# [Users] Grid structure inconsistency & errors in surface mask locations

Erik Schnetter schnetter at cct.lsu.edu
Mon Apr 19 10:49:33 CDT 2021

```Konrad

One generic way to approach this is to "walk" the solution from a
simple set of parameters (which you understand) to where you want them
to be, in small steps, so that you don't lose the horizons along the
way.

You'd start with a well-known setup, or at least a symmetric setup
(equal masses, equal distances), and then gradually change the
parameters. As you learn how the horizons that are found look like,
you can give better initial guesses for the next parameter set.

Unfortunately, the Einstein Toolkit does not contain a script that
would help you with this; you'll have to do that yourself.

Since you were worried about the resolution, and since this is only
about finding the horizon, you can use a much smaller domain (it only
needs to contain the horizons you're looking for), you can disable
mesh refinement, and you can use a rather high resolution. You can
certainly run this on a single workstation, and possibly even on a
laptop.

-erik

On Mon, Apr 19, 2021 at 10:39 AM Konrad Topolski
<k.topolski2 at student.uw.edu.pl> wrote:
>
> I have been trying a variety of initial guesses, but to no avail.
>
> The setup is, in short:
> par_b = 3.0
> target_m_plus = 0.25
> target_m_minus = 0.75
>
> And the smaller black hole's apparent horizon gets detected easily, which cannot be said of the bigger one. My estimates for the AHFinderDirect were at -1.05 to -0.95 for the center of the ellipsoid/sphere, with initial guesses for the x_radius ranging from 0.25 to 0.4, with y_radius, z _radius about 10-20% smaller.
>
> Am I still not getting enough resolution? The refinement levels in my finest setup around x=-1 are:
>
> Carpet::max_refinement_levels    =8
> CarpetRegrid2::movement_threshold_1 =   0.16
> CarpetRegrid2::num_levels_2         =  8
> CarpetRegrid2::position_x_2         = -1.0
> CarpetRegrid2::radius_2[ 1]         =  64.0
> CarpetRegrid2::radius_2[ 2]         =  16.0
> CarpetRegrid2::radius_2[ 3]         =   8.0
> CarpetRegrid2::radius_2[ 4]         =   4.0
> CarpetRegrid2::radius_2[ 5]         =   3.0
> CarpetRegrid2::radius_2[ 6]         =   2.5
> CarpetRegrid2::radius_2[ 7]         =   2.0
> CarpetRegrid2::movement_threshold_2 =   0.16
>
> I do not know a-priori if this apparent horizon is best approximated by an ellipsoid (can one see this before performing the numerical simulation?) or not, but I don't know if I'm on the right track.
> It seems weird to me that I cannot find the x=-1 AH, even though the corresponding black hole it is ~3 times more massive.
>
> In another attempt, I tried to copy the philosophy of the setup in the GW150914.rpar file,
> setting the center_offset for the two punctures and other parameters, but that didn't work either.
> I took
> target_mp=0.75, target_mm=0.25,
> separation D=6, so xp=D*mm = 1.5, xm=D*mp=-4.5,
> par_b=D/2 = 3.0, center_offset=xp-half_D=-1.5
> (all here: https://gist.github.com/konrad-topolski/1c208415d3edac2f428ded637153f639)
> I'm starting to doubt if any of these setups is correct.
>
> By the way, can I somehow redirect the outputs of all MPI processes to one file at real time, better still, override the creation of CCTK_Proc files altogether?
>
> Best regards
>
>
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> Users at einsteintoolkit.org
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--
Erik Schnetter <schnetter at cct.lsu.edu>
http://www.perimeterinstitute.ca/personal/eschnetter/
```