ngravs: Distinct gravitational interactions in GADGET-2
  We discuss an extension of the massively parallel cosmological simulation
code GADGET-2, which now enables investigation of multiple and distinct
gravitational force laws, provided they are dominated by a constant scaling of
the Newtonian force. In addition to simplifying investigations of a universally
modified force law, the ngravs extension provides a foundation for
state-of-the-art collisionless cosmological simulations of exotic gravitational
scenarios. We briefly review the algorithms used by GADGET-2, and present our
extension to multiple gravities, highlighting additional features that
facilitate consideration of exotic force laws. We discuss the accuracy and
performance of the ngravs extension, both internally and with an unaltered
GADGET-2, in the relevant operational modes. The ngravs extension is publicly
released to the research community.
