Merge branch 'particle_splitting' into 'master'

Particle splitting "a la EAGLE"

See merge request !984

doc/RTD/source/ParameterFiles/parameter_description.rst

@@ -373,6 +373,15 @@ prevent the smoothing length from going below this value in dense
 environments. This will lead to smoothing over more particles than specified
 by :math:`\eta`.
 
+The optional parameter ``particle_splitting`` (Default: 0) activates the
+splitting of overly massive particles into 2. By switching this on, the code
+will loop over all the particles at every tree rebuild and split the particles
+with a mass above a fixed threshold into two copies that are slightly shifted
+(by a randomly orientated vector of norm :math:`0.2h`). Their masses and other
+relevant particle-carried quantities are then halved. The mass threshold for
+splitting is set by the parameter ``particle_splitting_mass_threshold`` which is
+specified using the internal unit system.
+
 The final set of parameters in this section determine the initial and minimum
 temperatures of the particles.
 
@@ -399,14 +408,17 @@ The full section to start a typical cosmological run would be:
 .. code:: YAML
 
    SPH:
-     resolution_eta:           1.2
-     CFL_condition:            0.1
-     h_tolerance:              1e-4
-     h_min_ratio:              0.1
-     initial_temperature:      273
-     minimal_temperature:      100
-     H_mass_fraction:          0.755
-     H_ionization_temperature: 1e4
+     resolution_eta:                     1.2
+     CFL_condition:                      0.1
+     h_tolerance:                        1e-4
+     h_min_ratio:                        0.1
+     h_max:                              1.    # U_L
+     initial_temperature:                273   # U_T
+     minimal_temperature:                100   # U_T
+     H_mass_fraction:                    0.755
+     H_ionization_temperature:           1e4   # U_T
+     particle_splitting:                 1 
+     particle_splitting_mass_threshold:  5e-3  # U_M
 
 .. _Parameters_Stars:
 

examples/EAGLE_ICs/EAGLE_12/eagle_12.yml

@@ -48,12 +48,14 @@ Gravity:
   
 # Parameters for the hydrodynamics scheme
 SPH:
-  resolution_eta:        1.2348   # Target smoothing length in units of the mean inter-particle separation (1.2348 == 48Ngbs with the cubic spline kernel).
-  h_min_ratio:           0.1      # Minimal smoothing in units of softening.
-  h_max:                 0.5      # Maximal softening in co-moving internal units.
-  CFL_condition:         0.1      # Courant-Friedrich-Levy condition for time integration.
-  minimal_temperature:   100.0    # (internal units)
-  initial_temperature:   268.7
+  resolution_eta:                    1.2348   # Target smoothing length in units of the mean inter-particle separation (1.2348 == 48Ngbs with the cubic spline kernel).
+  h_min_ratio:                       0.1      # Minimal smoothing in units of softening.
+  h_max:                             0.5      # Maximal softening in co-moving internal units.
+  CFL_condition:                     0.1      # Courant-Friedrich-Levy condition for time integration.
+  minimal_temperature:               100.0    # (internal units)
+  initial_temperature:               268.7    # (internal units)
+  particle_splitting:                1        # Particle splitting is ON
+  particle_splitting_mass_threshold: 7e-4     # (internal units, i.e. 7e6 Msun ~ 4x initial gas particle mass)
 
 # Parameters of the stars neighbour search
 Stars:

examples/EAGLE_ICs/EAGLE_25/eagle_25.yml

@@ -48,12 +48,14 @@ Gravity:
   
 # Parameters for the hydrodynamics scheme
 SPH:
-  resolution_eta:        1.2348   # Target smoothing length in units of the mean inter-particle separation (1.2348 == 48Ngbs with the cubic spline kernel).
-  h_min_ratio:           0.1      # Minimal smoothing in units of softening.
-  h_max:                 0.5      # Maximal softening in co-moving internal units.
-  CFL_condition:         0.1      # Courant-Friedrich-Levy condition for time integration.
-  minimal_temperature:   100.0    # (internal units)
-  initial_temperature:   268.7
+  resolution_eta:                    1.2348   # Target smoothing length in units of the mean inter-particle separation (1.2348 == 48Ngbs with the cubic spline kernel).
+  h_min_ratio:                       0.1      # Minimal smoothing in units of softening.
+  h_max:                             0.5      # Maximal softening in co-moving internal units.
+  CFL_condition:                     0.1      # Courant-Friedrich-Levy condition for time integration.
+  minimal_temperature:               100.0    # (internal units)
+  initial_temperature:               268.7    # (internal units)
+  particle_splitting:                1        # Particle splitting is ON
+  particle_splitting_mass_threshold: 7e-4     # (internal units, i.e. 7e6 Msun ~ 4x initial gas particle mass)
 
