cluster.separation()

cluster.separation([separation])

If separation is specified, sets the separation accessor to the specified function and returns this cluster layout. If separation is not specified, returns the current separation accessor, which defaults to:

function separation(a, b) {
  return a.parent == b.parent ? 1 : 2;
}

The separation accessor is used to separate neighboring leaves. The separation function is passed two leaves a and b, and must return the desired separation. The nodes are typically siblings, though the nodes may be more distantly related if the layout decides to place such nodes adjacent.