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<div style="color: rgb(0, 0, 0); font-family: 微软雅黑; font-size: 14px; line-height: 1.5;">Dear nicola,</div><div style="color: rgb(0, 0, 0); font-family: 微软雅黑; font-size: 14px; line-height: 1.5;"> Thank you so much for explanation!</div><div style="color: rgb(0, 0, 0); font-family: 微软雅黑; font-size: 14px; line-height: 1.5;"> I continue investigation on Carbon. This time, I choose graphene.</div><div style="color: rgb(0, 0, 0); font-family: 微软雅黑; font-size: 14px; line-height: 1.5;"> According SSSP's data on converge pattern of Carbon here : <a href="http://materialscloud.org/sssp/results/C_conv_patt.png" _src="http://materialscloud.org/sssp/results/C_conv_patt.png">http://materialscloud.org/sssp/results/C_conv_patt.png</a> . I think we can conclude that at least for phonon calculation, GBRV is better than 031PAW, and 031PAW is better than 100PAW. And I check the input structure of SSSP, the converge pattern is based on graphite which is close to graphene.</div><div style="color: rgb(0, 0, 0); font-family: 微软雅黑; font-size: 14px; line-height: 1.5;"><br></div><div style="color: rgb(0, 0, 0); font-family: 微软雅黑; font-size: 14px; line-height: 1.5;"> So I did calculation, I choose 4 pseudopotentials</div><div style="color: rgb(0, 0, 0); font-family: 微软雅黑; font-size: 14px; line-height: 1.5;"> 1. C.pz-n-kjpaw_psl.1.0.0.UPF which is not PBE</div><div style="color: rgb(0, 0, 0); font-family: 微软雅黑; font-size: 14px; line-height: 1.5;"> 2. C.pbe-n-kjpaw_psl.0.1.UPF from pslibrary 0.3.1</div><div style="color: rgb(0, 0, 0); font-family: 微软雅黑; font-size: 14px; line-height: 1.5;"> 3. C.pbe-n-kjpaw_psl.1.0.0.UPF from pslibrary 1.0.0</div><div style="color: rgb(0, 0, 0); font-family: 微软雅黑; font-size: 14px; line-height: 1.5;"> 4. c_pbe_v1.2.uspp.F.UPF from GBRV</div><div style="color: rgb(0, 0, 0); font-family: 微软雅黑; font-size: 14px; line-height: 1.5;"><br></div><div style="color: rgb(0, 0, 0); font-family: 微软雅黑; font-size: 14px; line-height: 1.5;"> I done calculation under same condition:</div><div style="color: rgb(0, 0, 0); font-family: 微软雅黑; font-size: 14px; line-height: 1.5;"> 1. scf : kmesh=8,8,1, conv_thr = 1.0d-10</div><div style="color: rgb(0, 0, 0); font-family: 微软雅黑; font-size: 14px; line-height: 1.5;"> 2. phonon : qmesh = 4,4,1, tr2_ph=1.d-14</div><div style="color: rgb(0, 0, 0); font-family: 微软雅黑; font-size: 14px; line-height: 1.5;"><br></div><div style="color: rgb(0, 0, 0); font-family: 微软雅黑; font-size: 14px; line-height: 1.5;"> first case: ecut=40, ecutrho=361</div><div style="color: rgb(0, 0, 0); font-family: 微软雅黑; font-size: 14px; line-height: 1.5;"> different sum rule</div><div style="color: rgb(0, 0, 0); font-family: 微软雅黑; font-size: 14px; line-height: 1.5;"> 1. sum rule =simple <a href="https://pasteboard.co/GE5roha.png" _src="https://pasteboard.co/GE5roha.png" style="line-height: 1.7;">https://pasteboard.co/GE5roha.png</a> </div><div style="color: rgb(0, 0, 0); font-family: 微软雅黑; font-size: 14px; line-height: 1.5;"> 2. sum rule =crystal <a href="https://pasteboard.co/GE5rBCd.png" _src="https://pasteboard.co/GE5rBCd.png" style="line-height: 1.7;">https://pasteboard.co/GE5rBCd.png</a> </div><div style="color: rgb(0, 0, 0); font-family: 微软雅黑; font-size: 14px; line-height: 1.5;"> observations:</div><div style="color: rgb(0, 0, 0); font-family: 微软雅黑; font-size: 14px; line-height: 1.5;"> 1. only pz lda potential got TA mode right. The others failed with negative frequency no matter what sum rule</div><div style="color: rgb(0, 0, 0); font-family: 微软雅黑; font-size: 14px; line-height: 1.5;"> 2. 