The twinning formation of InP nanowires (NWs) is systematically investigated based on our empirical potential, which incorporates electrostatic energy due to valence bond and ionic charges. The calculated energy difference between zinc blende and wurtzite structures shows that the stable crystal structure depends on the NW diameter. The Monte-Carlo calculations to determine the NW stacking sequence also show that the nuclei formation at the top layer of growing NW crucially affects the twinning formation: The averaged periodicity in the nucleation model in which the nucleus consists of facet atoms at the top layer of NW is almost proportional to the NW size, while that in the nucleation model consisting of a small number of atoms is independent of the NW size. These results are consistent with the observation of rotational twins in the NWs grown by the vapor-liquid-solid mechanism and by the selective-area growth, implying that our approach is feasible to clarify the structural trends of InP NWs. (c) 2007 Elsevier B.V. All rights reserved.