diff --git a/examples/graph/02_random_nodes.html b/examples/graph/02_random_nodes.html
index 3d4b5a96..b4e5ee9d 100755
--- a/examples/graph/02_random_nodes.html
+++ b/examples/graph/02_random_nodes.html
@@ -74,6 +74,7 @@
         nodes: nodes,
         edges: edges
       };
+        /*
       var options = {
         nodes: {
           shape: 'circle'
@@ -83,6 +84,13 @@
         },
         stabilize: false
       };
+      */
+    var options = {
+    edges: {
+        length: 50
+    },
+    stabilize: false
+    };
       graph = new vis.Graph(container, data, options);
 
       // add event listeners
diff --git a/src/graph/Graph.js b/src/graph/Graph.js
index 4e508177..e6d3e3ab 100644
--- a/src/graph/Graph.js
+++ b/src/graph/Graph.js
@@ -39,6 +39,14 @@ function Graph (container, data, options) {
         highlight: {
           border: '#2B7CE9',
           background: '#D2E5FF'
+        },
+        cluster: {
+          border: '#000000',
+          background: '#F0F0F0',
+          highlight: {
+            border: '#FF0FFF',
+            background: '#FF000F'
+          }
         }
       },
       borderColor: '#2B7CE9',
@@ -63,6 +71,13 @@ function Graph (container, data, options) {
         altLength: undefined
       }
     },
+    clustering: {
+      clusterLength: 30,      // threshold length for clustering
+      zoomOffset: 0.1,
+      widthGrowth: 15,        // growth factor = ((parent_size + child_size) / parent_size) * widthGrowthFactor
+      heightGrowth: 15,       // growth factor = ((parent_size + child_size) / parent_size) * heightGrowthFactor
+      massTransferCoefficient: 0.1 // parent.mass += massTransferCoefficient * child.mass
+    },
     minForce: 0.05,
     minVelocity: 0.02,   // px/s
     maxIterations: 1000  // maximum number of iteration to stabilize
@@ -72,6 +87,8 @@ function Graph (container, data, options) {
   this.node_indices = [];     // the node indices list is used to speed up the computation of the repulsion fields
   this.nodes = {};            // object with Node objects
   this.edges = {};            // object with Edge objects
+  this.scale = 1;             // defining the global scale variable in the constructor
+  this.previous_scale = 1;    // use this to check if the zoom operation is in or out
   // TODO: create a counter to keep track on the number of nodes having values
   // TODO: create a counter to keep track on the number of nodes currently moving
   // TODO: create a counter to keep track on the number of edges having values
@@ -130,7 +147,184 @@ function Graph (container, data, options) {
 
