We conjecture an approximate expression for the free energy in the thermodynamic limit of the classical square lattice Ising model in a uniform (real) magnetic field. The zero-field result is well known due to Onsager for more than eighty years, but no such result exists for a nonzero magnetic field on a regular lattice. We verify our conjecture using numerical tensor renormalization group (TRG) methods and find good agreement with a maximum deviation of $\sim2\%$ from the numerical results for the free energy across all $\beta$ and real magnetic field, $h$.
Categories: hep-lat cond-mat.stat-mech hep-th