If h = 0, it is very easy to obtain the free energy in the case of free boundary condition, i.e. when H ( σ σ ) = − − J ( σ σ 1 σ σ 2 + ⋯ ⋯ + σ σ L − − 1 σ σ L ) . Then the model factorizes under the change of variables σ σ j ′ = σ σ j σ σ j − − 1 , j ≥ ≥ 2.