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<p>Dear Houcine,</p>
<p><br>
</p>
<p>First of all, which exchange-correlation functional are you using? If you are using LDA, that may explain the underestimation of the lattice constants. Taking the experimental lattice and relaxing the structure without relaxing the cell ('relax' instead
of 'vc-relax') is a valid approach. I personally prefer to relax both, provided that the level of theory I am using gives the lattice constants that are accurate enough. The system doesn't look to me like something to which I would directly apply DFT-D3, but
I haven't studied this type of materials.<br>
</p>
<p><br>
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<p>Best wishes,</p>
<p>Marko<br>
</p>
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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 후신 부지드 <bouzid@skku.edu><br>
<b>Sent:</b> Friday, December 20, 2019 2:57:28 PM<br>
<b>To:</b> users@lists.quantum-espresso.org<br>
<b>Subject:</b> [QE-users] Large difference between experimental lattice constant and the relaxed one w/ and w/o dft-d3</font>
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<p><span id="rc">Dear QE users,</span></p>
<p><span id="rc">I'm working on a layered material (SnS2) slightly doped with transition metals (TM)</span></p>
<p><span id="rc">the experimental c lattice constant of this Trigonal system is around 6.88 A as measured by XRD (slightly less with Mn doping)</span></p>
<p><span id="rc">DFT calculation without any dispersion force correction gives 6.49 A</span></p>
<p><span id="rc">by including a dft-d3 correction during the relaxation it is even less ~ 5.95
<br>
</span></p>
<p><span id="rc">- Any advice what value should I consider ? It is correct to take the experimental value without any relaxation ?</span></p>
<p><span id="rc">- Same inquiry about London dispersion force correction, is it needed to use it in this case for next steps (scf and nscf caculations) since it is giving a big difference compared with the experimental value ?</span></p>
<p><span id="rc">Any help is appreciated!<br>
</span></p>
<p><span id="rc">Best Regards</span></p>
<p><span id="rc"><br>
</span></p>
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<p><span style="color: rgb(0, 37, 222); font-family: Calibri, sans-serif; font-size: 10pt; background-color: rgb(255, 255, 255);">Houcine BOUZID
</span><span style="color: rgb(0, 37, 222); font-family: Calibri, sans-serif; font-size: 10pt; background-color: rgb(255, 255, 255);"><br>
Sungkyunkwan University, <br>
Korea. </span></p>
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