   // draw data
   this.setData(data);
-}
+};
+
+/**
+ * This function checks if the zoom action is in or out.
+ * If out, check if we can form clusters, if in, check if we can open clusters.
+ * This function is only called from _zoom()
+ *
+ * @private
+ */
+Graph.prototype._updateClusters = function() {
+  if (this.previous_scale > this.scale) { // zoom out
+    this._formClusters();
+  }
+  else if (this.previous_scale < this.scale) { // zoom out
+    this._openClusters();
+  }
+  this._updateNodeIndexList();
+
+  for (var nid in this.nodes) {
+    var node = this.nodes[nid];
+    node.label = String(node.remaining_edges);
+  }
+
+  this.previous_scale = this.scale;
+};
+
+/**
+ * This function loops over all nodes in the node_indices list. For each node it checks if it is a cluster and if it
+ * has to be opened based on the current zoom level.
+ *
+ * @private
+ */
+Graph.prototype._openClusters = function() {
+  for (var i = 0; i < this.node_indices.length; i++) {
+    var node = this.nodes[this.node_indices[i]];
+    this._expandClusterNode(node,true);
+  }
+};
+
+/**
+ * This function checks if a node has to be opened. This is done by checking the zoom level.
+ * If the node contains child nodes, this function is recursively called on the child nodes as well.
+ * This recursive behaviour is optional and can be set by the recursive argument.
+ *
+ * @param node        | Node object: to check for cluster and expand
+ * @param recursive   | Boolean: enable or disable recursive calling
+ * @private
+ */
+Graph.prototype._expandClusterNode = function(node, recursive) {
+  if (node.formation_scale != undefined) {
+    if (this.scale > node.formation_scale) {
+      for (var contained_node_id in node.contained_nodes) {
+        if (node.contained_nodes.hasOwnProperty(contained_node_id)) {
+          // put the child node back in the global nodes object
+          this.nodes[contained_node_id] = node.contained_nodes[contained_node_id];
+
+          var child_node = this.nodes[contained_node_id];
+
+          // remove mass from child node from parent node
+          node.mass -= this.constants.clustering.massTransferCoefficient * this.nodes[contained_node_id].mass;
+
+          // decrease the size again
+          node.cluster_size -= child_node.cluster_size;
+          var grow_coefficient = this.constants.clustering.zoomOffset + (node.cluster_size + child_node.cluster_size) / node.cluster_size;
+          node.width -= grow_coefficient * this.constants.clustering.widthGrowth;
+          node.height -= grow_coefficient * this.constants.clustering.heightGrowth;
+          node.fontSize -= 1 * child_node.cluster_size;
+
+          // check if a further expansion step is possible if recursivity is enabled
+          if (recursive == true) {
+            this._expandClusterNode(child_node,true);
+          }
+        }
+      }
+
+      // put the edges back in the global this.edges
+      for (var contained_edge_id in node.contained_edges) {
+        if (node.contained_edges.hasOwnProperty(contained_edge_id)) {
+          this.edges[contained_edge_id] = node.contained_edges[contained_edge_id];
+          node.remaining_edges += 1;
+        }
+      }
+
+      // reset the cluster settings of this node
+      node.resetCluster();
+    }
+  }
+};
+
+/**
+ * This function checks if any nodes at the end of their trees have edges below a threshold length
+ * This function is called only from _updateClusters()
+ *
+ * @private
+ */
+Graph.prototype._formClusters = function() {
+  var min_length = this.constants.clustering.clusterLength/this.scale;
+
+  var dx,dy,length,
+      edges = this.edges;
+
+  // create an array of edge ids
+  var edges_id_array = []
+  for (var id in edges) {
+    if (edges.hasOwnProperty(id)) {
+      edges_id_array.push(id);
+    }
+  }
+
+  // check if any edges are shorter than min_length and start the clustering
+  // the clustering favours the node with the larger mass
+  for (var i = 0; i < edges_id_array.length; i++) {
+    var edge = edges[edges_id_array[i]];
+    if (edge.connected) {
+      dx = (edge.to.x - edge.from.x);
+      dy = (edge.to.y - edge.from.y);
+      length = Math.sqrt(dx * dx + dy * dy);
+
+
+      if (length < min_length) {
+        // checking for clustering possiblities
+
+        // first check which node is larger
+        if (edge.to.mass > edge.from.mass) {
+          var parent_node = edge.to
+          var child_node = edge.from
+        }
+        else {
+          var parent_node = edge.from
+          var child_node = edge.to
+        }
+
+        // we allow clustering from outside in, ideally the child node in on the outside
+        // if we do not cluster from outside in, we would have to reconnect edges or keep a second set of edges for the
+        // clusters. This will also have to be altered in the force calculation and rendering.
+        // This method is non-destructive and does not require a second set of data.
+        if (child_node.remaining_edges == 1) {
+          this._addToCluster(parent_node,child_node,edge);
+          delete this.edges[edges_id_array[i]];
+        }
+        else if (parent_node.remaining_edges == 1) {
+          this._addToCluster(child_node,parent_node,edge);
+          delete this.edges[edges_id_array[i]];
+        }
+      }
+    }
+  }
+};
+
+/**
+ * This function adds the childnode to the parentnode, creating a cluster if it is not already.
+ * This function is called only from _updateClusters()
+ *
+ * @param parent_node     | Node object: this is the node that will house the child node
+ * @param child_node      | Node object: this node will be deleted from the global this.nodes and stored in the parent node
+ * @param edge            | Edge object: this edge will be deleted from the global this.edges and stored in the parent node
+ * @private
+ */
+Graph.prototype._addToCluster = function(parent_node, child_node, edge) {
+  // join child node and edge in parent node
+  parent_node.contained_nodes[child_node.id] = child_node;
+  parent_node.contained_edges[child_node.id] = edge;
+
+  if (this.nodes.hasOwnProperty(child_node.id)) {
+    delete this.nodes[child_node.id];
+  }
+
+
+  var grow_coefficient = this.constants.clustering.zoomOffset + (parent_node.cluster_size + child_node.cluster_size) / parent_node.cluster_size;
+  parent_node.mass += this.constants.clustering.massTransferCoefficient * child_node.mass;
+  parent_node.width += grow_coefficient * this.constants.clustering.widthGrowth;
+  parent_node.height += grow_coefficient * this.constants.clustering.heightGrowth;
+  parent_node.remaining_edges -= 1;
+  parent_node.formation_scale = this.scale;
+  parent_node.cluster_size += child_node.cluster_size;
+  parent_node.fontSize += 1 * child_node.cluster_size;
+};
+
 
 /**
  * Update the this.node_indices with the most recent node index list
@@ -144,7 +338,7 @@ Graph.prototype._updateNodeIndexList = function() {
       this.node_indices.push(idx);
     }
   }
-}
+};
 
 /**
  * Set nodes and edges, and optionally options as well.
@@ -580,6 +774,7 @@ Graph.prototype._zoom = function(scale, pointer) {
 
   this._setScale(scale);
   this._setTranslation(tx, ty);
+  this._updateClusters();
   this._redraw();
 
   return scale;
@@ -1578,19 +1773,25 @@ Graph.prototype._calculateForces = function() {
       dy = node2.y - node1.y;
       distance = Math.sqrt(dx * dx + dy * dy);
 
-      //if (distance < 10*minimumDistance) {
+      //
+      if (distance < 2*minimumDistance) { // at 2.0 * the minimum distance, the force is 0.000045
         angle = Math.atan2(dy, dx);
 
-        // TODO: correct factor for repulsing force
-        //repulsingForce = 2 * Math.exp(-5 * (distance * distance) / (dmin * dmin) ); // TODO: customize the repulsing force
-        //repulsingForce = Math.exp(-1 * (distance * distance) / (dmin * dmin) ); // TODO: customize the repulsing force
-        repulsingForce = 1 / (1 + Math.exp((distance / minimumDistance - 1) * steepness)); // TODO: customize the repulsing force
+        if (distance < 0.5*minimumDistance) { // at 0.5 * the minimum distance, the force is 0.993307
+          repulsingForce = 1.0;
+        }
+        else {
+          // TODO: correct factor for repulsing force
+          //repulsingForce = 2 * Math.exp(-5 * (distance * distance) / (dmin * dmin) ); // TODO: customize the repulsing force
+          //repulsingForce = Math.exp(-1 * (distance * distance) / (dmin * dmin) ); // TODO: customize the repulsing force
+          repulsingForce = 1 / (1 + Math.exp((distance / minimumDistance - 1) * steepness)); // TODO: customize the repulsing force
+        }
         fx = Math.cos(angle) * repulsingForce;
         fy = Math.sin(angle) * repulsingForce;
 
         node1._addForce(-fx, -fy);
         node2._addForce(fx, fy);
-     // }
+      }
     }
   }
 
