[Users] Stop MoL after tolerance is reached

[Haas, Roland]

Hello Erik,

as to how to decide to stop: the simplest way would be to compute a
pointwise stopping criterion (maybe magnitude of the right-hand-side or
so) then do a reduction on it using the CCTK_Reduce function
(assuming you are using Carpet), see eg. the code in Hydro_Analysis to
compute the center of mass: 

https://bitbucket.org/einsteintoolkit/einsteinanalysis/src/master/Hydro_Analysis/src/CoreCentroid.c#lines-100

the function itself is documented here:

https://www.einsteintoolkit.org/usersguide/UsersGuidech9.html#verbatim-63

Once you have decided to stop, terminate the simulation by calling
CCTK_TerminateNext
(https://www.einsteintoolkit.org/referencemanual/ReferenceManualch2.html#x4-192000A2)
making sure to have enabled the option to write a checkpoint when
terminating (io::checkpoint_on_terminate = yes).

You can read data from the checkpoint as initial data using the
filereader interface 

https://www.einsteintoolkit.org/thornguide/CactusBase/IOUtil/documentation.html#x1-1000010

and if you need to change grid structure, you can use the
ReadInterpolate thorn
(https://github.com/rhaas80/ReadInterpolate/tree/master/ReadInterpolate),
which is however a bit more complex to use.

If you need to change data in between what was checkpointed and what
needs to be evolved you will have to add routines to the INITIAL bin
after IOUtil_RecoverIDFromDatafiles (see IOUtil's schedule.ccl).

Hope this helps getting you started.

Yours,
Roland

> Dear all
> 
> I'm trying to construct Brill wave initial data by transforming the
> elliptic pde equation for the conformal factor in to a wave-like equation
> with a source. After reaching a stationary state, I want to stop the
> evolution and give the stationary state as an initial data for the BSSN
> system.
> 
> I'm evolving the wave-like equation with MoL and my question is, How can I
> stop the MoL evolution if the solution reaches the numerical tolerance and
> then pass it as initial data?
> 
> Erik



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