2-DIMENSIONAL MELTING REVISITED - MOLECULAR-DYNAMICS SIMULATIONS INITIATED WITH OPTICAL MICROSCOPY DATA

The two-dimensional melting behaviour of the inverse-twelfth-power sys tem and its dependence on boundary and initial conditions has been re- examined with long molecular dynamics simulation runs. The system cons ists of 1225 particles, which in a first series of runs is being simul ated under standard periodic boundary conditions. It is feasible to re concile some central simulation results with the experimental data of Murray et al., 1987, Phys. Rev. Lett., 58, 1200 for possible two-stage melting of submicron-sized charged colloidal spheres in liquid water. In another series of simulations, the particles are confined to a rou ghly rectangular cell with irregularly shaped walls. The energy as a f unction of density is found to be very sensitive to the choice of boun dary conditions. The dependence on the initial conditions is studied b y setting up the latter simulations with different sets of starting co ordinates-some of them probably those of a hexatic phase-which were di rectly obtained from Murray et al.'s experimental data for colloidal s uspensions. Using the short ranged r-12 potential, molecular dynamics simulations of these configurations were examined over five million ti me steps each. The initial order, even if experimentally found to be c onsistent with that of a hexatic phase, is not seen to undergo any rap id breakup. This suggests that kinetic bottlenecks limiting equilibrat ion are still important, even for runs of this length.