Comment (by Roland Haas):

There’s also a bug in that setting dtalp requires computing spatial derivatives (via SummationByParts Diff_Gv aliased function) so that it can only be computed in the interior, but no SYNC or boundary condition is applied. The easiest fix to this (since boundary conditions applied to initial data are tricky) would likely be to compute alp on a grid larger than the Cactus grid so that dx_alp can be computed on the whole Cactus grid.

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Ticket URL: https://bitbucket.org/einsteintoolkit/tickets/issues/2631/meudon_binns-fails-if-initial_dtshift-or