 # Parameters of the stars neighbour search
 Stars:

examples/EAGLE_ICs/EAGLE_50/eagle_50.yml

@@ -48,12 +48,14 @@ Gravity:
 
 # Parameters for the hydrodynamics scheme
 SPH:
-  resolution_eta:        1.2348   # Target smoothing length in units of the mean inter-particle separation (1.2348 == 48Ngbs with the cubic spline kernel).
-  h_min_ratio:           0.1      # Minimal smoothing in units of softening.
-  h_max:                 0.5      # Maximal softening in co-moving internal units.
-  CFL_condition:         0.1      # Courant-Friedrich-Levy condition for time integration.
-  minimal_temperature:   100.0    # (internal units)
-  initial_temperature:   268.7
+  resolution_eta:                    1.2348   # Target smoothing length in units of the mean inter-particle separation (1.2348 == 48Ngbs with the cubic spline kernel).
+  h_min_ratio:                       0.1      # Minimal smoothing in units of softening.
+  h_max:                             0.5      # Maximal softening in co-moving internal units.
+  CFL_condition:                     0.1      # Courant-Friedrich-Levy condition for time integration.
+  minimal_temperature:               100.0    # (internal units)
+  initial_temperature:               268.7    # (internal units)
+  particle_splitting:                1        # Particle splitting is ON
+  particle_splitting_mass_threshold: 7e-4     # (internal units, i.e. 7e6 Msun ~ 4x initial gas particle mass)
 
 # Parameters of the stars neighbour search
 Stars:

examples/EAGLE_ICs/README

@@ -11,6 +11,9 @@ the following changes have been made:
    been changed to 1.79 ckpc. This follows the recommendations of
    Ludlow et al. 2019. Old values were 0.7 pkpc and 2.69 ckpc for
    all the particle species.
+ - SPH particles with a mass larger than 7*10^6 Msun (~4x the initial
+   gas particle mass) are now split into 2 equal mass particles
+   within the smoothing length of the original particle.
  - The metallicity-dependent density threshold for star formation
    uses the smoothed metallicities and not the raw metallicities
    any more.

examples/EAGLE_low_z/EAGLE_12/eagle_12.yml

@@ -54,11 +54,13 @@ Gravity:
   
 # Parameters for the hydrodynamics scheme
 SPH:
-  resolution_eta:        1.2348   # Target smoothing length in units of the mean inter-particle separation (1.2348 == 48Ngbs with the cubic spline kernel).
-  h_min_ratio:           0.1      # Minimal smoothing in units of softening.
-  h_max:                 0.5      # Maximal softening in co-moving internal units.
-  CFL_condition:         0.1      # Courant-Friedrich-Levy condition for time integration.
-  minimal_temperature:   100      # (internal units)
+  resolution_eta:        1.2348           # Target smoothing length in units of the mean inter-particle separation (1.2348 == 48Ngbs with the cubic spline kernel).
+  h_min_ratio:           0.1              # Minimal smoothing in units of softening.
+  h_max:                 0.5              # Maximal softening in co-moving internal units.
+  CFL_condition:         0.1              # Courant-Friedrich-Levy condition for time integration.
+  minimal_temperature:   100              # (internal units)
+  particle_splitting:    1
+  particle_splitting_mass_threshold: 7e-4 # Internal units (i.e. 7e6 Msun ~ 4 times the initial gas mass)
 
 # Parameters of the stars neighbour search
 Stars:

examples/EAGLE_low_z/EAGLE_25/eagle_25.yml

@@ -62,11 +62,13 @@ Gravity:
 
