Top panels: element abundances over depth following from two different treatments of radiative acceleration by photoionization (T_eff = 54 000 K, log g = 8.2). Left: the whole photon momentum is transferred to the ion. Right: only a fraction is transferred, according to Eq. (3). Bottom panel: difference. The peaks correspond to a shift of the steep regions of the abundance pattern by one depth point in the model atmosphere.