Numerical backward integration of two point masses representing the clusters in the external Milky Way potential. (a) The run mesh (fragment) for cluster age T = 50 Myr: small squares represent individual runs with different present-day (t = 0) LOS velocities. Coloured squares mark runs with primordial separations smaller than 15 pc. The black plus and star denote the central V_r values and run #(53,61) given in panel b. Rimmed circles mark runs with negative primordial energy E_K. The dashed black line and the orange rectangle depict the line of equal LOS velocities and the runs within ±1σ from mid values. (b) Typical curves of separation S(t). (c, d) The cumulative number of runs vs. the primordial separation below 15 pc for 40 and 50 Myr. The dashed lines show all runs within 1σ-rectangle, and the solid lines show only runs with negative energy E_K (bound). Colours of the lines code the total mass M of the clusters.