NN lattice-gas energy is $\epsilon_1\! +\! 2\epsilon_{RT}$ and, more significantly the effective NNN interaction energy is $\epsilon_2\! +\! \epsilon_{RT}$. Thus, $\epsilon_{RT}$ must be attractive (negative) if it is to help account for the discrepancy in Fig. 2 of Ref.  between the model and experiment. Furthermore, by revisiting the configurations discussed in the penultimate paragraph of the Introduction, we find that the kink energy $\varepsilon_k$ becomes $-\scriptstyle{\frac{1}{2}}\textstyle{\epsilon_1} -\! \epsilon_{RT}$. Thus, for a repulsive $\epsilon_{RT}$, $|\epsilon_1|$ will be larger than predicted by an analysis of, e.g., step-edge diffusivity that neglects $\epsilon_{RT}$. Lastly, the close-packed edge energy, i.e. the $T\! =\! 0$ line tension $\beta(0) \! =\! -\scriptstyle{\frac{1}{2}}\textstyle{\epsilon_1}  -\! \epsilon_2$, becomes $-\scriptstyle{\frac{1}{2}}\textstyle{\epsilon_1} - \! \epsilon_2 -\! 2\epsilon_{RT}$

Discussion and Conclusions
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We now turn to experimental information about the interactions, followed by comments on the limited available calculations of them, often recapitulating the discussion in Ref. . All the experiments are predicated on the belief that at 320K there is sufficient mobility to allow equilibrium to be achieved. If the NNN interactions are to explain at least partially the high stiffness of experiment compared to Ising theory, the NNN interaction must be attractive and a substantial fraction of $\epsilon_1$. Since compact islands do 