@@ -1645,7 +1846,8 @@ Graph.prototype._calculateForces = function() {
     }
   }
 
-  /* TODO: re-implement repulsion of edges
+  // TODO: re-implement repulsion of edges
+
    // repulsing forces between edges
    var minimumDistance = this.constants.edges.distance,
    steepness = 10; // higher value gives steeper slope of the force around the given minimumDistance
@@ -1680,7 +1882,7 @@ Graph.prototype._calculateForces = function() {
    edges[l2].to._addForce(fx, fy);
    }
    }
-   */
+
 };
 
 
@@ -1721,16 +1923,9 @@ Graph.prototype._discreteStepNodes = function() {
  */
 Graph.prototype.start = function() {
   if (this.moving) {
-    var start = window.performance.now();
-
     this._calculateForces();
     this._discreteStepNodes();
 
-    var end = window.performance.now();
-    var time = end - start;
-    console.log('Execution time: ' + time);
-
-
     var vmin = this.constants.minVelocity;
     this.moving = this._isMoving(vmin);
   }
@@ -1741,8 +1936,23 @@ Graph.prototype.start = function() {
       var graph = this;
       this.timer = window.setTimeout(function () {
         graph.timer = undefined;
+
+        // benchmark the calculation
+        var start = window.performance.now();
         graph.start();
+        // Optionally call this twice for faster convergence
+        // graph.start();
+        var end = window.performance.now();
+        var time = end - start;
+       // console.log('Simulation time: ' + time);
+
+
+        start = window.performance.now();
         graph._redraw();
+        end = window.performance.now();
+        time = end - start;
+      //  console.log('Drawing time: ' + time);
+
       }, this.refreshRate);
     }
   }
diff --git a/src/graph/Node.js b/src/graph/Node.js
index 934c1399..e7ce4206 100644
--- a/src/graph/Node.js
+++ b/src/graph/Node.js
@@ -25,6 +25,7 @@
 function Node(properties, imagelist, grouplist, constants) {
   this.selected = false;
 
+
   this.edges = []; // all edges connected to this node
   this.group = constants.nodes.group;
 
@@ -50,6 +51,10 @@ function Node(properties, imagelist, grouplist, constants) {
   this.imagelist = imagelist;
   this.grouplist = grouplist;
 
+  // creating the variables for clustering
+  this.resetCluster();
+  this.remaining_edges = 0;
+
   this.setProperties(properties, constants);
 
   // mass, force, velocity
@@ -62,6 +67,17 @@ function Node(properties, imagelist, grouplist, constants) {
   this.damping = 0.9; // damping factor
 };
 
+/**
+ * (re)setting the clustering variables and objects
+ */
+Node.prototype.resetCluster = function() {
+  // clustering variables
+  this.formation_scale = undefined; // this is used to determine when to open the cluster
+  this.cluster_size = 1;            // this signifies the total amount of nodes in this cluster
+  this.contained_nodes = {};
+  this.contained_edges = {};
+};
+
 /**
  * Attach a edge to the node
  * @param {Edge} edge
@@ -70,6 +86,8 @@ Node.prototype.attachEdge = function(edge) {
   if (this.edges.indexOf(edge) == -1) {
     this.edges.push(edge);
   }
+  this.remaining_edges = this.edges.length;
+  this.remaining_edges_tmp = this.edges.length;
   this._updateMass();
 };
 
@@ -82,6 +100,8 @@ Node.prototype.detachEdge = function(edge) {
   if (index != -1) {
     this.edges.splice(index, 1);
   }
+  this.remaining_edges = this.edges.length;
+  this.remaining_edges_tmp = this.edges.length;
   this._updateMass();
 };
 
@@ -192,6 +212,14 @@ Node.parseColor = function(color) {
       highlight: {
         border: color,
         background: color
+      },
+      cluster: {
+        border: color,
+        background: color,
+        highlight: {
+          border: color,
+          background: color
+        }
       }
     };
     // TODO: automatically generate a nice highlight color
@@ -200,6 +228,7 @@ Node.parseColor = function(color) {
     c = {};
     c.background = color.background || 'white';
     c.border = color.border || c.background;
+
     if (util.isString(color.highlight)) {
       c.highlight = {
         border: color.highlight,
@@ -211,6 +240,32 @@ Node.parseColor = function(color) {
       c.highlight.background = color.highlight && color.highlight.background || c.background;
       c.highlight.border = color.highlight && color.highlight.border || c.border;
     }
+
+    // check if cluster colorgroup has been defined
+    if (util.isString(color.cluster)) {
+      c.cluster = {
+        border: color.cluster,
+        background: color.cluster
+      }
+    }
+    else {
+      c.cluster = {};
+      c.cluster.background = color.cluster && color.cluster.background || c.background;
+      c.cluster.border = color.cluster && color.cluster.border || c.border;
+    }
+
+    // check if cluster highlight colorgroup has been defined
+    if (util.isString(color.cluster.highlight)) {
+      c.cluster.highlight = {
+        border: color.cluster.highlight,
+        background: color.cluster.highlight
+      }
+    }
+    else {
+      c.cluster.highlight = {};
+      c.cluster.highlight.background = color.cluster.highlight && color.cluster.highlight.background || c.background;
+      c.cluster.highlight.border = color.cluster.highlight && color.cluster.highlight.border || c.border;
+    }
   }
   return c;
 };
@@ -220,7 +275,8 @@ Node.parseColor = function(color) {
  */
 Node.prototype.select = function() {
   this.selected = true;
-  this._reset();
+  // why do this?
+  // this._reset();
 };
 
 /**
@@ -228,7 +284,8 @@ Node.prototype.select = function() {
  */
 Node.prototype.unselect = function() {
   this.selected = false;
-  this._reset();
+  // why do this?
+  // this._reset();
 };
 