 # Parameters for the hydrodynamics scheme
 SPH:
-  resolution_eta:        1.2348   # Target smoothing length in units of the mean inter-particle separation (1.2348 == 48Ngbs with the cubic spline kernel).
-  h_min_ratio:           0.1      # Minimal smoothing in units of softening.
-  h_max:                 0.5      # Maximal softening in co-moving internal units.
-  CFL_condition:         0.1      # Courant-Friedrich-Levy condition for time integration.
-  minimal_temperature:   100      # (internal units)
+  resolution_eta:        1.2348           # Target smoothing length in units of the mean inter-particle separation (1.2348 == 48Ngbs with the cubic spline kernel).
+  h_min_ratio:           0.1              # Minimal smoothing in units of softening.
+  h_max:                 0.5              # Maximal softening in co-moving internal units.
+  CFL_condition:         0.1              # Courant-Friedrich-Levy condition for time integration.
+  minimal_temperature:   100              # (internal units)
+  particle_splitting:    1
+  particle_splitting_mass_threshold: 7e-4 # Internal units (i.e. 7e6 Msun ~ 4 times the initial gas mass)
 
 # Parameters of the stars neighbour search
 Stars:

examples/EAGLE_low_z/EAGLE_50/eagle_50.yml

@@ -53,11 +53,13 @@ Gravity:
 
 # Parameters for the hydrodynamics scheme
 SPH:
-  resolution_eta:        1.2348   # Target smoothing length in units of the mean inter-particle separation (1.2348 == 48Ngbs with the cubic spline kernel).
-  h_min_ratio:           0.1      # Minimal smoothing in units of softening.
-  h_max:                 0.5      # Maximal softening in co-moving internal units.
-  CFL_condition:         0.1      # Courant-Friedrich-Levy condition for time integration.
-  minimal_temperature:   100      # (internal units)
+  resolution_eta:        1.2348           # Target smoothing length in units of the mean inter-particle separation (1.2348 == 48Ngbs with the cubic spline kernel).
+  h_min_ratio:           0.1              # Minimal smoothing in units of softening.
+  h_max:                 0.5              # Maximal softening in co-moving internal units.
+  CFL_condition:         0.1              # Courant-Friedrich-Levy condition for time integration.
+  minimal_temperature:   100              # (internal units)
+  particle_splitting:    1
+  particle_splitting_mass_threshold: 7e-4 # Internal units (i.e. 7e6 Msun ~ 4 times the initial gas mass)
 
 # Parameters of the stars neighbour search
 Stars:

examples/EAGLE_low_z/EAGLE_6/eagle_6.yml

@@ -63,11 +63,13 @@ Gravity:
 
 # Parameters for the hydrodynamics scheme
 SPH:
-  resolution_eta:        1.2348   # Target smoothing length in units of the mean inter-particle separation (1.2348 == 48Ngbs with the cubic spline kernel).
-  h_min_ratio:           0.1      # Minimal smoothing in units of softening.
-  h_max:                 0.5      # Maximal softening in co-moving internal units.
-  CFL_condition:         0.1      # Courant-Friedrich-Levy condition for time integration.
-  minimal_temperature:   100      # (internal units)
+  resolution_eta:        1.2348           # Target smoothing length in units of the mean inter-particle separation (1.2348 == 48Ngbs with the cubic spline kernel).
+  h_min_ratio:           0.1              # Minimal smoothing in units of softening.
+  h_max:                 0.5              # Maximal softening in co-moving internal units.
+  CFL_condition:         0.1              # Courant-Friedrich-Levy condition for time integration.
+  minimal_temperature:   100              # (internal units)
+  particle_splitting:    1
+  particle_splitting_mass_threshold: 7e-4 # Internal units (i.e. 7e6 Msun ~ 4 times the initial gas mass)
 
 # Parameters of the stars neighbour search
 Stars:

examples/parameter_example.yml

@@ -28,26 +28,28 @@ Cosmology:
 
