Together with a student of mine we are trying to use the CCZ4 formulation implemented in McLachlan. We saw that most parameters are the same as in BSSN, but there are damping factors that need to be selected.

In Alic et al 2013 (https://link.aps.org/doi/10.1103/PhysRevD.88.064049) they mention 3 damping factors (k1, k2, and k3), but in the param.ccl of McLachlan/ML_CCZ4 only k1 and k2 are mentioned (assuming they are the same).

Is it because k3 is hardcoded to the value suggested in Alic et al 2013? Or does McLachlan/ML_CCZ4 use a different set of evolution equations (in which case, is there a reference)?

We were able to find some example parameter files (for example the ones from the Parma group https://einstein.pr.infn.it/gravity/Research/BNS2015.html) and we found that they used indeed k1 and k2 from Alic et al 2013:

ML_CCZ4::dampk1 = 0.05
ML_CCZ4::dampk2 = 0

We also saw that they added ML_CCZ4::ML_Theta to the Dissipation vars and set

ML_CCZ4::apply_dissipation = "never".

The other parameters look the same as when using BSSN.

Since I never used ML_CCZ4 I want to be sure that I'm setting the parameters correctly and the damping factors seem to be the most delicate point.

Thanks,

Bruno

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

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

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