# Many-body force

The many-body (or *n*-body) force applies mutually amongst all nodes. It can be used to simulate gravity (attraction) if the strength is positive, or electrostatic charge (repulsion) if the strength is negative. This implementation uses a quadtree and the Barnes–Hut approximation to greatly improve performance; the accuracy can be customized using the theta parameter.

Unlike the link force, which only affect two linked nodes, the charge force is global: every node affects every other node, even if they are on disconnected subgraphs.

## forceManyBody()

Source · Creates a new many-body force with the default parameters.

`const manyBody = d3.forceManyBody().strength(-100);`

*manyBody*.strength(*strength*)

Source · If *strength* is specified, sets the strength accessor to the specified number or function, re-evaluates the strength accessor for each node, and returns this force. A positive value causes nodes to attract each other, similar to gravity, while a negative value causes nodes to repel each other, similar to electrostatic charge. If *strength* is not specified, returns the current strength accessor, which defaults to:

```
function strength() {
return -30;
}
```

The strength accessor is invoked for each node in the simulation, being passed the *node* and its zero-based *index*. The resulting number is then stored internally, such that the strength of each node is only recomputed when the force is initialized or when this method is called with a new *strength*, and not on every application of the force.

*manyBody*.theta(*theta*)

Source · If *theta* is specified, sets the Barnes–Hut approximation criterion to the specified number and returns this force. If *theta* is not specified, returns the current value, which defaults to 0.9.

To accelerate computation, this force implements the Barnes–Hut approximation which takes O(*n* log *n*) per application where *n* is the number of nodes. For each application, a quadtree stores the current node positions; then for each node, the combined force of all other nodes on the given node is computed. For a cluster of nodes that is far away, the charge force can be approximated by treating the cluster as a single, larger node. The *theta* parameter determines the accuracy of the approximation: if the ratio *w* / *l* of the width *w* of the quadtree cell to the distance *l* from the node to the cell’s center of mass is less than *theta*, all nodes in the given cell are treated as a single node rather than individually.

*manyBody*.distanceMin(*distance*)

Source · If *distance* is specified, sets the minimum distance between nodes over which this force is considered. If *distance* is not specified, returns the current minimum distance, which defaults to 1. A minimum distance establishes an upper bound on the strength of the force between two nearby nodes, avoiding instability. In particular, it avoids an infinitely-strong force if two nodes are exactly coincident; in this case, the direction of the force is random.

*manyBody*.distanceMax(*distance*)

Source · If *distance* is specified, sets the maximum distance between nodes over which this force is considered. If *distance* is not specified, returns the current maximum distance, which defaults to infinity. Specifying a finite maximum distance improves performance and produces a more localized layout.