 /**
@@ -490,9 +547,15 @@ Node.prototype._drawBox = function (ctx) {
   this.left = this.x - this.width / 2;
   this.top = this.y - this.height / 2;
 
-  ctx.strokeStyle = this.selected ? this.color.highlight.border : this.color.border;
-  ctx.fillStyle = this.selected ? this.color.highlight.background : this.color.background;
-  ctx.lineWidth = this.selected ? 2.0 : 1.0;
+  if (this.cluster_size > 1) {
+    ctx.strokeStyle = this.selected ? this.color.cluster.highlight.border : this.color.cluster.border;
+    ctx.fillStyle = this.selected ? this.color.cluster.highlight.background : this.color.cluster.background;
+  }
+  else {
+    ctx.strokeStyle = this.selected ? this.color.highlight.border : this.color.border;
+    ctx.fillStyle = this.selected ? this.color.highlight.background : this.color.background;
+  }
+  ctx.lineWidth = (this.selected ? 2.0 : 1.0) + (this.cluster_size > 1) ? 2.0 : 0.0;
   ctx.roundRect(this.left, this.top, this.width, this.height, this.radius);
   ctx.fill();
   ctx.stroke();
@@ -516,9 +579,15 @@ Node.prototype._drawDatabase = function (ctx) {
   this.left = this.x - this.width / 2;
   this.top = this.y - this.height / 2;
 
-  ctx.strokeStyle = this.selected ? this.color.highlight.border : this.color.border;
-  ctx.fillStyle = this.selected ? this.color.highlight.background : this.color.background;
-  ctx.lineWidth = this.selected ? 2.0 : 1.0;
+  if (this.cluster_size > 1) {
+    ctx.strokeStyle = this.selected ? this.color.cluster.highlight.border : this.color.cluster.border;
+    ctx.fillStyle = this.selected ? this.color.cluster.highlight.background : this.color.cluster.background;
+  }
+  else {
+    ctx.strokeStyle = this.selected ? this.color.highlight.border : this.color.border;
+    ctx.fillStyle = this.selected ? this.color.highlight.background : this.color.background;
+  }
+  ctx.lineWidth = (this.selected ? 2.0 : 1.0) + (this.cluster_size > 1) ? 2.0 : 0.0;
   ctx.database(this.x - this.width/2, this.y - this.height*0.5, this.width, this.height);
   ctx.fill();
   ctx.stroke();
@@ -544,9 +613,15 @@ Node.prototype._drawCircle = function (ctx) {
   this.left = this.x - this.width / 2;
   this.top = this.y - this.height / 2;
 
-  ctx.strokeStyle = this.selected ? this.color.highlight.border : this.color.border;
-  ctx.fillStyle = this.selected ? this.color.highlight.background : this.color.background;
-  ctx.lineWidth = this.selected ? 2.0 : 1.0;
+  if (this.cluster_size > 1) {
+    ctx.strokeStyle = this.selected ? this.color.cluster.highlight.border : this.color.cluster.border;
+    ctx.fillStyle = this.selected ? this.color.cluster.highlight.background : this.color.cluster.background;
+  }
+  else {
+    ctx.strokeStyle = this.selected ? this.color.highlight.border : this.color.border;
+    ctx.fillStyle = this.selected ? this.color.highlight.background : this.color.background;
+  }
+  ctx.lineWidth = (this.selected ? 2.0 : 1.0) + (this.cluster_size > 1) ? 2.0 : 0.0;
   ctx.circle(this.x, this.y, this.radius);
   ctx.fill();
   ctx.stroke();
@@ -571,9 +646,15 @@ Node.prototype._drawEllipse = function (ctx) {
   this.left = this.x - this.width / 2;
   this.top = this.y - this.height / 2;
 
-  ctx.strokeStyle = this.selected ? this.color.highlight.border : this.color.border;
-  ctx.fillStyle = this.selected ? this.color.highlight.background : this.color.background;
-  ctx.lineWidth = this.selected ? 2.0 : 1.0;
+  if (this.cluster_size > 1) {
+    ctx.strokeStyle = this.selected ? this.color.cluster.highlight.border : this.color.cluster.border;
+    ctx.fillStyle = this.selected ? this.color.cluster.highlight.background : this.color.cluster.background;
+  }
+  else {
+    ctx.strokeStyle = this.selected ? this.color.highlight.border : this.color.border;
+    ctx.fillStyle = this.selected ? this.color.highlight.background : this.color.background;
+  }
+  ctx.lineWidth = (this.selected ? 2.0 : 1.0) + (this.cluster_size > 1) ? 2.0 : 0.0;
   ctx.ellipse(this.left, this.top, this.width, this.height);
   ctx.fill();
   ctx.stroke();
@@ -615,9 +696,15 @@ Node.prototype._drawShape = function (ctx, shape) {
   this.left = this.x - this.width / 2;
   this.top = this.y - this.height / 2;
 
