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 < [email protected]> 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: [email protected] 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 ----------------------------------------------------------------------
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