 # Parameters for the hydrodynamics scheme
 SPH:
-  resolution_eta:            1.2348   # Target smoothing length in units of the mean inter-particle separation (1.2348 == 48Ngbs with the cubic spline kernel).
-  CFL_condition:             0.1      # Courant-Friedrich-Levy condition for time integration.
-  use_mass_weighted_num_ngb: 0        # (Optional) Are we using the mass-weighted definition of the number of neighbours?
-  h_tolerance:               1e-4     # (Optional) Relative accuracy of the Netwon-Raphson scheme for the smoothing lengths.
-  h_max:                     10.      # (Optional) Maximal allowed smoothing length in internal units. Defaults to FLT_MAX if unspecified.
-  h_min_ratio:               0.       # (Optional) Minimal allowed smoothing length in units of the softening. Defaults to 0 if unspecified.
-  max_volume_change:         1.4      # (Optional) Maximal allowed change of kernel volume over one time-step.
-  max_ghost_iterations:      30       # (Optional) Maximal number of iterations allowed to converge towards the smoothing length.
-  initial_temperature:       0        # (Optional) Initial temperature (in internal units) to set the gas particles at start-up. Value is ignored if set to 0.
-  minimal_temperature:       0        # (Optional) Minimal temperature (in internal units) allowed for the gas particles. Value is ignored if set to 0.
-  H_mass_fraction:           0.755    # (Optional) Hydrogen mass fraction used for initial conversion from temp to internal energy. Default value is derived from the physical constants.
-  H_ionization_temperature:  1e4      # (Optional) Temperature of the transition from neutral to ionized Hydrogen for primoridal gas.
-  viscosity_alpha:           0.8      # (Optional) Override for the initial value of the artificial viscosity. In schemes that have a fixed AV, this remains as alpha throughout the run.
-  viscosity_alpha_max:       2.0      # (Optional) Maximal value for the artificial viscosity in schemes that allow alpha to vary.
-  viscosity_alpha_min:       0.1      # (Optional) Minimal value for the artificial viscosity in schemes that allow alpha to vary.
-  viscosity_length:          0.1      # (Optional) Decay length for the artificial viscosity in schemes that allow alpha to vary.
-  diffusion_alpha:           0.0      # (Optional) Override the initial value for the thermal diffusion coefficient in schemes with thermal diffusion.
-  diffusion_beta:            0.01     # (Optional) Override the decay/rise rate tuning parameter for the thermal diffusion.
-  diffusion_alpha_max:       1.0      # (Optional) Override the maximal thermal diffusion coefficient that is allowed for a given particle.
-  diffusion_alpha_min:       0.0      # (Optional) Override the minimal thermal diffusion coefficient that is allowed for a given particle.
+  resolution_eta:                    1.2348   # Target smoothing length in units of the mean inter-particle separation (1.2348 == 48Ngbs with the cubic spline kernel).
+  CFL_condition:                     0.1      # Courant-Friedrich-Levy condition for time integration.
+  use_mass_weighted_num_ngb:         0        # (Optional) Are we using the mass-weighted definition of the number of neighbours?
+  h_tolerance:                       1e-4     # (Optional) Relative accuracy of the Netwon-Raphson scheme for the smoothing lengths.
+  h_max:                             10.      # (Optional) Maximal allowed smoothing length in internal units. Defaults to FLT_MAX if unspecified.
+  h_min_ratio:                       0.       # (Optional) Minimal allowed smoothing length in units of the softening. Defaults to 0 if unspecified.
+  max_volume_change:                 1.4      # (Optional) Maximal allowed change of kernel volume over one time-step.
+  max_ghost_iterations:              30       # (Optional) Maximal number of iterations allowed to converge towards the smoothing length.
+  particle_splitting:                1        # (Optional) Are we splitting particles that are too massive (default: 0)
+  particle_splitting_mass_threshold: 7e-4     # (Optional) Mass threshold for particle splitting (in internal units)
+  initial_temperature:               0        # (Optional) Initial temperature (in internal units) to set the gas particles at start-up. Value is ignored if set to 0.
+  minimal_temperature:               0        # (Optional) Minimal temperature (in internal units) allowed for the gas particles. Value is ignored if set to 0.
+  H_mass_fraction:                   0.755    # (Optional) Hydrogen mass fraction used for initial conversion from temp to internal energy. Default value is derived from the physical constants.
+  H_ionization_temperature:          1e4      # (Optional) Temperature of the transition from neutral to ionized Hydrogen for primoridal gas.
+  viscosity_alpha:                   0.8      # (Optional) Override for the initial value of the artificial viscosity. In schemes that have a fixed AV, this remains as alpha throughout the run.
+  viscosity_alpha_max:               2.0      # (Optional) Maximal value for the artificial viscosity in schemes that allow alpha to vary.
+  viscosity_alpha_min:               0.1      # (Optional) Minimal value for the artificial viscosity in schemes that allow alpha to vary.
+  viscosity_length:                  0.1      # (Optional) Decay length for the artificial viscosity in schemes that allow alpha to vary.
+  diffusion_alpha:                   0.0      # (Optional) Override the initial value for the thermal diffusion coefficient in schemes with thermal diffusion.
+  diffusion_beta:                    0.01     # (Optional) Override the decay/rise rate tuning parameter for the thermal diffusion.
+  diffusion_alpha_max:               1.0      # (Optional) Override the maximal thermal diffusion coefficient that is allowed for a given particle.
+  diffusion_alpha_min:               0.0      # (Optional) Override the minimal thermal diffusion coefficient that is allowed for a given particle.
 
