[Users] question about ML_CCZ4 parameters

[Bruno Giacomazzo]

Michael and  Federico,
   thanks a lot for your emails. This information is very helpful.

Cheers,
Bruno


Il giorno gio 2 giu 2022 alle ore 16:30 Federico Guercilena <
guercilena.federico at gmail.com> ha scritto:

> Dear Bruno,
>
> contrary to Erik, I was not around when ML_CCZ4 was implemented, but I
> used it (or tried to use it) rather extensively.
>
> Regarding the parameter named k3 in the papers, I'm pretty sure that's the
> one named GammaShift in the code. It is not hard coded, you can change it
> in the parfile, but its default value is 0.5 . As noted in the two original
> CCZ4 papers, this value breaks the covariance of the system (it should be
> set to 1. for it to be restored), but it allows to handle BH singularities.
>
> The values of 0.05 and 0 for k1 and k2 (which do correspond to the k1 and
> k2 of the papers) are "standard" as far as I can tell. Another value I've
> seen a lot in parfiles for k1 is 0.036, while as far as I know k2 has
> always been set to 0. I cannot recall any real difference in BNS runs with
> k1=0.05 or k1=0.036, but I never tried to systematically tune these
> parameters to "optimal" values.
>
> Finally, I want to note that I had a lot of subtle problems in using
> ML_CCZ4 that made me switch to ML_BSSN. These would not be outright
> crashes, but larger than expected oscillations of stars that in the long
> term made some simulations unusable to get realistic data, despite the GR
> constraints violations being much lower than ML_BSSN. When I first
> encountered this problem, I somehow got the impression that they might be
> related to some conflict with boundary conditions, but now I'm starting to
> think that they are related to the values of k1, k2 and k3 not being tuned
> properly, or maybe even by the system not being strongly hyperbolic (note
> that as far as I know, there is no actual proof of strong hyperbolicity for
> the original version of CCZ4 as implemented in McLachaln in the literature.
> Other versions of the system have been proved hyperbolic, but not this one).
>
> Actually, if you and your student took the time to do some tuning and
> managed to improve the performance of this code, that would be pretty
> interesting in its own right, I think.
>
> I hope this was of some help.
>
> Best,
> Federico
>
>
>

-- 

Prof. Bruno Giacomazzo
Department of Physics
University of Milano-Bicocca
Piazza della Scienza 3
20126 Milano
Italy

email: bruno.giacomazzo at unimib.it
phone: (+39) 02 6448 2321
web: http://www.brunogiacomazzo.org

----------------------------------------------------------------------
There are only 10 types of people in the world:
Those who understand binary, and those who don't
----------------------------------------------------------------------
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://lists.einsteintoolkit.org/pipermail/users/attachments/20220602/6962f6c6/attachment.html