Sketch of the code coupling. The code runs in blocks, starting at block 1. For illustration purposes the total number of blocks is 12 in this sketch; in reality, there are many more blocks in one simulation. After some planetesimals have formed during the first run of the Lagrangian dust evolution code (top row), we switch to the N-body code and inject the formed planetesimals (middle row). After a fixed time interval we switch back to the Lagrangian dust code and update the value for the pebble accretion efficiency as was output by the N-body code. This “zig-zag” behaviour continues until all planetesimals have left the N-body code, either because they have been accreted by bigger objects, or because they have grown into bigger objects and have migrated across the inner radius of the N-body domain r_domain (see text formore details). The final stage of our calculation concerns the growth of the protoplanets that have crossed r_domain, and is discussed in Sect. 2.5.