 # Parameters of the stars neighbour search
 Stars:

src/Makefile.am

@@ -76,7 +76,7 @@ AM_SOURCES = space.c runner_main.c runner_doiact_hydro.c runner_doiact_limiter.c
     runner_doiact_stars.c runner_doiact_black_holes.c runner_ghost.c runner_recv.c \
     runner_sort.c runner_drift.c runner_black_holes.c runner_time_integration.c \
     runner_doiact_hydro_vec.c runner_others.c\
-    queue.c task.c cell.c engine.c engine_maketasks.c \
+    queue.c task.c cell.c engine.c engine_maketasks.c engine_split_particles.c \
     engine_marktasks.c engine_drift.c engine_unskip.c engine_collect_end_of_step.c \
     engine_redistribute.c engine_fof.c serial_io.c timers.c debug.c scheduler.c \
     proxy.c parallel_io.c units.c common_io.c single_io.c multipole.c version.c map.c \

src/chemistry/EAGLE/chemistry.h

@@ -403,6 +403,21 @@ __attribute__((always_inline)) INLINE static void chemistry_add_bpart_to_bpart(
   bp_data->iron_mass_from_SNIa += swallowed_data->iron_mass_from_SNIa;
 }
 
+/**
+ * @brief Split the metal content of a particle into n pieces
+ *
+ * We only need to split the fields that are not fractions.
+ *
+ * @param p The #part.
+ * @param n The number of pieces to split into.
+ */
+__attribute__((always_inline)) INLINE static void chemistry_split_part(
+    struct part* p, const double n) {
+  p->chemistry_data.mass_from_SNIa /= n;
+  p->chemistry_data.mass_from_SNII /= n;
+  p->chemistry_data.mass_from_AGB /= n;
+}
+
 /**
  * @brief Returns the total metallicity (metal mass fraction) of the
  * star particle to be used in feedback/enrichment related routines.

src/chemistry/GEAR/chemistry.h

@@ -229,7 +229,9 @@ __attribute__((always_inline)) INLINE static void chemistry_first_init_spart(
  */
 __attribute__((always_inline)) INLINE static void chemistry_bpart_from_part(
     struct chemistry_bpart_data* bp_data,
-    const struct chemistry_part_data* p_data, const double gas_mass) {}
+    const struct chemistry_part_data* p_data, const double gas_mass) {
+  error("Loic: to be implemented");
+}
 
 /**
  * @brief Add the chemistry data of a gas particle to a black hole.
@@ -242,7 +244,9 @@ __attribute__((always_inline)) INLINE static void chemistry_bpart_from_part(
  */
 __attribute__((always_inline)) INLINE static void chemistry_add_part_to_bpart(
     struct chemistry_bpart_data* bp_data,
-    const struct chemistry_part_data* p_data, const double gas_mass) {}
+    const struct chemistry_part_data* p_data, const double gas_mass) {
+  error("Loic: to be implemented");
+}
 
 /**
  * @brief Add the chemistry data of a black hole to another one.
@@ -254,6 +258,19 @@ __attribute__((always_inline)) INLINE static void chemistry_add_part_to_bpart(
  */
 __attribute__((always_inline)) INLINE static void chemistry_add_bpart_to_bpart(
     struct chemistry_bpart_data* bp_data,
-    const struct chemistry_bpart_data* swallowed_data) {}
+    const struct chemistry_bpart_data* swallowed_data) {
+  error("Loic: to be implemented");
+}
+
+/**
+ * @brief Split the metal content of a particle into n pieces
+ *
+ * @param p The #part.
+ * @param n The number of pieces to split into.
+ */
+__attribute__((always_inline)) INLINE static void chemistry_split_part(
+    struct part* p, const double n) {
+  error("Loic: to be implemented");
+}
 
