Abstract: The distance between non-bonded atom pairs have been calculated by use of analytical projective geometry method. The distance of non-bonded atom pairs attached to the adjacent molecular chain atoms, rab(φ_i), is given asThe distance between non-bonded atom pairs attached to the non-adjacent molecular chain atoms, ra b(φ_i,φ_i+1) is given asThe method suggested by the present author is simple but valid only for those chain molecules without double bonds.