<html><head><meta http-equiv="content-type" content="text/html; charset=us-ascii"></head><body style="overflow-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;">On Nov 19, 2025, at 10:26, Steven Brandt via Users <users@einsteintoolkit.org> wrote:<br><div><blockquote type="cite"><br><div><div class="moz-cite-prefix" style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none;">On 11/19/2025 8:24 AM, Erik Schnetter wrote:<br></div><blockquote type="cite" cite="mid:C98218AB-873C-4B74-8378-91B0BBAAAAD1@gmail.com" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none;">Miguel<div><br></div><div>If I recall correctly, Ian Hinder studied convergence of black hole simulations with subcycling in time in great detail. The Einstein Toolkit gallery example for GW150914 contains the respective distilled knowledge. <a href="https://einsteintoolkit.org/gallery/bbh/index.html" moz-do-not-send="true" class="moz-txt-link-freetext">https://einsteintoolkit.org/gallery/bbh/index.html</a></div><div><br></div><div>Some important details that I recall:</div><div>- You can regrid only when the fine and coarse grids are aligned</div><div>- You cannot use time interpolation at all. You need to use enough buffer zones for all the RK substeps for all the fine timesteps for each coarse time step. With 3 ghost zones and RK4 you need 21 buffer zones.</div></blockquote><p style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none;">Does no time interpolation mean no dense output? That didn't exist when Ian did these tests, right?</p></div></blockquote><div>Output doesn't affect time evolution, so it doesn't matter which way you output things. Of course, if you use second-order accurate interpolation to output a quantity you cannot expect 4th order convergence for these quantities. If you output time-interpolated values of e.g. the lapse then you should check convergence only for the fine grid values of the lapse there, not for the interpolated coarse grid values.</div><div><br></div><div>-erik</div><div><br></div></div><div>-- </div><div>Erik Schnetter <schnetter@gmail.com></div><div>http://www.perimeterinstitute.ca/personal/eschnetter/</div><div><br></div></body></html>