Comment (by Roland Haas):

Running the kasner.par file from einsteeinexamples with a number of previous ET releases:

• ET_2019_03 "Proca"
• ET_2013_05 "Gauss"
• ET_2011_10 "Maxwell": aborts with NaN found in step 0
• ET_2011_05 "Curie"

shows the same behaviour for all of them, namely exponential growth i n gxx.

Note that Curie is the release before the parfile shows up and also the release before the ET paper showing the Kasner example, Maxwell is the release just after and the first one to contain the parfile.

This turns out to be due to the fact that ML_BSSN, while it extends ADMBASE::lapse_evolution_method actually ignores that parameter and evolves the lapse with its default gauge condition. This is despite the parfile setting:

ADMBase::lapse_evolution_method = "static"
ADMBase::shift_evolution_method = "static"
ADMBase::dtlapse_evolution_method = "static"
ADMBase::dtshift_evolution_method = "static"

It can be fixed by choosing gauge evolution parameters such that all RHS for the gauge quantities are zero:

# ML_BSSN ignores gaug
--

Ticket URL: https://bitbucket.org/einsteintoolkit/tickets/issues/2373/kasner-example-from-et-paper-cannot-be