I don’t know if this is a bug or a feature. Consider for example the parameter lapse_evolution_method. Lean extends the available keywords by adding LeanBSSNMoL. However, this parameter is never checked in the code and, as long as evolution_method = “LeanBSSNMoL”, the code evolves the lapse (as you can see in the files that set the rhs to zero and compute the rhss). Quickly grepping in ML_BSSN seems to indicate that the Lean is not alone in this doing this.

If this is the intended behavior, I find it very counter intuitive: why would Lean extend lapse_evolution_method if it is never used? Since the keyword exist, my expectation is that the evolution is activated only when the LeanBSSNMoL is selected, so if I set lapse_evolution_method to static I would like to have a fixed lapse. The same is true for the shift_evolution_method.

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Ticket URL: https://bitbucket.org/einsteintoolkit/tickets/issues/2487/_evolution_method-leanbssnmol-in-lean-does