<tt>BIFROST</tt>: simulating compact subsystems in star clusters using a hierarchical fourth-order forward symplectic integrator code

We present BIFROST, an extended version of the GPU-accelerated hierarchical fourth-order forward symplectic integrator code FROST. BIFROST (BInaries in FROST) can efficiently evolve collisional stellar systems with arbitrary binary fractions up to $f_\mathrm{bin}=100\%$ by using secular and regularised integration for binaries, triples, multiple or small clusters around black holes within framework. Post-Newtonian (PN) terms order PN3.5 are included equations motion compact subsystems optional three-body spin-dependent terms. PN1.0 interactions computed everywhere simulation domain. The has several merger criteria (gravitational-wave inspirals, tidal disruption events object collisions) addition relativistic recoil kicks mergers. show that $N$ particles scaling remains good $N_\mathrm{GPU} \sim 40\times N / 10^6$ GPUs increasing 100 per cent hardly increase running time (less than a factor $\sim 1.5$). also validate numerical accuracy presenting number star simulations most extreme ones including core collapse two intermediate mass kick.