Reset the value of rho_dh to something sensible if no neighbours are found when imposing h=h_max

src/hydro/Default/hydro.h

@@ -210,9 +210,6 @@ __attribute__((always_inline)) INLINE static void hydro_end_density(
 
   const float irho = 1.f / p->rho;
 
-  /* Compute the derivative term */
-  p->rho_dh = 1.f / (1.f + hydro_dimension_inv * p->h * p->rho_dh * irho);
-
   /* Finish calculation of the velocity curl components */
   p->density.rot_v[0] *= h_inv_dim_plus_one * irho;
   p->density.rot_v[1] *= h_inv_dim_plus_one * irho;
@@ -222,6 +219,24 @@ __attribute__((always_inline)) INLINE static void hydro_end_density(
   p->density.div_v *= h_inv_dim_plus_one * irho;
 }
 
+/**
+ * @brief Sets all particle fields to sensible values when the #part has 0 ngbs.
+ *
+ * @param p The particle to act upon
+ * @param xp The extended particle data to act upon
+ */
+__attribute__((always_inline)) INLINE static void hydro_part_has_no_neighbours(
+    struct part *restrict p, struct xpart *restrict xp) {
+
+  /* Re-set problematic values */
+  p->rho_dh = 0.f;
+  p->density.wcount_dh = 0.f;
+  p->density.div_v = 0.f;
+  p->density.rot_v[0] = 0.f;
+  p->density.rot_v[1] = 0.f;
+  p->density.rot_v[2] = 0.f;
+}
+
 /**
  * @brief Prepare a particle for the force calculation.
  *
@@ -249,6 +264,9 @@ __attribute__((always_inline)) INLINE static void hydro_prepare_force(
   const float fc = p->force.soundspeed =
       sqrtf(hydro_gamma * hydro_gamma_minus_one * u);
 
+  /* Compute the derivative term */
+  p->rho_dh = 1.f / (1.f + hydro_dimension_inv * p->h * p->rho_dh / p->rho);
+
   /* Compute the P/Omega/rho2. */
   xp->omega = 1.0f + hydro_dimension_inv * h * p->rho_dh / p->rho;
   p->force.P_over_rho2 = u * hydro_gamma_minus_one / (p->rho * xp->omega);

src/hydro/Gadget2/hydro.h

@@ -224,6 +224,24 @@ __attribute__((always_inline)) INLINE static void hydro_end_density(
   p->density.div_v *= h_inv_dim_plus_one * rho_inv;
 }
 
+/**
+ * @brief Sets all particle fields to sensible values when the #part has 0 ngbs.
+ *
+ * @param p The particle to act upon
+ * @param xp The extended particle data to act upon
+ */
+__attribute__((always_inline)) INLINE static void hydro_part_has_no_neighbours(
+    struct part *restrict p, struct xpart *restrict xp) {
+
+  /* Re-set problematic values */
+  p->density.rho_dh = 0.f;
+  p->density.wcount_dh = 0.f;
+  p->density.div_v = 0.f;
+  p->density.rot_v[0] = 0.f;
+  p->density.rot_v[1] = 0.f;
+  p->density.rot_v[2] = 0.f;
+}
+
 /**
  * @brief Prepare a particle for the force calculation.
  *

src/hydro/Gizmo/hydro.h

@@ -366,6 +366,19 @@ __attribute__((always_inline)) INLINE static void hydro_end_density(
   p->density.wcount_dh *= p->density.wcorr;
 }
 
+/**
+ * @brief Sets all particle fields to sensible values when the #part has 0 ngbs.
+ *
+ * @param p The particle to act upon
+ * @param xp The extended particle data to act upon
+ */
+__attribute__((always_inline)) INLINE static void hydro_part_has_no_neighbours(
+    struct part* restrict p, struct xpart* restrict xp) {
+
+  /* Re-set problematic values */
+  p->density.wcount_dh = 0.f;
+}
+
 /**
  * @brief Prepare a particle for the gradient calculation.
  *

src/hydro/Minimal/hydro.h

@@ -227,6 +227,20 @@ __attribute__((always_inline)) INLINE static void hydro_end_density(
   p->density.wcount_dh *= h_inv * kernel_gamma * kernel_norm;
 }
 
+/**
+ * @brief Sets all particle fields to sensible values when the #part has 0 ngbs.
+ *
+ * @param p The particle to act upon
+ * @param xp The extended particle data to act upon
+ */
+__attribute__((always_inline)) INLINE static void hydro_part_has_no_neighbours(
+    struct part *restrict p, struct xpart *restrict xp) {
+
+  /* Re-set problematic values */
+  p->density.rho_dh = 0.f;
+  p->density.wcount_dh = 0.f;
+}
+
 /**
  * @brief Prepare a particle for the force calculation.
  *

src/hydro/PressureEntropy/hydro.h

@@ -236,6 +236,25 @@ __attribute__((always_inline)) INLINE static void hydro_end_density(
   p->density.div_v *= h_inv_dim_plus_one * rho_inv;
 }
 
+/**
+ * @brief Sets all particle fields to sensible values when the #part has 0 ngbs.
+ *
+ * @param p The particle to act upon
+ * @param xp The extended particle data to act upon
+ */
+__attribute__((always_inline)) INLINE static void hydro_part_has_no_neighbours(
+    struct part *restrict p, struct xpart *restrict xp) {
+
+  /* Re-set problematic values */
+  p->density.rho_dh = 0.f;
+  p->density.wcount_dh = 0.f;
+  p->density.pressure_dh = 0.f;  // MATTHIEU: to be checked
+  p->density.div_v = 0.f;
+  p->density.rot_v[0] = 0.f;
+  p->density.rot_v[1] = 0.f;
+  p->density.rot_v[2] = 0.f;
+}
+
 /**
  * @brief Prepare a particle for the force calculation.
  *

src/hydro/Shadowswift/hydro.h

@@ -238,6 +238,19 @@ __attribute__((always_inline)) INLINE static void hydro_end_density(
 #endif
 }
 
+/**
+ * @brief Sets all particle fields to sensible values when the #part has 0 ngbs.
+ *
+ * @param p The particle to act upon
+ * @param xp The extended particle data to act upon
+ */
+__attribute__((always_inline)) INLINE static void hydro_part_has_no_neighbours(
+    struct part* restrict p, struct xpart* restrict xp) {
+
+  /* Re-set problematic values */
+  p->density.wcount_dh = 0.f;
+}
+
 /**
  * @brief Prepare a particle for the gradient calculation.
  *

src/runner.c

@@ -708,6 +708,10 @@ void runner_do_ghost(struct runner *r, struct cell *c, int timer) {
 
             /* Ok, this particle is a lost cause... */
             p->h = hydro_h_max;
+
+            /* Do some damage control if no neighbours at all were found */
+            if (p->density.wcount == kernel_root * kernel_norm)
+              hydro_part_has_no_neighbours(p, xp);
           }
         }