Use only min mass gas in time step

doc/RTD/source/ParameterFiles/parameter_description.rst

@@ -601,9 +601,13 @@ the start and end times or scale factors from the parameter file.
 
 * Dimensionless pre-factor of the maximal allowed displacement:
   ``max_dt_RMS_factor`` (default: ``0.25``)
-
-This value rarely needs altering. See the theory documents for its
-precise meaning.
+* Whether or not only the gas particle masses should be considered for
+  the baryon component of the calculation: ``dt_RMS_use_gas_only`` (default: ``0``)
+  
+These values rarely need altering. The second parameter is only
+meaningful if a subgrid model produces star (or other) particles with
+masses substantially smaller than the gas masses. See the theory
+documents for the precise meanings.
 
 A full time-step section for a non-cosmological run would be:
 
@@ -620,10 +624,11 @@ Whilst for a cosmological run, one would need:
 .. code:: YAML
 
   TimeIntegration:
-    dt_max:            1e-4
-    dt_min:            1e-10
-    max_dt_RMS_factor: 0.25     # Default optional value
-
+    dt_max:              1e-4
+    dt_min:              1e-10
+    max_dt_RMS_factor:   0.25     # Default optional value
+    dt_RMS_use_gas_only: 0        # Default optional value
+    
 .. _Parameters_ICs:
 
 Initial Conditions

examples/GEAR/AgoraCosmo/agora_cosmo.yml

@@ -14,6 +14,7 @@ TimeIntegration:
   dt_min:     1e-16 # The minimal time-step size of the simulation (in internal units).
   dt_max:     0.1  # The maximal time-step size of the simulation (in internal units).
   max_dt_RMS_factor: 0.25  # (Optional) Dimensionless factor for the maximal displacement allowed based on the RMS velocities.
+  dt_RMS_use_gas_only: 1 
 
 # Cosmological parameters
 Cosmology:

examples/GEAR/ZoomIn/zoom_in.yml

@@ -19,7 +19,8 @@ TimeIntegration:
   dt_min:     1e-16 # The minimal time-step size of the simulation (in internal units).
   dt_max:     0.1  # The maximal time-step size of the simulation (in internal units).
   max_dt_RMS_factor: 0.25  # (Optional) Dimensionless factor for the maximal displacement allowed based on the RMS velocities.
-
+  dt_RMS_use_gas_only: 1 
+  
 # Cosmological parameters
 Cosmology:
   h:              0.673        # Reduced Hubble constant

examples/parameter_example.yml

@@ -143,12 +143,13 @@ Scheduler:
 
 # Parameters governing the time integration (Set dt_min and dt_max to the same value for a fixed time-step run.)
 TimeIntegration:
-  time_begin:        0.    # The starting time of the simulation (in internal units).
-  time_end:          1.    # The end time of the simulation (in internal units).
-  dt_min:            1e-6  # The minimal time-step size of the simulation (in internal units).
-  dt_max:            1e-2  # The maximal time-step size of the simulation (in internal units).
-  max_dt_RMS_factor: 0.25  # (Optional) Dimensionless factor for the maximal displacement allowed based on the RMS velocities.
-
+  time_begin:          0.    # The starting time of the simulation (in internal units).
+  time_end:            1.    # The end time of the simulation (in internal units).
+  dt_min:              1e-6  # The minimal time-step size of the simulation (in internal units).
+  dt_max:              1e-2  # The maximal time-step size of the simulation (in internal units).
+  max_dt_RMS_factor:   0.25  # (Optional) Dimensionless factor for the maximal displacement allowed based on the RMS velocities.
+  dt_RMS_use_gas_only: 0     # (Optional) When computing the max RMS dt, should only the gas particles be considered in the baryon component calculation?
+  
 # Parameters governing the snapshots
 Snapshots:
   basename:   output      # Common part of the name of output files.

src/engine.c

@@ -2820,6 +2820,8 @@ void engine_init(
   e->dt_max_RMS_displacement = FLT_MAX;
   e->max_RMS_displacement_factor = parser_get_opt_param_double(
       params, "TimeIntegration:max_dt_RMS_factor", 0.25);
+  e->max_RMS_dt_use_only_gas = parser_get_opt_param_int(
+      params, "TimeIntegration:dt_RMS_use_gas_only", 0);
   e->dt_kick_grav_mesh_for_io = 0.f;
   e->a_first_statistics =
       parser_get_opt_param_double(params, "Statistics:scale_factor_first", 0.1);
@@ -3115,9 +3117,12 @@ void engine_recompute_displacement_constraint(struct engine *e) {
     /* Baryon case */
     if (N_b > 0.f) {
 
-      /* Minimal mass for the baryons */
-      const float min_mass_b =
-          min4(min_mass[0], min_mass[3], min_mass[4], min_mass[5]);
+      /* Minimal mass for the bayons */
+      float min_mass_b;
+      if (e->max_RMS_dt_use_only_gas)
+        min_mass_b = min_mass[0];
+      else
+        min_mass_b = min4(min_mass[0], min_mass[3], min_mass[4], min_mass[5]);
 
       /* Inter-particle sepration for the baryons */
       const float d_b = cbrtf(min_mass_b / (Ob * rho_crit0));

src/engine.h

@@ -163,6 +163,10 @@ struct engine {
   /* Dimensionless factor for the RMS time-step condition. */
   double max_RMS_displacement_factor;
 
+  /* When computing the max RMS dt, should only the gas particles
+   * be considered as the baryon component? */
+  int max_RMS_dt_use_only_gas;
+
   /* Time of the simulation beginning */
   double time_begin;