100paw get worse when sum rule is crystal</div><div style="color: rgb(0, 0, 0); font-family: 微软雅黑; font-size: 14px; line-height: 1.5;"><br></div><div style="color: rgb(0, 0, 0); font-family: 微软雅黑; font-size: 14px; line-height: 1.5;"> second case: ecut=50, ecutrho=500</div><div style="color: rgb(0, 0, 0); font-family: 微软雅黑; font-size: 14px; line-height: 1.5;"> different sum rule</div><div style="color: rgb(0, 0, 0); font-family: 微软雅黑; font-size: 14px; line-height: 1.5;"> 1. sum rule = simple <a href="https://pasteboard.co/GE4OvmB.png" _src="https://pasteboard.co/GE4OvmB.png" style="line-height: 1.5;">https://pasteboard.co/GE4OvmB.png</a></div><div style="color: rgb(0, 0, 0); font-family: 微软雅黑; font-size: 14px; line-height: 1.5;"> 2. sum rule = crystal <a href="https://pasteboard.co/GE4OSFKa.png" _src="https://pasteboard.co/GE4OSFKa.png" style="line-height: 1.5;">https://pasteboard.co/GE4OSFKa.png</a></div><div style="color: rgb(0, 0, 0); font-family: 微软雅黑; font-size: 14px; line-height: 1.5;"> observations:</div><div style="color: rgb(0, 0, 0); font-family: 微软雅黑; font-size: 14px; line-height: 1.5;"> 1. for sum rule = simple, only pz lda got TA mode right</div><div style="color: rgb(0, 0, 0); font-family: 微软雅黑; font-size: 14px; line-height: 1.5;"> 2. for sum rule = crystal, surprisingly, 100paw also got TA mode right this time</div><div style="color: rgb(0, 0, 0); font-family: 微软雅黑; font-size: 14px; line-height: 1.5;"> </div><div style="color: rgb(0, 0, 0); font-family: 微软雅黑; font-size: 14px; line-height: 1.5;"> So, several questions</div><div style="color: rgb(0, 0, 0); font-family: 微软雅黑; font-size: 14px; line-height: 1.5;"> 1. According to SSSP result, at ecut=40, dual=8, phonon dispersion is already quite converged GBRV and 031paw. But even set ecut=50, they doesn't get TA mode right. However, 100PAW got TA mode right at ecut=50. But, SSSP pick GBRV as best pseudopotential. This confused me. I seems that what is best for one structure is not generally best for other structure, and in this case, the best pseudopotential always give negative frequency. Is it because that graphene is too special? Or more generally is two dimensional material special case and not suited for SSSP result?</div><div style="color: rgb(0, 0, 0); font-family: 微软雅黑; font-size: 14px; line-height: 1.5;"> 2. For ecut=40, crystal sum rule make 100paw worse, however for ecut=50, crystal sum rule make 100paw right. I remember I saw somewhere in the forum that crystal sum rule should be generally better than simple. However, in this case, crystal sum rule is sometimes good, sometimes bad. This makes choosing sum rule some kind of arbitrary. Can we think crystal rule is better than simple rule when crystal generates more negative frequncy? Can we choose sum rule just because it removes all negative freq and makes thing better looking? </div><div style="color: rgb(0, 0, 0); font-family: 微软雅黑; font-size: 14px; line-height: 1.5;"> 3. For graphene, LDA always gives correct TA mode, while PBE is quite uncontrollable even at large ecut if we didn't choose the "right" pseudopotential. But I also see sayings that PBE is better for interface because it contains gradient. So I am wondering do we really need to stick with PBE when study two dimensional material? What do we loss if we use LDA, does it affect for example accuracy of electron-phonon coupling calculation?