Molecular dynamics simulations of model polyethylene chains were performed. The time evolutions of chain conformation for a range of molecular weights are studied.The process of chain relaxation and collapse includes many stages. Local clusters formed and were relatively stable for a long time

in addition

the local order structure still exists in the last compact coil. The results verified that the modification of force field for demonstration of stages in the folding process is not necessary. When the number of chain units is less than 1000 CH2 units

the local collapse stage is difficult to be observed

just as the results reported by Kavassalis et al..Nevertheless

as the chain became longer

the stage of collapse is clearly characterized by the average number of clusters. The following feature are found:first

the transition between different stages of chain folding became sharper as the chain becme olnger; second

the clusters inclused the local order structure became more stable as the chain became olnger. Based on our analysis

local collapse stages can be characterized by the number of clusters. The cluster structure was the essential feature of the local collapse

and such structure will be maintained in the following collapse structure. There is an interesting increase of the torsion energy for system during the coalescence of local clusters in the relatively long chains. The increase of the torsion energy may lead to the expansion of the radius of gyration.