Gravitational potential and acceleration. Left: non-axisymmetric component of the Milky Way external potential. Right: slices through the centre of the galaxy of the acceleration parallel to the Galactic plane in simulations F3000HD (left column) and MW3000HD (middle column) and their relative difference (right column) at t ≈ 2500 Myr. The first row corresponds to the contribution to the acceleration from the external potential, and the second is the contribution from the self-gravity of the gas and the newly created stars.