The Lagrangian numerical relativity code SPHINCS_BSSN_v1.0

We present version 1.0 of our Lagrangian numerical relativity code SPHINCS_BSSN . This code evolves the full set of Einstein equations, but contrary to other numerical relativity codes, it evolves the matter fluid via Lagrangian particles in the framework of a high-accuracy version of smooth particle hydrodynamics (SPH). The major new elements introduced here are: (i) a new method to map the stress–energy tensor (known at the particles) to the spacetime mesh, based on a local regression estimate; (ii) additional measures that ensure the robust evolution of a neutron star through its collapse to a black hole; and (iii) further refinements in how we place the SPH particles for our initial data. The latter are implemented in our code SPHINCS_ID which now, in addition to LORENE , can also couple to initial data produced by the initial data library FUKA . We discuss several simulations of neutron star mergers performed with SPHINCS_BSSN_v1.0 , including irrotational cases with and without prompt collapse and a system where only one of the stars has a large spin (χ = 0.5).