Apply the entropy floor after the kick operation.

src/hydro/Gadget2/hydro.h

@@ -306,10 +306,11 @@ hydro_get_physical_internal_energy_dt(const struct part *restrict p,
  * We assume a constant density for the conversion to entropy.
  *
  * @param p The particle of interest.
- * @param du_dt The new time derivative of the internal energy.
+ * @param du_dt The new time derivative of the comoving internal energy.
  */
 __attribute__((always_inline)) INLINE static void
-hydro_set_comoving_internal_energy_dt(struct part *restrict p, float du_dt) {
+hydro_set_comoving_internal_energy_dt(struct part *restrict p,
+                                      const float du_dt) {
 
   p->entropy_dt = gas_entropy_from_internal_energy(p->rho, du_dt);
 }
@@ -321,15 +322,29 @@ hydro_set_comoving_internal_energy_dt(struct part *restrict p, float du_dt) {
  *
  * @param p The particle of interest.
  * @param cosmo Cosmology data structure
- * @param du_dt The time derivative of the internal energy.
+ * @param du_dt The time derivative of the physical internal energy.
  */
 __attribute__((always_inline)) INLINE static void
 hydro_set_physical_internal_energy_dt(struct part *restrict p,
                                       const struct cosmology *restrict cosmo,
-                                      float du_dt) {
+                                      const float du_dt) {
   p->entropy_dt =
       gas_entropy_from_internal_energy(p->rho * cosmo->a3_inv, du_dt);
 }
+/**
+ * @brief Sets the physical entropy of a particle
+ *
+ * @param p The particle of interest.
+ * @param cosmo Cosmology data structure
+ * @param s The physical entropy
+ */
+__attribute__((always_inline)) INLINE static void hydro_set_physical_entropy(
+    struct part *p, struct xpart *xp, const struct cosmology *cosmo,
+    const float entropy) {
+
+  /* Note there is no conversion from physical to comoving entropy */
+  xp->entropy_full = entropy;
+}
 
 /**
  * @brief Computes the hydro time-step of a given particle

src/kick.h

@@ -79,6 +79,7 @@ __attribute__((always_inline)) INLINE static void kick_part(
     struct part *restrict p, struct xpart *restrict xp, double dt_kick_hydro,
     double dt_kick_grav, double dt_kick_therm, double dt_kick_corr,
     const struct cosmology *cosmo, const struct hydro_props *hydro_props,
+    const struct entropy_floor_properties *entropy_floor_props,
     integertime_t ti_start, integertime_t ti_end) {
 
 #ifdef SWIFT_DEBUG_CHECKS
@@ -114,6 +115,13 @@ __attribute__((always_inline)) INLINE static void kick_part(
   hydro_kick_extra(p, xp, dt_kick_therm, dt_kick_grav, dt_kick_hydro,
                    dt_kick_corr, cosmo, hydro_props);
   if (p->gpart != NULL) gravity_kick_extra(p->gpart, dt_kick_grav);
+
+  /* Verify that the particle is not below the entropy floor */
+  const float floor = entropy_floor(p, cosmo, entropy_floor_props);
+  if (hydro_get_physical_entropy(p, xp, cosmo) < floor) {
+    hydro_set_physical_entropy(p, xp, cosmo, floor);
+    hydro_set_physical_internal_energy_dt(p, cosmo, 0.f);
+  }
 }
 
 /**

src/runner.c

@@ -1679,6 +1679,7 @@ void runner_do_kick1(struct runner *r, struct cell *c, int timer) {
   const struct engine *e = r->e;
   const struct cosmology *cosmo = e->cosmology;
   const struct hydro_props *hydro_props = e->hydro_properties;
+  const struct entropy_floor_properties *entropy_floor = e->entropy_floor;
   const int with_cosmology = (e->policy & engine_policy_cosmology);
   struct part *restrict parts = c->hydro.parts;
   struct xpart *restrict xparts = c->hydro.xparts;
@@ -1746,7 +1747,7 @@ void runner_do_kick1(struct runner *r, struct cell *c, int timer) {
 
         /* do the kick */
         kick_part(p, xp, dt_kick_hydro, dt_kick_grav, dt_kick_therm,
-                  dt_kick_corr, cosmo, hydro_props, ti_begin,
+                  dt_kick_corr, cosmo, hydro_props, entropy_floor, ti_begin,
                   ti_begin + ti_step / 2);
 
         /* Update the accelerations to be used in the drift for hydro */
@@ -1853,6 +1854,7 @@ void runner_do_kick2(struct runner *r, struct cell *c, int timer) {
   const struct engine *e = r->e;
   const struct cosmology *cosmo = e->cosmology;
   const struct hydro_props *hydro_props = e->hydro_properties;
+  const struct entropy_floor_properties *entropy_floor = e->entropy_floor;
   const int with_cosmology = (e->policy & engine_policy_cosmology);
   const int count = c->hydro.count;
   const int gcount = c->grav.count;
@@ -1916,8 +1918,8 @@ void runner_do_kick2(struct runner *r, struct cell *c, int timer) {
 
         /* Finish the time-step with a second half-kick */
         kick_part(p, xp, dt_kick_hydro, dt_kick_grav, dt_kick_therm,
-                  dt_kick_corr, cosmo, hydro_props, ti_begin + ti_step / 2,
-                  ti_begin + ti_step);
+                  dt_kick_corr, cosmo, hydro_props, entropy_floor,
+                  ti_begin + ti_step / 2, ti_begin + ti_step);
 
 #ifdef SWIFT_DEBUG_CHECKS
         /* Check that kick and the drift are synchronized */