MONTE-CARLO SIMULATION OF CRYSTALLINE POLYETHYLENE

We consider here the problem of constructing an efficient algorithm fo r a classical Monte Carlo simulation of crystalline polyethylene with unconstrained bond lengths and angles. This macromolecular crystal, pr esents a particular example of a system with many different energy sca les, ranging from soft ones represented by nonbonded van der Waals int eractions, to stiff ones, represented in particular by bond stretching . A proper sampling of all the energy scales poses a problem and it is shown that a standard Metropolis algorithm employing just local moves is not very efficient at low temperatures. As a solution it is propos ed to employ also global moves consisting of displacements of the cent er of mass of the whole chains in all three spatial directions as well as rotations of the chains around an axis parallel to the It-axis, wh ich act on the degrees of freedom associated with the chain packing. I t is shown that by properly alternating such global moves with standar d local moves, the statistical inefficiency of the algorithm is consid erably reduced.