-  ctx.strokeStyle = this.selected ? this.color.highlight.border : this.color.border;
-  ctx.fillStyle = this.selected ? this.color.highlight.background : this.color.background;
-  ctx.lineWidth = this.selected ? 2.0 : 1.0;
+  if (this.cluster_size > 1) {
+    ctx.strokeStyle = this.selected ? this.color.cluster.highlight.border : this.color.cluster.border;
+    ctx.fillStyle = this.selected ? this.color.cluster.highlight.background : this.color.cluster.background;
+  }
+  else {
+    ctx.strokeStyle = this.selected ? this.color.highlight.border : this.color.border;
+    ctx.fillStyle = this.selected ? this.color.highlight.background : this.color.background;
+  }
+  ctx.lineWidth = (this.selected ? 2.0 : 1.0) + (this.cluster_size > 1) ? 2.0 : 0.0;
 
   ctx[shape](this.x, this.y, this.radius);
   ctx.fill();
diff --git a/vis.js b/vis.js
index 3aa33749..ff11893e 100644
--- a/vis.js
+++ b/vis.js
@@ -4,8 +4,8 @@
  *
  * A dynamic, browser-based visualization library.
  *
- * @version @@version
- * @date    @@date
+ * @version 0.3.0-SNAPSHOT
+ * @date    2014-01-08
  *
  * @license
  * Copyright (C) 2011-2013 Almende B.V, http://almende.com
@@ -12472,6 +12472,7 @@ if (typeof CanvasRenderingContext2D !== 'undefined') {
 function Node(properties, imagelist, grouplist, constants) {
   this.selected = false;
 
+
   this.edges = []; // all edges connected to this node
   this.group = constants.nodes.group;
 
@@ -12497,6 +12498,10 @@ function Node(properties, imagelist, grouplist, constants) {
   this.imagelist = imagelist;
   this.grouplist = grouplist;
 
+  // creating the variables for clustering
+  this.resetCluster();
+  this.remaining_edges = 0;
+
   this.setProperties(properties, constants);
 
   // mass, force, velocity
@@ -12509,6 +12514,17 @@ function Node(properties, imagelist, grouplist, constants) {
   this.damping = 0.9; // damping factor
 };
 
+/**
+ * (re)setting the clustering variables and objects
+ */
+Node.prototype.resetCluster = function() {
+  // clustering variables
+  this.formation_scale = undefined; // this is used to determine when to open the cluster
+  this.cluster_size = 1;            // this signifies the total amount of nodes in this cluster
+  this.contained_nodes = {};
+  this.contained_edges = {};
+};
+
 /**
  * Attach a edge to the node
  * @param {Edge} edge
@@ -12517,6 +12533,8 @@ Node.prototype.attachEdge = function(edge) {
   if (this.edges.indexOf(edge) == -1) {
     this.edges.push(edge);
   }
+  this.remaining_edges = this.edges.length;
+  this.remaining_edges_tmp = this.edges.length;
   this._updateMass();
 };
 
@@ -12529,6 +12547,8 @@ Node.prototype.detachEdge = function(edge) {
   if (index != -1) {
     this.edges.splice(index, 1);
   }
+  this.remaining_edges = this.edges.length;
+  this.remaining_edges_tmp = this.edges.length;
   this._updateMass();
 };
 
@@ -12639,6 +12659,14 @@ Node.parseColor = function(color) {
       highlight: {
         border: color,
         background: color
+      },
+      cluster: {
+        border: color,
+        background: color,
+        highlight: {
+          border: color,
+          background: color
+        }
       }
     };
     // TODO: automatically generate a nice highlight color
@@ -12647,6 +12675,7 @@ Node.parseColor = function(color) {
     c = {};
     c.background = color.background || 'white';
     c.border = color.border || c.background;
+
     if (util.isString(color.highlight)) {
       c.highlight = {
         border: color.highlight,
@@ -12658,6 +12687,32 @@ Node.parseColor = function(color) {
       c.highlight.background = color.highlight && color.highlight.background || c.background;
       c.highlight.border = color.highlight && color.highlight.border || c.border;
     }
+
+    // check if cluster colorgroup has been defined
+    if (util.isString(color.cluster)) {
+      c.cluster = {
+        border: color.cluster,
+        background: color.cluster
+      }
+    }
+    else {
+      c.cluster = {};
+      c.cluster.background = color.cluster && color.cluster.background || c.background;
+      c.cluster.border = color.cluster && color.cluster.border || c.border;
+    }
+
+    // check if cluster highlight colorgroup has been defined
+    if (util.isString(color.cluster.highlight)) {
+      c.cluster.highlight = {
+        border: color.cluster.highlight,
+        background: color.cluster.highlight
+      }
+    }
+    else {
+      c.cluster.highlight = {};
+      c.cluster.highlight.background = color.cluster.highlight && color.cluster.highlight.background || c.background;
+      c.cluster.highlight.border = color.cluster.highlight && color.cluster.highlight.border || c.border;
+    }
   }
   return c;
 };
@@ -12667,7 +12722,8 @@ Node.parseColor = function(color) {
  */
 Node.prototype.select = function() {
   this.selected = true;
-  this._reset();
+  // why do this?
+  // this._reset();
 };
 
 /**
@@ -12675,7 +12731,8 @@ Node.prototype.select = function() {
  */
 Node.prototype.unselect = function() {
   this.selected = false;
-  this._reset();
+  // why do this?
+  // this._reset();
 };
 
 /**
@@ -12937,9 +12994,15 @@ Node.prototype._drawBox = function (ctx) {
   this.left = this.x - this.width / 2;
   this.top = this.y - this.height / 2;
 
-  ctx.strokeStyle = this.selected ? this.color.highlight.border : this.color.border;
-  ctx.fillStyle = this.selected ? this.color.highlight.background : this.color.background;
-  ctx.lineWidth = this.selected ? 2.0 : 1.0;
+  if (this.cluster_size > 1) {
+    ctx.strokeStyle = this.selected ? this.color.cluster.highlight.border : this.color.cluster.border;
+    ctx.fillStyle = this.selected ? this.color.cluster.highlight.background : this.color.cluster.background;
+  }
+  else {
+    ctx.strokeStyle = this.selected ? this.color.highlight.border : this.color.border;
+    ctx.fillStyle = this.selected ? this.color.highlight.background : this.color.background;
+  }
+  ctx.lineWidth = (this.selected ? 2.0 : 1.0) + (this.cluster_size > 1) ? 2.0 : 0.0;
   ctx.roundRect(this.left, this.top, this.width, this.height, this.radius);
   ctx.fill();
   ctx.stroke();
@@ -12963,9 +13026,15 @@ Node.prototype._drawDatabase = function (ctx) {
   this.left = this.x - this.width / 2;
   this.top = this.y - this.height / 2;
 
