We derive exact finite-size corrections for the free energy $F$ of the Ising model on the ${ \cal M} \times 2 { \cal N}$ square lattice with Brascamp-Kunz boundary conditions. We calculate ratios $r_p(\rho)$ of $p$th coefficients of F for the infinitely long cylinder (${ \cal M} \to \infty$) and the infinitely long Brascamp-Kunz strip (${ \cal N} \to \infty$) at varying values of the aspect ratio $\rho={ (\cal M}+1) / 2{ \cal N}$. Like previous studies have shown for the two-dimensional dimer model, the limiting values $p \to \infty$ of $r_p(\rho)$ exhibit abrupt anomalous behaviour at certain values of $\rho$. These critical values of $\rho$ and the limiting values of the finite-size-expansion-coefficient ratios differ, however, between the two models.
Categories: cond-mat.stat-mech