New function initialising the extra arrays just after reading in the ICs

src/cell.c

@@ -564,6 +564,7 @@ void cell_init_parts(struct cell *c, void *data) {
     xp[i].v_full[0] = p[i].v[0];
     xp[i].v_full[1] = p[i].v[1];
     xp[i].v_full[2] = p[i].v[2];
+    hydro_first_init_part(&p[i], &xp[i]);
     hydro_init_part(&p[i]);
     hydro_reset_acceleration(&p[i]);
   }

src/hydro/Default/hydro.h

@@ -38,6 +38,18 @@ __attribute__((always_inline)) INLINE static float hydro_compute_timestep(
   return fminf(dt_cfl, dt_u_change);
 }
 
+/**
+ * @brief Initialises the particles for the first time
+ *
+ * This function is called only once just after the ICs have been
+ * read in to do some conversions.
+ *
+ * @param p The particle to act upon
+ * @param xp The extended particle data to act upon
+ */
+__attribute__((always_inline))
+INLINE static void hydro_first_init_part(struct part* p, struct xpart* xp) {}
+
 /**
  * @brief Prepares a particle for the density calculation.
  *

src/hydro/Gadget2/hydro.h

@@ -41,6 +41,18 @@ __attribute__((always_inline)) INLINE static float hydro_compute_timestep(
   return fminf(dt_cfl, dt_accel);
 }
 
+/**
+ * @brief Initialises the particles for the first time
+ *
+ * This function is called only once just after the ICs have been
+ * read in to do some conversions.
+ *
+ * @param p The particle to act upon
+ * @param xp The extended particle data to act upon
+ */
+__attribute__((always_inline))
+INLINE static void hydro_first_init_part(struct part* p, struct xpart* xp) {}
+
 /**
  * @brief Prepares a particle for the density calculation.
  *

src/hydro/Minimal/hydro.h

@@ -35,6 +35,21 @@ __attribute__((always_inline)) INLINE static float hydro_compute_timestep(
   return dt_cfl;
 }
 
+/**
+ * @brief Initialises the particles for the first time
+ *
+ * This function is called only once just after the ICs have been
+ * read in to do some conversions.
+ *
+ * @param p The particle to act upon
+ * @param xp The extended particle data to act upon
+ */
+__attribute__((always_inline))
+INLINE static void hydro_first_init_part(struct part* p, struct xpart* xp) {
+
+  xp->u_full = p->u;
+}
+
 /**
  * @brief Prepares a particle for the density calculation.
  *

src/hydro/Minimal/hydro_part.h

@@ -26,6 +26,9 @@ struct xpart {
   /* Velocity at the last full step. */
   float v_full[3];
 
+  /* Thermal energy at the last full step. */
+  float u_full;
+
 } __attribute__((aligned(xpart_align)));
 
 /* Data of a single particle. */