-  ctx.strokeStyle = this.selected ? this.color.highlight.border : this.color.border;
-  ctx.fillStyle = this.selected ? this.color.highlight.background : this.color.background;
-  ctx.lineWidth = this.selected ? 2.0 : 1.0;
+  if (this.cluster_size > 1) {
+    ctx.strokeStyle = this.selected ? this.color.cluster.highlight.border : this.color.cluster.border;
+    ctx.fillStyle = this.selected ? this.color.cluster.highlight.background : this.color.cluster.background;
+  }
+  else {
+    ctx.strokeStyle = this.selected ? this.color.highlight.border : this.color.border;
+    ctx.fillStyle = this.selected ? this.color.highlight.background : this.color.background;
+  }
+  ctx.lineWidth = (this.selected ? 2.0 : 1.0) + (this.cluster_size > 1) ? 2.0 : 0.0;
   ctx.database(this.x - this.width/2, this.y - this.height*0.5, this.width, this.height);
   ctx.fill();
   ctx.stroke();
@@ -12991,9 +13060,15 @@ Node.prototype._drawCircle = function (ctx) {
   this.left = this.x - this.width / 2;
   this.top = this.y - this.height / 2;
 
-  ctx.strokeStyle = this.selected ? this.color.highlight.border : this.color.border;
-  ctx.fillStyle = this.selected ? this.color.highlight.background : this.color.background;
-  ctx.lineWidth = this.selected ? 2.0 : 1.0;
+  if (this.cluster_size > 1) {
+    ctx.strokeStyle = this.selected ? this.color.cluster.highlight.border : this.color.cluster.border;
+    ctx.fillStyle = this.selected ? this.color.cluster.highlight.background : this.color.cluster.background;
+  }
+  else {
+    ctx.strokeStyle = this.selected ? this.color.highlight.border : this.color.border;
+    ctx.fillStyle = this.selected ? this.color.highlight.background : this.color.background;
+  }
+  ctx.lineWidth = (this.selected ? 2.0 : 1.0) + (this.cluster_size > 1) ? 2.0 : 0.0;
   ctx.circle(this.x, this.y, this.radius);
   ctx.fill();
   ctx.stroke();
@@ -13018,9 +13093,15 @@ Node.prototype._drawEllipse = function (ctx) {
   this.left = this.x - this.width / 2;
   this.top = this.y - this.height / 2;
 
-  ctx.strokeStyle = this.selected ? this.color.highlight.border : this.color.border;
-  ctx.fillStyle = this.selected ? this.color.highlight.background : this.color.background;
-  ctx.lineWidth = this.selected ? 2.0 : 1.0;
+  if (this.cluster_size > 1) {
+    ctx.strokeStyle = this.selected ? this.color.cluster.highlight.border : this.color.cluster.border;
+    ctx.fillStyle = this.selected ? this.color.cluster.highlight.background : this.color.cluster.background;
+  }
+  else {
+    ctx.strokeStyle = this.selected ? this.color.highlight.border : this.color.border;
+    ctx.fillStyle = this.selected ? this.color.highlight.background : this.color.background;
+  }
+  ctx.lineWidth = (this.selected ? 2.0 : 1.0) + (this.cluster_size > 1) ? 2.0 : 0.0;
   ctx.ellipse(this.left, this.top, this.width, this.height);
   ctx.fill();
   ctx.stroke();
@@ -13062,9 +13143,15 @@ Node.prototype._drawShape = function (ctx, shape) {
   this.left = this.x - this.width / 2;
   this.top = this.y - this.height / 2;
 
-  ctx.strokeStyle = this.selected ? this.color.highlight.border : this.color.border;
-  ctx.fillStyle = this.selected ? this.color.highlight.background : this.color.background;
-  ctx.lineWidth = this.selected ? 2.0 : 1.0;
+  if (this.cluster_size > 1) {
+    ctx.strokeStyle = this.selected ? this.color.cluster.highlight.border : this.color.cluster.border;
+    ctx.fillStyle = this.selected ? this.color.cluster.highlight.background : this.color.cluster.background;
+  }
+  else {
+    ctx.strokeStyle = this.selected ? this.color.highlight.border : this.color.border;
+    ctx.fillStyle = this.selected ? this.color.highlight.background : this.color.background;
+  }
+  ctx.lineWidth = (this.selected ? 2.0 : 1.0) + (this.cluster_size > 1) ? 2.0 : 0.0;
 
   ctx[shape](this.x, this.y, this.radius);
   ctx.fill();
@@ -14005,6 +14092,14 @@ function Graph (container, data, options) {
         highlight: {
           border: '#2B7CE9',
           background: '#D2E5FF'
+        },
+        cluster: {
+          border: '#000000',
+          background: '#F0F0F0',
+          highlight: {
+            border: '#FF0FFF',
+            background: '#FF000F'
+          }
         }
       },
       borderColor: '#2B7CE9',
@@ -14029,6 +14124,13 @@ function Graph (container, data, options) {
         altLength: undefined
       }
     },
+    clustering: {
+      clusterLength: 30,      // threshold length for clustering
+      zoomOffset: 0.1,
+      widthGrowth: 15,        // growth factor = ((parent_size + child_size) / parent_size) * widthGrowthFactor
+      heightGrowth: 15,       // growth factor = ((parent_size + child_size) / parent_size) * heightGrowthFactor
+      massTransferCoefficient: 0.1 // parent.mass += massTransferCoefficient * child.mass
+    },
     minForce: 0.05,
     minVelocity: 0.02,   // px/s
     maxIterations: 1000  // maximum number of iteration to stabilize
@@ -14038,6 +14140,8 @@ function Graph (container, data, options) {
   this.node_indices = [];     // the node indices list is used to speed up the computation of the repulsion fields
   this.nodes = {};            // object with Node objects
   this.edges = {};            // object with Edge objects
+  this.scale = 1;             // defining the global scale variable in the constructor
+  this.previous_scale = 1;    // use this to check if the zoom operation is in or out
   // TODO: create a counter to keep track on the number of nodes having values
   // TODO: create a counter to keep track on the number of nodes currently moving
   // TODO: create a counter to keep track on the number of edges having values
@@ -14096,7 +14200,184 @@ function Graph (container, data, options) {
 
