  We extend the state-of-the-art N-body code PKDGRAV3 with the inclusion of
mesh-free gas hydrodynamics for cosmological simulations. Two new hydrodynamic
solvers have been implemented, the mesh-less finite volume and mesh-less finite
mass methods. The solvers manifestly conserve mass, momentum and energy, and
have been validated with a wide range of standard test simulations, including
cosmological simulations. We also describe improvements to PKDGRAV3 that have
been implemented for performing hydrodynamic simulations. These changes have
been made with efficiency and modularity in mind, and provide a solid base for
the implementation of the required modules for galaxy formation and evolution
physics and future porting to GPUs. The code is released in a public
repository, together with the documentation and all the test simulations
presented in this work.
