Record r_max and CoM of each multipole at rebuild time.

src/multipole.h

@@ -176,9 +176,15 @@ struct gravity_tensors {
       /*! Centre of mass of the matter dsitribution */
       double CoM[3];
 
+      /*! Centre of mass of the matter dsitribution at the last rebuild */
+      double CoM_rebuild[3];
+
       /*! Upper limit of the CoM<->gpart distance */
       double r_max;
 
+      /*! Upper limit of the CoM<->gpart distance at the last rebuild */
+      double r_max_rebuild;
+
       /*! Multipole mass */
       struct multipole m_pole;
 
@@ -2537,4 +2543,30 @@ __attribute__((always_inline)) INLINE static int gravity_multipole_accept(
   return (r2 > (r_crit_a + r_crit_b) * (r_crit_a + r_crit_b));
 }
 
+/**
+ * @brief Checks whether a cell-cell interaction can be appromixated by a M-M
+ * interaction using the r_max at rebuild time.
+ *
+ * @param ma The #multipole of the first #cell.
+ * @param mb The #multipole of the second #cell.
+ * @param theta_crit_inv The inverse of the critical opening angle.
+ */
+__attribute__((always_inline)) INLINE static int
+gravity_multipole_accept_rebuild(const struct gravity_tensors *ma,
+                                 const struct gravity_tensors *mb,
+                                 double theta_crit_inv) {
+
+  const double r_crit_a = ma->r_max_rebuild * theta_crit_inv;
+  const double r_crit_b = mb->r_max_rebuild * theta_crit_inv;
+
+  const double dx = ma->CoM_rebuild[0] - mb->CoM_rebuild[0];
+  const double dy = ma->CoM_rebuild[1] - mb->CoM_rebuild[1];
+  const double dz = ma->CoM_rebuild[2] - mb->CoM_rebuild[2];
+
+  const double r2 = dx * dx + dy * dy + dz * dz;
+
+  /* Multipole acceptance criterion (Dehnen 2002, eq.10) */
+  return (r2 > (r_crit_a + r_crit_b) * (r_crit_a + r_crit_b));
+}
+
 #endif /* SWIFT_MULTIPOLE_H */

src/runner.c

@@ -1392,8 +1392,7 @@ void runner_do_end_force(struct runner *r, struct cell *c, int timer) {
         if (e->policy & engine_policy_self_gravity) {
 
           /* Check that this gpart has interacted with all the other
-           * particles
-           * (via direct or multipoles) in the box */
+           * particles (via direct or multipoles) in the box */
           gp->num_interacted++;
           if (gp->num_interacted != (long long)e->s->nr_gparts)
             error(

src/runner_doiact_grav.h

@@ -638,8 +638,8 @@ void runner_do_grav_long_range(struct runner *r, struct cell *ci, int timer) {
     if (ci == cj || cj->gcount == 0) continue;
 
     /* Check the multipole acceptance criterion */
-    if (gravity_multipole_accept(ci->multipole, cj->multipole,
-                                 theta_crit_inv)) {
+    if (gravity_multipole_accept_rebuild(ci->multipole, cj->multipole,
+                                         theta_crit_inv)) {
       /* Go for a (non-symmetric) M-M calculation */
       runner_dopair_grav_mm(r, ci, cj);
     }

src/space.c

@@ -2115,6 +2115,10 @@ void space_split_recursive(struct space *s, struct cell *c,
 
       /* Take minimum of both limits */
       c->multipole->r_max = min(r_max, sqrt(dx * dx + dy * dy + dz * dz));
+      c->multipole->r_max_rebuild = c->multipole->r_max;
+      c->multipole->CoM_rebuild[0] = c->multipole->CoM[0];
+      c->multipole->CoM_rebuild[1] = c->multipole->CoM[1];
+      c->multipole->CoM_rebuild[2] = c->multipole->CoM[2];
     } /* Deal with gravity */
   }
 
@@ -2194,6 +2198,10 @@ void space_split_recursive(struct space *s, struct cell *c,
         c->multipole->r_max = 0.;
       }
     }
+    c->multipole->r_max_rebuild = c->multipole->r_max;
+    c->multipole->CoM_rebuild[0] = c->multipole->CoM[0];
+    c->multipole->CoM_rebuild[1] = c->multipole->CoM[1];
+    c->multipole->CoM_rebuild[2] = c->multipole->CoM[2];
   }
 
   /* Set the values for this cell. */