Energetic particle irradiation of solids can cause surface ultra-smoothening , self-organized nanoscale pattern formation, or degradation of the structural integrity of nuclear reactor components. Periodic patterns including high-aspect ratio quantum dots have stimulated interest in this method as a means of sub-lithographic nanofabrication. However, despite much research, there is little fundamental understanding of the mechanisms governing the selection of smooth or patterned surfaces, and precisely which physical effects cause observed transtions between different regimes has remained a matter of speculation.

Here, using only parameter-free molecular dynamics simulations of single-ion impacts as input into a multi-scale analysis, we report the first ab-initio prediction of the mechanism governing transition from corrugated surfaces to flatness, obtaining good agreement with experimental observations. In the process, we show that -- contrary to previous assumptions -- these phenomena are dominated by the impact-induced
redistribution of target atoms that are not sputtered away, and that
erosive effects are essentially irrelevant. The predictions are of
particular relevance in the context of tungsten plasma-facing fusion
reactor walls which, despite a sputter erosion rate that is essentially
zero, develop a mysterious nanoscale topography leading to surface degradation.  Our results suggest that if redistribution dominates stability selection, then an extremely low sputter erosion rate is an insufficient design criterion for such materials.