Comment (by Gabriele Bozzola):

The way I personally interpret the parameter lapse_evolution_method (and shift_evolution_method) is “the lapse will be evolved according to the method specified”. Therefore, my expectation is that that if it is not set to LeanBSSNMoL (or ML_BSSN), Lean (or ML_BSSN) should not evolve the lapse. In this way I can set it to static (evolve the metric in a fixed gauge, which is why I needed this), or develop my thorn to evolve with my personal choice. However, I don’t know if this is the intended behavior or not. Maybe evolution_method implies that metric, lapse, and shift have to be evolved in the same way.

In any case, it is easy trivial to implement the behavior I describe: the simplest way is to put a conditional in front of the code that writes rhs_lapse and rhs_shift checking if lapse_evolution_method is LeanBSSNMoL. A better solution should also avoid computing/syncing stuff that is not needed.

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