<div dir="ltr"><div><div><div>Dear Sergey,<br><br></div>Thanks a lot for your reply!<br><br></div>Best regards,<br></div>Yun<br></div><br><div class="gmail_quote"><div dir="ltr">Sergey Chulkov <<a href="mailto:sergeya...@gmail.com">sergeya...@gmail.com</a>> 于2018年8月27日周一上午6:06写道：<br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr">Hi Yun,<br><br>The purpose of the convergence test is to find the lowest possible value of the plane-wave cutoff that still gives you an accurate total energy. The actual accurate total energy may be higher then the "minimal" energy across all possible values of the plain-wave cutoff.<br><br>In case of sparse real-space grids (small cutoff) there is no uniform dependence between the total energy and the value of the plain-wave cutoff. The main reason is that different energy components are computed using different basis set representations. For example, a smaller plain-wave cutoff can potentially give you an underestimated exchange-correlation energy, but overestimated electrostatic energy between nuclei and electrons which can finally lead to a lower total energy.<br><br>For a reasonable guess for the cutoff value you can refer to this page (<a href="https://www.cp2k.org/faq:cutoff" target="_blank">https://www.cp2k.org/faq:cutoff</a>). The relative cutoff of 50 Ry is usually a good choice, the relative cutoff of 60 Ry is typically enough in most cases. <br><br>Best wishes,<br>Sergey<br><br>On Friday, August 24, 2018 at 3:29:24 PM UTC+1, <a href="mailto:any...@gmail.com" target="_blank">any...@gmail.com</a> wrote:<blockquote class="gmail_quote" style="margin:0;margin-left:0.8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr">Dear CP2K developers and users,<br><br>I did the convergence tests with cp2k 3.0 for hBN system, following the instructions of <a href="https://www.cp2k.org/howto:converging_cutoff" rel="nofollow" target="_blank">https://www.cp2k.org/howto:converging_cutoff</a> . The energy values and the NG on grids are as follows. I wonder why higher cutoff values give fairly higher energies, for example, the total energy for cutoff 350 ry is higher than 300 ry, I thought in principle, higher cutoff values could have more stable energy (lower value), at least not that higher than the number with lower cutoffs. But here in my case, it's not, can anybody help me to see why? Is 250Ry and rel_cutoff of 70 Ry here a good choice? <br><br>Also, when one starts a calculation, should the convergence tests always be taken with different values of cut off and rel_cutoff? or it's safe to just use larger values such as (cutoff 400 ry, rel_cutoff 80ry)? <br><br>Thanks a lot in advance for all your suggestions.<br><br>Best regards,<br>Yun<br><br># Grid cutoff vs total energy<br># Date: Fri Aug 24 13:38:48 CEST 2018<br># PWD: /home/Convergence_test<br># REL_CUTOFF = 60<br># Cutoff (Ry) | Total Energy (Ha) | NG on grid 1 | NG on grid 2 | NG on grid 3 | NG on grid 4<br> 50.00 -833.3666229006 559488 81536 0 0<br> 100.00 -824.4110369450 457024 180160 3840 0<br> 150.00 -824.3925595993 424576 134912 81536 0<br> 200.00 -824.3899074807 345088 210560 81536 3840<br> 250.00 -824.3979520649 294272 162752 180160 3840<br> 300.00 -824.3920162797 294272 162752 180160 3840<br> 350.00 -824.3858376968 271744 185280 119488 64512<br> 400.00 -824.3841314539 271744 152832 134912 81536<br> 450.00 -824.3831328479 271744 152832 134912 81536<br> 500.00 -824.3833590835 236416 184320 138752 81536<br><br><br># Rel Grid cutoff vs total energy<br># Date: Fri Aug 24 14:54:23 CEST 2018<br># PWD: /home/Convergence_test/Rel_cutoff<br># CUTOFF = 250<br># Rel Cutoff (Ry) | Total Energy (Ha) | NG on grid 1 | NG on grid 2 | NG on grid 3 | NG on grid 4<br> 10.00 -824.9353843726 24384 212032 184320 220288<br> 20.00 -824.4674485372 117312 176960 162752 184000<br> 30.00 -824.3980791451 236416 184320 138752 81536<br> 40.00 -824.3980825574 271744 152832 134912 81536<br> 50.00 -824.3979520649 294272 162752 180160 3840<br> 60.00 -824.3979520649 294272 162752 180160 3840<br> 70.00 -824.3979509738 345088 210560 81536 3840<br> 80.00 -824.3979509738 345088 210560 81536 3840<br> 90.00 -824.3979509738 420736 138752 79616 1920<br> 100.00 -824.3979509738 424576 134912 81536 0<br><br><br></div></blockquote></div> <p></p> -- <br> You received this message because you are subscribed to a topic in the Google Groups "cp2k" group.<br> To unsubscribe from this topic, visit <a href="https://groups.google.com/d/topic/cp2k/acB9J8aFjRU/unsubscribe" target="_blank">https://groups.google.com/d/topic/cp2k/acB9J8aFjRU/unsubscribe</a>.<br> To unsubscribe from this group and all its topics, send an email to <a href="mailto:cp2k+uns...@googlegroups.com" target="_blank">cp2k+uns...@googlegroups.com</a>.<br> To post to this group, send email to <a href="mailto:cp...@googlegroups.com" target="_blank">cp...@googlegroups.com</a>.<br> Visit this group at <a href="https://groups.google.com/group/cp2k" target="_blank">https://groups.google.com/group/cp2k</a>.<br> For more options, visit <a href="https://groups.google.com/d/optout" target="_blank">https://groups.google.com/d/optout</a>.<br> </blockquote></div>