diff --git a/data/models/dimers random walk.xml b/data/models/dimers random walk.xml
index 277f9ef..a7c9efc 100644
--- a/data/models/dimers random walk.xml	
+++ b/data/models/dimers random walk.xml	
@@ -116,3 +116,4 @@
     </transitions>
 
 </gem-graph-model>
+
diff --git a/include/model.h b/include/model.h
index 1891e3a..b278694 100644
--- a/include/model.h
+++ b/include/model.h
@@ -52,15 +52,14 @@
 
 struct space_unit_t
 {
-    bool lock;
-    struct arrow_t **sites;     // Array of struct arrow_t* elements
+    uint id;
+    struct arrow_t *sites[];     // Array of struct arrow_t elements
                                 // - lenght is multiplicity
 };
 
 struct space_t
 {
     // Dimensions of space.
-    // Note that a value 0 is not allowed, minimum is 1
     int x_dim;
     int y_dim;
     int z_dim;
diff --git a/src/model.c b/src/model.c
index 09b96ea..b2077d3 100644
--- a/src/model.c
+++ b/src/model.c
@@ -421,6 +421,8 @@ bool model_get_next_conditions (struct model_t *self, char *new_cond_id)
     xmlAttr *attribute;
     xmlChar *value;
 
+    printlog("NEW CALL : cur_node = %p\n", cur_node);
+
     if (cur_node == NULL) {
         // Get first state
         cur_node = model_get_node(self, (xmlChar *)"conditions");
@@ -433,6 +435,8 @@ bool model_get_next_conditions (struct model_t *self, char *new_cond_id)
             return false;
     }
 
+    printlog("END CALL : cur_node = %p\n", cur_node);
+
     // Lookup in properties
     if (model_get_node_str_attrib(cur_node, "id", new_cond_id))
         return true;
@@ -453,7 +457,7 @@ bool model_get_next_condition (struct model_t *self,
     char temp_char[25];
     uint check = 0; // bit field checker
 
-    //printf("NEW CALL : cur_node = %p\n", cur_node);
+    printlog("NEW CALL : cur_node = %p\n", cur_node);
 
     assert(new_condition);
     assert(cond_id);
@@ -520,8 +524,8 @@ bool model_get_next_condition (struct model_t *self,
         }
     }
 
-    //printf("DURING CALL : cur_node = %p\n", cur_node);
-    //printf("DURING CALL : cur_node->name = %s\n", cur_node->name);
+    printlog("DURING CALL : cur_node = %p\n", cur_node);
+    printlog("DURING CALL : cur_node->name = %s\n", cur_node->name);
 
     // Lookup in properties
     if (cur_node && cur_node->properties) {
@@ -653,22 +657,16 @@ bool model_load (struct model_t *self)
 
     // Dimensions
     self->space = calloc (1, sizeof(struct space_t));
-    self->space->x_dim = 1;
-    self->space->y_dim = 1;
-    self->space->z_dim = 1;
+    self->space->x_dim = 0;
+    self->space->y_dim = 0;
+    self->space->z_dim = 0;
 
     switch(self->dimension) {
     case 3:
-        // even in 1D, we must be able to see a grid, so prevent to be 0
-        if (self->space->z_dim)
             self->space->z_dim = model_get_dim_value (self, "z");
     case 2:
-        // even in 1D, we must be able to see a grid, so prevent to be 0
-        if (self->space->y_dim)
             self->space->y_dim = model_get_dim_value (self, "y");
     case 1:
-        // even in 1D, we must be able to see a grid, so prevent to be 0
-        if (self->space->x_dim)
             self->space->x_dim = model_get_dim_value (self, "x");
 
         printlog("x_dim=%d, y_dim=%d, z_dim=%d\n",
@@ -685,7 +683,8 @@ bool model_load (struct model_t *self)
     self->space->units = calloc (self->space->x_dim
                                     * self->space->y_dim
                                     * self->space->z_dim,
-                                 sizeof(struct space_unit_t));
+                                 sizeof(struct space_unit_t)
+                                 + self->multiplicity * sizeof(struct arrow_t*));
 
     // Multiplicity
     self->multiplicity = model_get_multiplicity(self);
@@ -705,20 +704,40 @@ bool model_load (struct model_t *self)
     printlog("Initial state : %s\n", &self->states[0].id);
 
     // Initial state arrows
-    self->arrows = arrow_new (self->arrows);
-
-    if (model_get_next_arrow(self,
-                         self->arrows,
-                         &self->states[0].id,
-                         self->dimension)) {
-        self->n_arrows++;
-
-        while (model_get_next_arrow(self,
-                                    arrow_new (self->arrows),
-                                    &self->states[0].id,
-                                    self->dimension)) {
-            self->n_arrows++;
+    while (arrow = arrow_new (self->arrows),
+           model_get_next_arrow(self,
+                                arrow,
+                                &self->states[0].id,
+                                self->dimension)) {
+        if (arrow->x > self->space->x_dim ||
+            arrow->y > self->space->y_dim ||
+            arrow->z > self->space->z_dim) {
+            printerr("Invalid coordinates for arrow %d (%d,%d,%d)\n",
+                self->n_arrows,
+                arrow->x,
+                arrow->y,
+                arrow->z);
+            return false;
         }
+
+        printlog("Loading arrow %d (%d,%d,%d, %d)\n",
+            self->n_arrows,
+            arrow->x,
+            arrow->y,
+            arrow->z,
+            arrow->site);
+
+        self->space->units [
+            arrow->x
+            + self->space->x_dim * arrow->y
+            + self->space->x_dim * self->space->y_dim * arrow->z
+        ].sites[arrow->site] = arrow;
+
+        if (self->n_arrows == 0) {
+            self->arrows = arrow;
+        }
+
+        self->n_arrows++;
     }
 
     printlog("Loaded %d arrows\n", self->n_arrows);