<div dir="ltr">Hi Nicholas,<div><br></div><div>I am seeing some definitions which are not totally comparable. For example, the primitive cell of Si has 2 atoms, and if you use a 13x13x13 kpoint grid, the comparable supercell using just the gamma point is a 13x13x13 supercell containing 4394 atoms. As you mention, for gamma point + supercell calculation, you use a cell with 250 atoms (5x5x5 supercell) which is comparable with a primitive cell calculation with 5x5x5 kpoint grid.</div><div><br></div><div>Abot the difference using different basis sets, well is not weird, the stress tensor calculation is highly dependent on the scf convergence and the accuracy of the wavefunction. </div><div><br></div><div>Another thing that caught my attention was the OT vs diagonalization comparison. Both probably give a pretty similar solution, but no equal... I would f¿prefer compare the primitive and supercell calculation using the same wavefunction solver, especially if you use smearing, which is not implemented for OT method, and could be a source of discrepancies (albeit in a semiconductor this is not so important, if you are using a GGA or LDA functional, the band gap is highly underestimated and the smearing affects the calculation).</div><div><br></div><div>Why are you using UKS instead of RKS?</div><div><br></div><div>Regards - Lucas Lodeiro</div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">El vie, 2 abr 2021 a las 15:14, Nicholas Winner (<<a href="mailto:nwi...@berkeley.edu">nwi...@berkeley.edu</a>>) escribió:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">Hello all,<div><br></div><div>I performed two cell optimizations on bulk Si, the first with a primitive cell and kpoints and the second using OT and a 250 atom supercell. Both structures started from the Materials Project (mp-149) lowest energy Fm-3m polymorph.</div><div><br></div><div>I found that while OT only took 3 cell opt steps, the kpoint calculation took 10 steps, which I found a little strange.</div><div><br></div><div>Upon investigation I found two things: </div><div>(a) the kpoint calculation, had a large initial pressure deviation of -92570 bar, compared to OT, which had an initial deviation of -708 bar at DZVP level and -2934 at TZV2PX level. Again these are the same structure, just kpoints vs supercell. It is a little odd to me that DZVP is closer to converged pressure, but I don't think its too strange.</div><div>(b) the kpoint calculation relaxed from Fm-3m to Im-3m space group, which is not the ground state polymorph.</div><div><br></div><div>My question is whether or not this issue is due to CP2K's kpoint implementation, or if this is something more fundamental, and how (if at all) it can be resolved. Below is the abridged DFT section for the kpoint calculation, which I assume is where the problem lies, but I can provide more info if needed.</div><div><br></div><div>Best,</div><div>Nick</div><div><p>&DFT</p>
<p> BASIS_SET_FILE_NAME BASIS_MOLOPT</p><p> BASIS_SET_FILE_NAME BASIS_MOLOPT_UCL</p>
<p> POTENTIAL_FILE_NAME GTH_POTENTIALS</p>
<p> UKS True</p>
<p> MULTIPLICITY 0</p>
<p> CHARGE 0</p>
<p> &QS</p>
<p> METHOD GPW</p>
<p> EPS_DEFAULT 1e-12</p>
<p> EXTRAPOLATION PS</p>
<p> &END QS</p>
<p> &SCF</p>
<p> MAX_SCF 100</p>
<p> EPS_SCF 1e-07</p>
<p> SCF_GUESS RESTART</p>
<p> MAX_ITER_LUMO 400</p>
<p> ADDED_MOS 100</p>
<p> &DIAGONALIZATION</p>
<p> &DAVIDSON</p>
<p> PRECONDITIONER FULL_ALL</p>
<p> &END DAVIDSON</p>
<p> &END DIAGONALIZATION</p>
<p> &MIXING</p>
<p> METHOD BROYDEN_MIXING</p>
<p> ALPHA 0.2</p>
<p> NBUFFER 5</p>
<p> &END MIXING</p>
<p> &SMEAR</p>
<p> ELEC_TEMP 300</p>
<p> METHOD FERMI_DIRAC</p>
<p> FIXED_MAGNETIC_MOMENT -100.0</p>
<p> &END SMEAR</p>
<p> &END SCF</p>
<p> &KPOINTS</p>
<p> SCHEME MONKHORST-PACK 13 13 13</p>
<p> EPS_GEO 1e-06</p>
<p> FULL_GRID False</p>
<p> PARALLEL_GROUP_SIZE -1</p>
<p> SYMMETRY False</p>
<p> UNITS B_VECTOR</p>
<p> VERBOSE True</p>
<p> WAVEFUNCTIONS COMPLEX</p>
<p> &END KPOINTS</p><p>&END DFT</p></div>
<p></p>
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