@@ -360,7 +360,7 @@ There’s now a nice [visual reference](https://github.com/d3/d3-ease/blob/maste

The force layout d3.layout.force has been renamed to d3.forceSimulation. The force simulation now uses [velocity Verlet integration](https://en.wikipedia.org/wiki/Verlet_integration#Velocity_Verlet) rather than position Verlet, tracking the nodes’ positions (*node*.x, *node*.y) and velocities (*node*.vx, *node*.vy) rather than their previous positions (*node*.px, *node*.py).

Rather than hard-coding a set of built-in forces, the force simulation is now extensible: you specify which forces you want! The approach affords greater flexibility through composition. The new forces are more flexible, too: force parameters can typically be configured per-node or per-link. There are separate positioning forces for [*x*](https://github.com/d3/d3-force#forceX) and [*y*](https://github.com/d3/d3-force#forceY) that replace *force*.gravity. The new [link force](https://github.com/d3/d3-force#forceLink) replaces *force*.linkStrength and employs better default heuristics to improve stability. The new [many-body force](https://github.com/d3/d3-force#forceManyBody) replaces *force*.charge and supports a new [minimum-distance parameter](https://github.com/d3/d3-force#manyBody_distanceMin) and performance improvements thanks to 4.0’s [new quadtrees](#quadtrees-d3-quadtree). There are also brand-new forces for [centering nodes](https://github.com/d3/d3-force#forceCenter) and [collision resolution](https://github.com/d3/d3-force#forceCollision).

Rather than hard-coding a set of built-in forces, the force simulation is now extensible: you specify which forces you want! The approach affords greater flexibility through composition. The new forces are more flexible, too: force parameters can typically be configured per-node or per-link. There are separate positioning forces for [*x*](https://github.com/d3/d3-force#forceX) and [*y*](https://github.com/d3/d3-force#forceY) that replace *force*.gravity; [*x*.x](https://github.com/d3/d3-force#x_x) and [*y*.y](https://github.com/d3/d3-force#y_y) replace *force*.size. The new [link force](https://github.com/d3/d3-force#forceLink) replaces *force*.linkStrength and employs better default heuristics to improve stability. The new [many-body force](https://github.com/d3/d3-force#forceManyBody) replaces *force*.charge and supports a new [minimum-distance parameter](https://github.com/d3/d3-force#manyBody_distanceMin) and performance improvements thanks to 4.0’s [new quadtrees](#quadtrees-d3-quadtree). There are also brand-new forces for [centering nodes](https://github.com/d3/d3-force#forceCenter) and [collision resolution](https://github.com/d3/d3-force#forceCollision).

The new forces and simulation have been carefully crafted to avoid nondeterminism. Rather than initializing nodes randomly, if the nodes do not have preset positions, they are placed in a phyllotaxis pattern:

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@@ -368,7 +368,7 @@ The new forces and simulation have been carefully crafted to avoid nondeterminis

Random jitter is still needed to resolve link, collision and many-body forces if there are coincident nodes, but at least in the common case, the force simulation (and the resulting force-directed graph layout) is now consistent across browsers and reloads. D3 no longer plays dice!

The force simulation has several new methods for greater control over heating, such as [*simulation*.alphaMin](https://github.com/d3/d3-force#simulation_alphaMin) and [*simulation*.alphaDecay](https://github.com/d3/d3-force#simulation_alphaDecay), and the internal timer. Calling [*simulation*.alpha](https://github.com/d3/d3-force#simulation_alpha) now has no effect on the internal timer, which is controlled independently via [*simulation*.stop](https://github.com/d3/d3-force#simulation_stop) and [*simulation*.restart](https://github.com/d3/d3-force#simulation_restart). As in 3.x, you can advance the simulation manually using [*simulation*.tick](https://github.com/d3/d3-force#simulation_tick). The *force*.friction parameter is replaced by *simulation*.velocityDecay. A new [*simulation*.alphaTarget](https://github.com/d3/d3-force#simulation_alphaTarget) method allows you to set the desired alpha (temperature) of the simulation, such that the simulation can be smoothly reheated during interaction, and then smoothly cooled again. This improves the stability of the graph during interaction.

The force simulation has several new methods for greater control over heating, such as [*simulation*.alphaMin](https://github.com/d3/d3-force#simulation_alphaMin) and [*simulation*.alphaDecay](https://github.com/d3/d3-force#simulation_alphaDecay), and the internal timer. Calling [*simulation*.alpha](https://github.com/d3/d3-force#simulation_alpha) now has no effect on the internal timer, which is controlled independently via [*simulation*.stop](https://github.com/d3/d3-force#simulation_stop) and [*simulation*.restart](https://github.com/d3/d3-force#simulation_restart). The force layout’s internal timer now starts automatically on creation, removing *force*.start. As in 3.x, you can advance the simulation manually using [*simulation*.tick](https://github.com/d3/d3-force#simulation_tick). The *force*.friction parameter is replaced by *simulation*.velocityDecay. A new [*simulation*.alphaTarget](https://github.com/d3/d3-force#simulation_alphaTarget) method allows you to set the desired alpha (temperature) of the simulation, such that the simulation can be smoothly reheated during interaction, and then smoothly cooled again. This improves the stability of the graph during interaction.

The force layout no longer depends on the [drag behavior](#dragging-d3-drag), though you can certainly create [draggable force-directed graphs](http://bl.ocks.org/mbostock/ad70335eeef6d167bc36fd3c04378048)! Set *node*.fx and *node*.fy to fix a node’s position. As an alternative to a [Voronoi](#voronoi-d3-voronoi) SVG overlay, you can now use [*simulation*.find](https://github.com/d3/d3-force#simulation_find) to find the closest node to a pointer.