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<p>Dear Dominik,</p>
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<p>This is the first time that I hear about negative Ueff. What I would do, I would just compute Ueff from first principles using the hp.x code of Quantum ESPRESSO: </p>
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Phys. Rev. B <span style="box-sizing: border-box; font-weight: 700; line-height: inherit;">98</span>, 085127 (2018).</h5>
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<div>Best regards,</div>
<div>Iurii</div>
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<div><span style="font-family: Calibri, Arial, Helvetica, sans-serif;">P.S.: Please do not forget to add your affiliation when posting to the pw_forum.</span><br>
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<font size="3" face="'Times New Roman', Times, serif" color="808080">--<br>
Dr. Iurii Timrov<br>
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<font size="3" face="'Times New Roman', Times, serif" color="808080"><font size="3" face="'Times New Roman', Times, serif" color="808080">Swiss Federal Institute of Technology Lausanne (EPFL<font color="808080"><font face="'Times New Roman', Times, serif">)</font></font></font><br>
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<div id="divRplyFwdMsg" dir="ltr"><font face="Calibri, sans-serif" style="font-size:11pt" color="#000000"><b>From:</b> users <users-bounces@lists.quantum-espresso.org> on behalf of Dominik Gresch <greschd@gmx.ch><br>
<b>Sent:</b> Tuesday, October 8, 2019 2:43:58 PM<br>
<b>To:</b> users@lists.quantum-espresso.org<br>
<b>Subject:</b> [QE-users] Negative dr2 error in DFT+U</font>
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<div>Dear QE community,</div>
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<div>I am trying to run DFT+U calculations (lda_plus_u_kind=0) with a negative Ueff = U - J. However, this frequently results in the following error being raised:</div>
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<p><span style="font-size:11pt"><span style="background:white"><span style="font-family:"Calibri",sans-serif"><span style="font-size:10.0pt"><span style="background:white"><span style="font-family:Consolas"><span style="color:#383a42">%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%<br>
Error in routine mix_rho (1):<br>
negative dr2<br>
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%</span></span></span></span></span></span></span></p>
<p>I have traced this to the <span style="font-size:10.5pt"><span style="font-family:Consolas">ns_ddot</span></span> function
<span style="font-size:10.5pt"><span style="font-family:"Calibri",sans-serif">in </span></span><span style="font-size:10.0pt"><span style="background:white"><span style="font-family:Consolas"><span style="color:black">PW/src/scf_mod.f90</span></span></span></span>,
which calculates a contribution to dr2 from the Hubbard term. Due to the negative U, this value also becomes negative. Consequently, the error can be avoided by changing <span style="font-size:11.0pt"><span style="font-family:Consolas">Hubbard_U(nt)</span></span> <span style="font-size:11.0pt"><span style="font-family:"Calibri",sans-serif">to </span></span><span style="font-size:11.0pt"><span style="font-family:Consolas">ABS(Hubbard_U(nt))</span></span>
in this function (in two places).</p>
<p>My question is this:</p>
<p>- Does this change make sense, or is the sign of ns_ddot relevant?</p>
<p>- Does it make sense physically to use negative U? These are empirical values, to match a known band structure.</p>
<p>Thank you and best regards,</p>
<p>Dominik Gresch</p>
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