</div><div style="color: rgb(0, 0, 0); font-family: 微软雅黑; font-size: 14px; line-height: 1.5;"><br></div><div style="color: rgb(0, 0, 0); font-family: 微软雅黑; font-size: 14px; line-height: 1.5;">-----------------------------------</div><div style="color: rgb(0, 0, 0); font-family: 微软雅黑; font-size: 14px; line-height: 1.5;">finally this is scf file I used for calculation</div><div style="color: rgb(0, 0, 0); font-family: 微软雅黑; font-size: 14px; line-height: 1.5;"><br></div><div><div><span style="line-height: 21px;">&CONTROL</span></div><div><span style="line-height: 21px;">prefix='graphene',</span></div><div><span style="line-height: 21px;">calculation='scf',</span></div><div><span style="line-height: 21px;">restart_mode='from_scratch',</span></div><div><span style="line-height: 21px;">wf_collect=.true.,</span></div><div><span style="line-height: 21px;">verbosity='high',</span></div><div><span style="line-height: 21px;">tstress=.true.,</span></div><div><span style="line-height: 21px;">tprnfor=.true.,</span></div><div><span style="line-height: 21px;">outdir='/qe_tmpdir',</span></div><div><span style="line-height: 21px;">pseudo_dir='/pseudo',</span></div><div><span style="line-height: 21px;">/</span></div><div><span style="line-height: 21px;">&SYSTEM</span></div><div><span style="line-height: 21px;"><span class="Apple-tab-span" style="white-space:pre"> </span>ibrav = 4,celldm(1)=4.6509939378d0, celldm(3)=8.1261173411d0,nat = 2,ntyp = 1,</span></div><div><span style="line-height: 21px;"><span class="Apple-tab-span" style="white-space:pre"> </span>ecutwfc = 40,ecutrho = 361,</span></div><div><span style="line-height: 21px;">/</span></div><div><span style="line-height: 21px;">&ELECTRONS</span></div><div><span style="line-height: 21px;"><span class="Apple-tab-span" style="white-space:pre"> </span>conv_thr = 1.0d-10, </span></div><div><span style="line-height: 21px;">/</span></div><div><span style="line-height: 21px;">ATOMIC_SPECIES</span></div><div><span style="line-height: 21px;">C 12.011 C.pbe-n-kjpaw_psl.1.0.0.UPF</span></div><div><span style="line-height: 21px;">ATOMIC_POSITIONS {crystal}</span></div><div><span style="line-height: 21px;"> C 0.0000000000d0 0.0000000000d0 0.0000000000d0</span></div><div><span style="line-height: 21px;"> C 0.3333333333d0 0.6666666667d0 0.0000000000d0</span></div><div><span style="line-height: 21px;">K_POINTS automatic </span></div><div><span style="line-height: 21px;">8 8 1 1 1 1</span></div></div><div style="color: rgb(0, 0, 0); font-family: 微软雅黑; font-size: 14px; line-height: 1.5;">--------------------------------------</div><div style="color: rgb(0, 0, 0); font-family: 微软雅黑; font-size: 14px; line-height: 1.5;"><br></div><div style="color: rgb(0, 0, 0); font-family: 微软雅黑; font-size: 14px; line-height: 1.5;">best regards</div><div style="color: rgb(0, 0, 0); font-family: 微软雅黑; font-size: 14px; line-height: 1.5;"> </div><div style="color: rgb(0, 0, 0); font-family: 微软雅黑; font-size: 14px; line-height: 1.5;"> </div><div style="color: rgb(0, 0, 0); font-family: 微软雅黑; font-size: 14px; line-height: 1.5;"> </div><div style="color: rgb(0, 0, 0); font-family: 微软雅黑; font-size: 14px; line-height: 1.5;"> </div>
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