The growth direction, diameter, and surface structure of semiconductor nanowires can be controlled by varying process conditions and metal catalysts. The ability to manipulate their structural properties on the atomic scale makes semiconductor nanowires attractive for a variety of noble applications in electronics, optoelectronics, and sensors. Previous studies have suggested mechanisms underlying the metal catalyzed growth of Si nanowires, involving Si diffusion into a metal catalyst, eutectic Si-catalyst alloy formation, and Si precipitation at the catalyst-nanowire interface. However, many fundamental aspects regarding how synthesis conditions influence the nanowire growth and structure are still uncertain. In this presentation, based on first principles-based atomistic modeling we will focus on addressing the formation of silicon precipitates in a gold particle and the dependence of nanowire growth direction on the gold particle size. Our simulation results clearly demonstrate that silicon atoms preferentially remain near the gold particle surface, indicating that silicon clusters may form and grow in the surface region. The underlying reasons for the surface segregation of silicon will be discussed. We have also determined the interface structure and energetics between gold and silicon with different orientations [(111), (110), (211), and (100)]. The results provide good insight into the preferred growth of silicon nanowires in the (111) direction. We will also discuss mechanisms underlying the change of growth direction from the (111) to (110) with decreasing the gold particle size.