Prevent the user from running with a mesh that is too small given r_cut_max.

src/gravity_properties.c

@@ -58,12 +58,17 @@ void gravity_props_init(struct gravity_props *p, struct swift_params *params,
   p->r_cut_min_ratio = parser_get_opt_param_float(
       params, "Gravity:r_cut_min", gravity_props_default_r_cut_min);
 
+  /* Some basic checks */
   if (p->mesh_size % 2 != 0)
     error("The mesh side-length must be an even number.");
 
   if (p->a_smooth <= 0.)
     error("The mesh smoothing scale 'a_smooth' must be > 0.");
 
+  if (2. * p->a_smooth * p->r_cut_max_ratio > p->mesh_size)
+    error("Mesh too small given r_cut_max. Should be at least %d cells wide.",
+          (int)(2. * p->a_smooth * p->r_cut_max_ratio) + 1);
+
   /* Time integration */
   p->eta = parser_get_param_float(params, "Gravity:eta");
 

src/mesh_gravity.c

@@ -574,7 +574,7 @@ void pm_mesh_init(struct pm_mesh* mesh, const struct gravity_props* props,
 
 #ifdef HAVE_FFTW
 
-  if (dim[0] != dim[1] || dim[0] != dim[1])
+  if (dim[0] != dim[1] || dim[0] != dim[2])
     error("Doing mesh-gravity on a non-cubic domain");
 
   const int N = props->mesh_size;
@@ -591,6 +591,9 @@ void pm_mesh_init(struct pm_mesh* mesh, const struct gravity_props* props,
   mesh->r_cut_max = mesh->r_s * props->r_cut_max_ratio;
   mesh->r_cut_min = mesh->r_s * props->r_cut_min_ratio;
 
+  if (2. * mesh->r_cut_max > box_size)
+    error("Mesh too small or r_cut_max too big for this box size");
+
   /* Allocate the memory for the combined density and potential array */
   mesh->potential = (double*)fftw_malloc(sizeof(double) * N * N * N);
   if (mesh->potential == NULL)