The Verlet integration is just this part here: https://github.com/mbostock/d3/blob/master/src/layout/force.js#L104-114 https://github.com/mbostock/d3/blob/mast

The Verlet integration is just this part here: https://github.com/mbostock/d3/blob/master/src/layout/force.js#L104-114 https://github.com/mbostock/d3/blob/master/src/layout/force.... The rest of the function is calculating default forces and constraints for graph layout; these can be disabled, and thanks to the resilience of Verlet integration, you can easily implement custom forces or constraints in your "tick" event listener (as Shan Carter did in the budget piece a few weeks ago). Given that it's a generalized physics engine, I like that @soulwire's code is organized into clean, modular units. That makes it easier to test and modify the internals. D3's force layout is specifically-tailored to graph layout, so I don't consider it necessary to make the implementation so modular; the requirements of the force layout are that it is fast by default, that it can be customized with incremental additional effort, and that it is convenient for the common cases. Also, with larger graphs numerical integration is one of the few places where JavaScript performance (not just rendering and DOM manipulation) actually matters; while unfortunate, it's often necessary to make a trade-off between generality and performance. Still, I'm considering more modular reusable forces and constraints for a future iteration of the force layout (perhaps similar to the older Protovis force layout). But since custom forces tend to be very… well… custom, I think the best option is simply to modify the nodes' positions as needed and let Verlet do the rest.