   // draw data
   this.setData(data);
-}
+};
+
+/**
+ * This function checks if the zoom action is in or out.
+ * If out, check if we can form clusters, if in, check if we can open clusters.
+ * This function is only called from _zoom()
+ *
+ * @private
+ */
+Graph.prototype._updateClusters = function() {
+  if (this.previous_scale > this.scale) { // zoom out
+    this._formClusters();
+  }
+  else if (this.previous_scale < this.scale) { // zoom out
+    this._openClusters();
+  }
+  this._updateNodeIndexList();
+
+  for (var nid in this.nodes) {
+    var node = this.nodes[nid];
+    node.label = String(node.remaining_edges);
+  }
+
+  this.previous_scale = this.scale;
+};
+
+/**
+ * This function loops over all nodes in the node_indices list. For each node it checks if it is a cluster and if it
+ * has to be opened based on the current zoom level.
+ *
+ * @private
+ */
+Graph.prototype._openClusters = function() {
+  for (var i = 0; i < this.node_indices.length; i++) {
+    var node = this.nodes[this.node_indices[i]];
+    this._expandClusterNode(node,true);
+  }
+};
+
+/**
+ * This function checks if a node has to be opened. This is done by checking the zoom level.
+ * If the node contains child nodes, this function is recursively called on the child nodes as well.
+ * This recursive behaviour is optional and can be set by the recursive argument.
+ *
+ * @param node        | Node object: to check for cluster and expand
+ * @param recursive   | Boolean: enable or disable recursive calling
+ * @private
+ */
+Graph.prototype._expandClusterNode = function(node, recursive) {
+  if (node.formation_scale != undefined) {
+    if (this.scale > node.formation_scale) {
+      for (var contained_node_id in node.contained_nodes) {
+        if (node.contained_nodes.hasOwnProperty(contained_node_id)) {
+          // put the child node back in the global nodes object
+          this.nodes[contained_node_id] = node.contained_nodes[contained_node_id];
+
+          var child_node = this.nodes[contained_node_id];
+
+          // remove mass from child node from parent node
+          node.mass -= this.constants.clustering.massTransferCoefficient * this.nodes[contained_node_id].mass;
+
+          // decrease the size again
+          node.cluster_size -= child_node.cluster_size;
+          var grow_coefficient = this.constants.clustering.zoomOffset + (node.cluster_size + child_node.cluster_size) / node.cluster_size;
+          node.width -= grow_coefficient * this.constants.clustering.widthGrowth;
+          node.height -= grow_coefficient * this.constants.clustering.heightGrowth;
+          node.fontSize -= 1 * child_node.cluster_size;
+
+          // check if a further expansion step is possible if recursivity is enabled
+          if (recursive == true) {
+            this._expandClusterNode(child_node,true);
+          }
+        }
+      }
+
+      // put the edges back in the global this.edges
+      for (var contained_edge_id in node.contained_edges) {
+        if (node.contained_edges.hasOwnProperty(contained_edge_id)) {
+          this.edges[contained_edge_id] = node.contained_edges[contained_edge_id];
+          node.remaining_edges += 1;
+        }
+      }
+
+      // reset the cluster settings of this node
+      node.resetCluster();
+    }
+  }
+};
+
+/**
+ * This function checks if any nodes at the end of their trees have edges below a threshold length
+ * This function is called only from _updateClusters()
+ *
+ * @private
+ */
+Graph.prototype._formClusters = function() {
+  var min_length = this.constants.clustering.clusterLength/this.scale;
+
+  var dx,dy,length,
+      edges = this.edges;
+
+  // create an array of edge ids
+  var edges_id_array = []
+  for (var id in edges) {
+    if (edges.hasOwnProperty(id)) {
+      edges_id_array.push(id);
+    }
+  }
+
+  // check if any edges are shorter than min_length and start the clustering
+  // the clustering favours the node with the larger mass
+  for (var i = 0; i < edges_id_array.length; i++) {
+    var edge = edges[edges_id_array[i]];
+    if (edge.connected) {
+      dx = (edge.to.x - edge.from.x);
+      dy = (edge.to.y - edge.from.y);
+      length = Math.sqrt(dx * dx + dy * dy);
+
+
+      if (length < min_length) {
+        // checking for clustering possiblities
+
+        // first check which node is larger
+        if (edge.to.mass > edge.from.mass) {
+          var parent_node = edge.to
+          var child_node = edge.from
+        }
+        else {
+          var parent_node = edge.from
+          var child_node = edge.to
+        }
+
+        // we allow clustering from outside in, ideally the child node in on the outside
+        // if we do not cluster from outside in, we would have to reconnect edges or keep a second set of edges for the
+        // clusters. This will also have to be altered in the force calculation and rendering.
+        // This method is non-destructive and does not require a second set of data.
+        if (child_node.remaining_edges == 1) {
+          this._addToCluster(parent_node,child_node,edge);
+          delete this.edges[edges_id_array[i]];
+        }
+        else if (parent_node.remaining_edges == 1) {
+          this._addToCluster(child_node,parent_node,edge);
+          delete this.edges[edges_id_array[i]];
+        }
+      }
+    }
+  }
+};
+
+/**
+ * This function adds the childnode to the parentnode, creating a cluster if it is not already.
+ * This function is called only from _updateClusters()
+ *
+ * @param parent_node     | Node object: this is the node that will house the child node
+ * @param child_node      | Node object: this node will be deleted from the global this.nodes and stored in the parent node
+ * @param edge            | Edge object: this edge will be deleted from the global this.edges and stored in the parent node
+ * @private
+ */
+Graph.prototype._addToCluster = function(parent_node, child_node, edge) {
+  // join child node and edge in parent node
+  parent_node.contained_nodes[child_node.id] = child_node;
+  parent_node.contained_edges[child_node.id] = edge;
+
+  if (this.nodes.hasOwnProperty(child_node.id)) {
+    delete this.nodes[child_node.id];
+  }
+
+
+  var grow_coefficient = this.constants.clustering.zoomOffset + (parent_node.cluster_size + child_node.cluster_size) / parent_node.cluster_size;
+  parent_node.mass += this.constants.clustering.massTransferCoefficient * child_node.mass;
+  parent_node.width += grow_coefficient * this.constants.clustering.widthGrowth;
+  parent_node.height += grow_coefficient * this.constants.clustering.heightGrowth;
+  parent_node.remaining_edges -= 1;
+  parent_node.formation_scale = this.scale;
+  parent_node.cluster_size += child_node.cluster_size;
+  parent_node.fontSize += 1 * child_node.cluster_size;
+};
+
 
