GADGET-4 is a massively parallel code for N-body/hydrodynamical cosmological simulations. It is a flexible code that can be applied to a variety of different types of simulations, offering a number of sophisticated simulation algorithms. An account of the numerical algorithms employed by the code is given in the original code paper, subsequent publications, and this documentation.
GADGET-4 was written mainly by Volker Springel, with important contributions and suggestions being made by numerous people, including Ruediger Pakmor, Oliver Zier, and Martin Reinecke.
For documentation of the code as well as the code paper, please refer to the code's web-site.