diff --git a/src/center.js b/src/center.js
index 0e9b1e1..b8ce38e 100644
--- a/src/center.js
+++ b/src/center.js
@@ -20,8 +20,8 @@ export default function(x, y) {
     }
   }
 
-  force.initialize = function(simulation) {
-    nodes = simulation.nodes();
+  force.initialize = function(_) {
+    nodes = _;
   };
 
   force.x = function(_) {
diff --git a/src/collide.js b/src/collide.js
index 123e0c1..591c315 100644
--- a/src/collide.js
+++ b/src/collide.js
@@ -46,9 +46,9 @@ export default function(radius) {
   }
 
   force.initialize = function(_) {
-    nodes = _.nodes();
-    var i, n = nodes.length, r;
-    radii = new Array(n), radiusMax = 0;
+    var i, n = (nodes = _).length, r;
+    radii = new Array(n);
+    radiusMax = 0;
     for (i = 0; i < n; ++i) {
       if ((radii[i] = r = +radius(nodes[i], i, nodes)) > radiusMax) {
         radiusMax = r;
diff --git a/src/contain.js b/src/contain.js
index 006c68b..e24d147 100644
--- a/src/contain.js
+++ b/src/contain.js
@@ -36,8 +36,8 @@ export default function(radius, x, y) {
     }
   }
 
-  force.initialize = function(simulation) {
-    nodes = simulation.nodes();
+  force.initialize = function(_) {
+    nodes = _;
     initialize();
   };
 
diff --git a/src/link.js b/src/link.js
index e31de57..b7c176a 100644
--- a/src/link.js
+++ b/src/link.js
@@ -67,8 +67,9 @@ export default function(links) {
     }
   }
 
-  force.initialize = function(simulation) {
-    nodes = simulation.nodes(), initialize();
+  force.initialize = function(_) {
+    nodes = _;
+    initialize();
   };
 
   force.links = function(_) {
diff --git a/src/manyBody.js b/src/manyBody.js
index 81463e2..733b263 100644
--- a/src/manyBody.js
+++ b/src/manyBody.js
@@ -88,7 +88,8 @@ export default function() {
   }
 
   force.initialize = function(_) {
-    nodes = _.nodes(), initialize();
+    nodes = _;
+    initialize();
   };
 
   force.strength = function(_) {
diff --git a/src/position.js b/src/position.js
index 951d785..7bdd8b4 100644
--- a/src/position.js
+++ b/src/position.js
@@ -32,8 +32,8 @@ export default function(x, y) {
     }
   }
 
-  force.initialize = function(simulation) {
-    nodes = simulation.nodes();
+  force.initialize = function(_) {
+    nodes = _;
     initialize();
   };
 
diff --git a/src/simulation.js b/src/simulation.js
index 521beda..276f999 100644
--- a/src/simulation.js
+++ b/src/simulation.js
@@ -68,7 +68,7 @@ export default function(nodes) {
   }
 
   function initializeForce(force) {
-    if (force.initialize) force.initialize(simulation);
+    if (force.initialize) force.initialize(nodes);
     return force;
   }