Evolution of the black hole binary parameters at the stage of the pre-bound and bound systems. Left panel: Separations, ΔR, between SMBHB for the numerical models A, B, and C as solid lines. The dotted colour lines represent the second initial randomisation seeds for the particle distributions. Horizontal black dashed lines represent the level of the Schwarzschild radii for SMBHB with summarised mass: M_SMBHB = M_BH1 + M_BH2. Middle pane: Evolution of the inverse semimajor axis for the same models. Black dashed lines show the simple linear fit for the ‘hardening’ of the orbits for A, B, and C models, respectively. Right panel: Evolution of eccentricity, e, for the SMBHB system for all numerical models.