The helicity of stable single helical carbon chains and iodine chains inside single-walled carbon nanotubes(SWCNTs)is studied by calculating the systematic van der Waals interaction energy.The results show that the optimal helical radius increases linearly with increasing tube radius,which produces a constant separation between the chain structure and the tube wall.The helical angle exhibits a ladder-like decrease with increasing tube radius,indicating that a large tube can produce a small helicity in the helical structures. The helicity of stable single helical carbon chains and iodine chains is studied by calculating the systematic van der Waals interaction energy. The results show that the optimal helical radius increases linearly with increasing tube radius, which produces a constant separation between the chain structure and the tube wall. helical angle exhibits a ladder-like decrease with increasing tube radius, indicating that a large tube can produce a small helicity in the helical structures.