 #endif /* SWIFT_CHEMISTRY_GEAR_H */

src/chemistry/none/chemistry.h

@@ -214,6 +214,17 @@ __attribute__((always_inline)) INLINE static void chemistry_add_bpart_to_bpart(
     struct chemistry_bpart_data* bp_data,
     const struct chemistry_bpart_data* swallowed_data) {}
 
+/**
+ * @brief Split the metal content of a particle into n pieces
+ *
+ * Nothing to do here.
+ *
+ * @param p The #part.
+ * @param n The number of pieces to split into.
+ */
+__attribute__((always_inline)) INLINE static void chemistry_split_part(
+    struct part* p, const double n) {}
+
 /**
  * @brief Returns the total metallicity (metal mass fraction) of the
  * star particle to be used in feedback/enrichment related routines.

src/cooling/Compton/cooling.h

@@ -308,6 +308,19 @@ __attribute__((always_inline)) INLINE static float cooling_get_radiated_energy(
   return xp->cooling_data.radiated_energy;
 }
 
+/**
+ * @brief Split the coolong content of a particle into n pieces
+ *
+ * @param p The #part.
+ * @param xp The #xpart.
+ * @param n The number of pieces to split into.
+ */
+static INLINE void cooling_split_part(struct part* p, struct xpart* xp,
+                                      double n) {
+
+  xp->cooling_data.radiated_energy /= n;
+}
+
 /**
  * @brief Initialises the cooling properties.
  *

src/cooling/EAGLE/cooling.c

@@ -642,6 +642,18 @@ __attribute__((always_inline)) INLINE float cooling_get_radiated_energy(
   return xp->cooling_data.radiated_energy;
 }
 
+/**
+ * @brief Split the coolong content of a particle into n pieces
+ *
+ * @param p The #part.
+ * @param xp The #xpart.
+ * @param n The number of pieces to split into.
+ */
+void cooling_split_part(struct part *p, struct xpart *xp, double n) {
+
+  xp->cooling_data.radiated_energy /= n;
+}
+
 /**
  * @brief Inject a fixed amount of energy to each particle in the simulation
  * to mimic Hydrogen reionization.

src/cooling/EAGLE/cooling.h

@@ -71,6 +71,8 @@ float cooling_get_temperature(
 
 float cooling_get_radiated_energy(const struct xpart *restrict xp);
 
+void cooling_split_part(struct part *p, struct xpart *xp, double n);
+
 void cooling_Hydrogen_reionization(const struct cooling_function_data *cooling,
                                    const struct cosmology *cosmo,
                                    struct space *s);

src/cooling/const_du/cooling.h

@@ -226,6 +226,19 @@ __attribute__((always_inline)) INLINE static float cooling_get_radiated_energy(
   return xp->cooling_data.radiated_energy;
 }
 
+/**
+ * @brief Split the coolong content of a particle into n pieces
+ *
+ * @param p The #part.
+ * @param xp The #xpart.
+ * @param n The number of pieces to split into.
+ */
+static INLINE void cooling_split_part(struct part* p, struct xpart* xp,
+                                      double n) {
+
+  xp->cooling_data.radiated_energy /= n;
+}
+
 /**
  * @brief Initialises the cooling function properties from the parameter file
  *

src/cooling/const_lambda/cooling.h

@@ -302,6 +302,19 @@ __attribute__((always_inline)) INLINE static float cooling_get_radiated_energy(
   return xp->cooling_data.radiated_energy;
 }
 
+/**
+ * @brief Split the coolong content of a particle into n pieces
+ *
+ * @param p The #part.
+ * @param xp The #xpart.
+ * @param n The number of pieces to split into.
+ */
+static INLINE void cooling_split_part(struct part* p, struct xpart* xp,
+                                      double n) {
+
+  xp->cooling_data.radiated_energy /= n;
+}
+
 /**
  * @brief Initialises the cooling properties.
  *

src/cooling/grackle/cooling.h

@@ -763,6 +763,19 @@ __attribute__((always_inline)) INLINE static float cooling_timestep(
   return FLT_MAX;
 }
 
+/**
+ * @brief Split the coolong content of a particle into n pieces
+ *
+ * @param p The #part.
+ * @param xp The #xpart.
+ * @param n The number of pieces to split into.
+ */
+INLINE static void cooling_split_part(struct part* p, struct xpart* xp,
+                                      double n) {
+
+  error("Loic: to be implemented");
+}
+
 /**
  * @brief Initialises the cooling unit system.
  *