Change of semi-major axis relative to the initial planetary positions for planetary cores with initially 20 Earth masses growing by gas accretion until they reach M_core = M_env, defined as the onset of runaway gas accretion. Top panel: change of semi-major axis for the different grain size distributions and bottom panel: change of semi-major axis for different chemical assumptions, where all simulations in the bottom plot feature α = 0.001 and u_f = 1 m s⁻¹. A value of (r₀ − r_f)∕r₀ = 0.0 implies thatthe planet does not migrate significantly while it is accreting gas. The orange line marks the value if the planet reaches the inner edge of the disc at 0.1 AU.