derivative decreases in a linear fashion with increasing β , except for a small roundoff region that we interpret as a finite-size effect. We then estimate β$_{c}$ as the point where dJ/dβ falls to zero, using the data in the linear region. The procedure for fixed β is analogous. The continuous transition points obtained by this procedure are quite robust with respect to changes in the region analyzed, as long as we exclude the roundoff region. Using the methods described above we construct a phase diagram based on the data for each system size studied; that for L = 500 is shown in Fig. 9. The vertical boundary at α = 0 . 905(2), and the horizontal boundary at β = 0 . 714(1) are obtained using the derivative method. (The polynomial method yields α = 0 . 8(1) and β = 0 . 67(5), respectively, for these boundaries.) The grey triangle in Fig. 9 represents a region on which we could not determine precisely the phase boundaries using either method, due to numerical uncertainty and finite-size effects. While the simplest interpretation is that the three phase boundaries meet at the point of intersection between the continuous transitions (i.e., horizontal and vertical) lines, the data in 16 FIG. 8: Current derivative for α = 2 . 0 and L = 200. 0 0.5 1 1.5 0.05 0.1 0.15 dJ/d