data-gr-id="98">fc</g> such that it gives minimum forces. But I am sure the forces will not be less than 2mRy/Bohr. Again this process I see a contradiction with Peter Sir's statement: forces should be less than 1mRy/<g class="gr_ gr_509 gr-alert gr_spell gr_inline_cards gr_run_anim ContextualSpelling ins-del multiReplace" id="509" data-gr-id="509">bohr</g> for phonon calculations and which I will not be having!!</div><div> </div><div>How much the ~2-3mRy/<g class="gr_ gr_921 gr-alert gr_spell gr_inline_cards gr_run_anim ContextualSpelling ins-del multiReplace" id="921" data-gr-id="921">bohr</g> forces on the atom affects the phonon band structure?</div><div>can I proceed for phonon bands with the structure having forces ~2-3 <g class="gr_ gr_1375 gr-alert gr_spell gr_inline_cards gr_run_anim ContextualSpelling ins-del multiReplace" id="1375" data-gr-id="1375">mRy</g>/Bohr?</div><div><br></div><div>Thank you very much!</div><div><br></div><div>Chin S.<br></div><div><br></div><div> </div><div><br></div><div><br></div><div> <br></div>
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<div>On Saturday 27 January 2018, 6:31:35 PM IST, Xavier Rocquefelte <xavier.rocquefelte@univ-rennes1.fr> wrote:</div>
<div><br></div>
<div><br></div>
<div><div id="ydp1c4de612yiv1933857513"><div>
<p>Dear Chin S. <br clear="none">
</p>
<p>Your system is metallic from the point of view of DFT which is not a surprise. <br clear="none">
</p>
<p>In your last calculation, which is the more accurate you simply catch this fact ... In other <g class="gr_ gr_68 gr-alert gr_gramm gr_inline_cards gr_disable_anim_appear Punctuation only-ins replaceWithoutSep" id="68" data-gr-id="68">words</g> in your previous <g class="gr_ gr_69 gr-alert gr_gramm gr_inline_cards gr_disable_anim_appear Punctuation only-ins replaceWithoutSep" id="69" data-gr-id="69">calculations</g> you had 0.1 eV gap due to <g class="gr_ gr_67 gr-alert gr_gramm gr_inline_cards gr_disable_anim_appear Grammar multiReplace" id="67" data-gr-id="67">less</g> <g class="gr_ gr_51 gr-alert gr_spell gr_inline_cards gr_disable_anim_appear ContextualSpelling ins-del multiReplace" id="51" data-gr-id="51">kpoints</g> and/or less accurate calculations. To really understand what is happening you should draw the DOS and bandstructure to see if you have band crossing at the Fermi level which could lead to occupancy of an expected empty band. If yes, it will have an impact on the chemical bond description and also on the phonons. <br clear="none">
</p>
<p>Depending on the system you will have to consider methods going beyond DFT, which in WIEN2k could be based on: <br clear="none">
</p>
<p>- DFT+U</p>
<p>- Onsite Hybrid functional</p>
<p>- Full hybrid <br clear="none">
</p>
<p>- TB-mBJ potential</p>
<p>If your band gap is based on charge transfer transitions you cannot <g class="gr_ gr_59 gr-alert gr_gramm gr_inline_cards gr_disable_anim_appear Grammar multiReplace" id="59" data-gr-id="59">used</g> DFT+U and onsite-hybrid. <br clear="none">Thus the less expensive approach will be TB-mBJ potential which will give the band gap but not the forces. <br clear="none">
</p>
<p>Thus only one option remains which is <g class="gr_ gr_46 gr-alert gr_gramm gr_inline_cards gr_disable_anim_appear Grammar only-ins doubleReplace replaceWithoutSep" id="46" data-gr-id="46">full</g> hybrid. <br clear="none">
</p>
<p>If your system is based on d-d <g class="gr_ gr_56 gr-alert gr_gramm gr_inline_cards gr_disable_anim_appear Punctuation only-del replaceWithoutSep" id="56" data-gr-id="56">transitions,</g> or highly correlated states, then you can <g class="gr_ gr_55 gr-alert gr_gramm gr_inline_cards gr_disable_anim_appear Grammar multiReplace" id="55" data-gr-id="55">used</g> DFT+U or Onsite-hybrid. <br clear="none">
</p>
<p>Hope it will help you. <br clear="none">
</p>
<p>Cheers</p>
<p>Xavier<br clear="none">
</p>
<p><br clear="none">
</p>
<div class="ydp1c4de612yiv1933857513moz-cite-prefix">Le 27/01/2018 à 12:28, chin Sabsu a écrit :<br clear="none">
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<div>Dear Stefaan Sir</div>
<div><br clear="none">
</div>
<div>Below are my updates:</div>
<div><br clear="none">
</div>
<div>A. For mesh size</div>
<div>I got your point and in all different distorted structure
(with different inequivalent atoms) I am having the same
number of electrons while the number of inequivalent atoms are different. NE varies from 154 to
616(1x1x1 cell) to 4928 (2x2x2 cell).</div>
<div>So I will divide xxx numk
by 4 and 32 in 1x1x1 and 2x2x2 supercell structure. Thanks
for this excellent recommendation!!!</div>
<div>For 2x2x2 supercell in phonopy calculations, we
need to go for mpi
jobs for certain cases!!<br clear="none">
</div>
<div><br clear="none">
</div>
<div>B. Now I have done some tests for my cubic perovskite and
below are my observations:</div>
<div><br clear="none">
</div>
<div>B1. For tolf 5 and -fc1 I for
FOR003 = ~4 mRy / bohr with
band gap ~0.1 eV<br clear="none">
</div>
<div>B2. For tolf 2 and - fc 0.1 forces
are almost same. and the band gap is still ~0.1ev<br clear="none">
</div>
<div><br clear="none">
</div>
<div>B3. Now to reduce the forces less then 0.1mRy/ bohr , I
reduced the tolf
to 0.5 and - fc
0.1 - 0.01. I got forces at the desired accurecy but
the band gap vanished and the system becomes metallic. The
experimentally reported band gap is 1.2eV<br clear="none">
</div>
<div>I used RKMAX
8, numk 2000, vxc 19.</div>
<div><br clear="none">
</div>
<div>What is your suggestion for point number B, why I am
getting the system metallic?</div>
<div>Can I use this relaxed structure for Phonopy?</div>
<div><br clear="none">
</div>
<div>Kind regards</div>
<div><br clear="none">
</div>
<div>Chin S.</div>
<div><br clear="none">
</div>
<div> <br clear="none">
</div>
<div>For t olf<br clear="none">
</div>
<div><br clear="none">
</div>
</div>
</div></div>
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