 /**
  * Update the this.node_indices with the most recent node index list
@@ -14110,7 +14391,7 @@ Graph.prototype._updateNodeIndexList = function() {
       this.node_indices.push(idx);
     }
   }
-}
+};
 
 /**
  * Set nodes and edges, and optionally options as well.
@@ -14546,6 +14827,7 @@ Graph.prototype._zoom = function(scale, pointer) {
 
   this._setScale(scale);
   this._setTranslation(tx, ty);
+  this._updateClusters();
   this._redraw();
 
   return scale;
@@ -15544,19 +15826,25 @@ Graph.prototype._calculateForces = function() {
       dy = node2.y - node1.y;
       distance = Math.sqrt(dx * dx + dy * dy);
 
-      //if (distance < 10*minimumDistance) {
+      //
+      if (distance < 2*minimumDistance) { // at 2.0 * the minimum distance, the force is 0.000045
         angle = Math.atan2(dy, dx);
 
-        // TODO: correct factor for repulsing force
-        //repulsingForce = 2 * Math.exp(-5 * (distance * distance) / (dmin * dmin) ); // TODO: customize the repulsing force
-        //repulsingForce = Math.exp(-1 * (distance * distance) / (dmin * dmin) ); // TODO: customize the repulsing force
-        repulsingForce = 1 / (1 + Math.exp((distance / minimumDistance - 1) * steepness)); // TODO: customize the repulsing force
+        if (distance < 0.5*minimumDistance) { // at 0.5 * the minimum distance, the force is 0.993307
+          repulsingForce = 1.0;
+        }
+        else {
+          // TODO: correct factor for repulsing force
+          //repulsingForce = 2 * Math.exp(-5 * (distance * distance) / (dmin * dmin) ); // TODO: customize the repulsing force
+          //repulsingForce = Math.exp(-1 * (distance * distance) / (dmin * dmin) ); // TODO: customize the repulsing force
+          repulsingForce = 1 / (1 + Math.exp((distance / minimumDistance - 1) * steepness)); // TODO: customize the repulsing force
+        }
         fx = Math.cos(angle) * repulsingForce;
         fy = Math.sin(angle) * repulsingForce;
 
         node1._addForce(-fx, -fy);
         node2._addForce(fx, fy);
-     // }
+      }
     }
   }
 
@@ -15611,7 +15899,8 @@ Graph.prototype._calculateForces = function() {
     }
   }
 
-  /* TODO: re-implement repulsion of edges
+  // TODO: re-implement repulsion of edges
+
    // repulsing forces between edges
    var minimumDistance = this.constants.edges.distance,
    steepness = 10; // higher value gives steeper slope of the force around the given minimumDistance
@@ -15646,7 +15935,7 @@ Graph.prototype._calculateForces = function() {
    edges[l2].to._addForce(fx, fy);
    }
    }
-   */
+
 };
 
 
@@ -15687,16 +15976,9 @@ Graph.prototype._discreteStepNodes = function() {
  */
 Graph.prototype.start = function() {
   if (this.moving) {
-    var start = window.performance.now();
-
     this._calculateForces();
     this._discreteStepNodes();
 
-    var end = window.performance.now();
-    var time = end - start;
-    console.log('Execution time: ' + time);
-
-
     var vmin = this.constants.minVelocity;
     this.moving = this._isMoving(vmin);
   }
@@ -15707,8 +15989,23 @@ Graph.prototype.start = function() {
       var graph = this;
       this.timer = window.setTimeout(function () {
         graph.timer = undefined;
+
+        // benchmark the calculation
+        var start = window.performance.now();
         graph.start();
+        // Optionally call this twice for faster convergence
+        // graph.start();
+        var end = window.performance.now();
+        var time = end - start;
+       // console.log('Simulation time: ' + time);
+
+
+        start = window.performance.now();
         graph._redraw();
+        end = window.performance.now();
+        time = end - start;
+      //  console.log('Drawing time: ' + time);
+
       }, this.refreshRate);
     }
   }