Smoothing length time derivative computed correctly in the Gadget-2 case

src/hydro/Gadget2/hydro.h

@@ -203,7 +203,7 @@ __attribute__((always_inline)) INLINE static void hydro_predict_extra(
  */
 __attribute__((always_inline))
     INLINE static void hydro_end_force(struct part* p) {
-
+  
   p->entropy_dt *=
       (const_hydro_gamma - 1.f) * powf(p->rho, -(const_hydro_gamma - 1.f));
 }

src/hydro/Gadget2/hydro_iact.h

@@ -197,7 +197,7 @@ __attribute__((always_inline)) INLINE static void runner_iact_force(
   const float r_inv = 1.0f / r;
 
   /* Get some values in local variables. */
-  // const float mi = pi->mass;
+  const float mi = pi->mass;
   const float mj = pj->mass;
   const float rhoi = pi->rho;
   const float rhoj = pj->rho;
@@ -280,6 +280,10 @@ __attribute__((always_inline)) INLINE static void runner_iact_force(
   pj->a[1] += acc * dx[1];
   pj->a[2] += acc * dx[2];
 
+  /* Get the time derivative for h. */
+  pi->force.h_dt -= mj * dvdr / rhoj * wi_dr;
+  pj->force.h_dt -= mi * dvdr / rhoi * wj_dr;
+  
   /* Update the signal velocity. */
   pi->v_sig = fmaxf(pi->v_sig, v_sig);
   pj->v_sig = fmaxf(pj->v_sig, v_sig);
@@ -366,6 +370,9 @@ __attribute__((always_inline)) INLINE static void runner_iact_nonsym_force(
   pi->a[1] -= acc * dx[1];
   pi->a[2] -= acc * dx[2];
 
+  /* Get the time derivative for h. */
+  pi->force.h_dt -= mj * dvdr / rhoj * wi_dr;
+  
   /* Update the signal velocity. */
   pi->v_sig = fmaxf(pi->v_sig, v_sig);
 

src/runner.c

@@ -851,8 +851,13 @@ void runner_dokick(struct runner *r, struct cell *c, int timer) {
       if (is_fixdt || p->t_end <= t_current) {
 
         /* First, finish the force loop */
+        p->force.h_dt *= p->h * 0.333333333f;
+
+        /* And do the same of the extra variable */
         hydro_end_force(p);
 
+        /* Now we are ready to compute the next time-step size */
+
         if (is_fixdt) {
 
           /* Now we have a time step, proceed with the kick */