Hello Hal,
Is there an existing thorn that allows you to read in initial grid data from a file? I see EinsteinInitialData/IDFileADM, but that does not actually implement any reading.
The file reader in thorn IOUtil will do that for you. See $CACTUS/repos/carpet/Carpet/CarpetIOHDF5/test/input_initial_data.par for an example.
Yours, Roland