This set of parameters, contained in the namelist block &BOUNDARY_PARAMS, is used to set up boundary conditions on the current simulation. If this namelist block is absent, periodic boundary conditions are assumed. Setting up other types of boundary conditions in RAMSES is quite complex. The user is invited to read the corresponding section. The default setting, corresponding to a periodic box, should be sufficient in most cases. The strategy to set up boundary conditions is based on using "ghost regions" outside the computational domain, where flow variables are carefully specified in order to mimic the effect of the chosen type of boundary. Note that the order in which boundary regions are specified in the namelist is very important, especially for reflexive or zero gradient boundaries. See Advanced boundary for more information on setting up such boundary conditions. Specific examples can be found in the namelist/ directory of the package.