Fraction of binaries retained in the nuclear star cluster (solid line) and average number of BHs ejected per BH merger (dotted line) as a function of the cluster escape speed. Here, the zero-age main-sequence distribution of masses is dN/ dM ∝ M ⁰, to account for mass segregation in the cluster center, where most interactions occur. We also assume a maximum BH mass of 20  M _☉ and a maximum main-sequence mass of 1  M _☉, but most results are robust against variations of these quantities. All runs are done with 100 realizations, which explains the lack of perfect smoothness. We see, as expected, that the retention fraction increases rapidly with escape speed, so that for nuclear star clusters, most binaries stay in the cluster until merger. We also see that at V _esc ~ 200 km s ⁻¹ and above, tens of percent of BH singles also stay in the cluster. This suggests a high merger efficiency.