Hello Ryn,

I am trying to do a cosmology simulation project using Cactus. For this project, I'm evolving an expanding homogeneous fluid in a box (FLRW spacetime), and I would like to use flat boundary conditions. I've attached the par file I am currently using. However, when I run this, the boundary cells do not seem to update at each time step to match the current value of the grid cells on the edge, as seen in this plot of the second time step output:

The parameter admbase_boundary_condition is only acted on if you set the evolution method to "static" (see schedule.ccl for routine ADMBase_Boundaries which is the only one using the parameter).

Since you have a BSSN evolution thorn enabled it will compute the ADM variables from the BSSN ones. So to set a certain boundary condition on the ADM variables you need to set McLachlan's options. Eg

ML_BSSN::rhs_boundary_condition = "scalar" # set RHS to 0 in boundary

ML_BSSN::ML_log_confac_bound = "flat" ML_BSSN::ML_metric_bound = "flat" ML_BSSN::ML_Gamma_bound = "flat" ML_BSSN::ML_trace_curv_bound = "flat" ML_BSSN::ML_curv_bound = "flat" ML_BSSN::ML_lapse_bound = "flat" ML_BSSN::ML_dtlapse_bound = "flat" ML_BSSN::ML_shift_bound = "flat" ML_BSSN::ML_dtshift_bound = "flat"

Though this may or may not actually make things crash with bad values ("flat" being an unusual boundary condition for the metric).

Yours, Roland