Population-synthesis codes are an unique tool to explore the parameter space of massive
binary star evolution and binary compact object (BCO) formation. Most population-synthesis
codes are based on the same stellar evolution model, limiting our ability to explore the main
uncertainties. Here, we present the new version of the code sevn, which overcomes this
issue by interpolating the main stellar properties from a set of pre-computed evolutionary
tracks. We describe the new interpolation and adaptive time-step algorithms of sevn, and the�…

ABSTRACT Population III (Pop. III) binary stars likely produced the first stellar-born binary
black hole (BBH) mergers in the Universe. Here, we quantify the main sources of uncertainty
for the merger rate density evolution and mass spectrum of Pop. III BBHs by considering four
different formation histories and 11 models of the initial orbital properties of Pop. III binary
stars. The uncertainty on the orbital properties affects the BBH merger rate density by up to
two orders of magnitude, models with shorter orbital periods leading to higher BBH merger�…