The hard hexagon model can be defined similarly on the square and honeycomb lattices. No exact solution is known for either of these models, but the critical point z c is near 3.7962 ± 0.0001 for the square lattice and 7.92 ± 0.08 for the honeycomb lattice; κ (1) is approximately 1.503048082... (OEIS : A085850) for the square lattice and 1.546440708... for the honeycomb lattice (Baxter 1999).