Chemical, jump, and tracer diffusion coefficients D, D(j), and D*, respectively, have been determined by Monte Carlo simulations on a square lattice for the case of nearest-neighbor attractive interactions. D was determined by the fluctuation autocorrelation method and by the Kubo-Green equation, using values of <(delta-N)2>/<N> determined from isotherms. <(delta-N)2>/<N> can only be determined above T(c) and, for this situation, the two methods yield identical D values within the error of the simulations. Distinct high- and low-T regimes for D are observed, because of the changes in <(delta-N)2)/<N> with T, with much higher activation energies and prefactors for T/T(c) < 1.4 than for T/T(c) > 1.4. For D* and D(j) much smaller effects are seen. The numerical values of activation energies suggest that diffusing atoms are either isolated atoms in the dilute lattice gas or those which become detached from corners of close-packed (1 X 1) islands below T(c), but have a higher nearest-neighbor complement in the dense lattice gas above T(c).
