Tricritical behaviour of the frustrated Ising antiferromagnet on the   honeycomb lattice
  We use the effective-field theory with correlations based on different cluster sizes to investigate phase diagrams of the frustrated Ising antiferromagnet on the honeycomb lattice with isotropic interactions of the strength $J_1 < 0$ between nearest-neighbour pairs and $J_2 < 0$ between next-nearest neighbour pairs of spins. We present results for the ground-state energy as a function of the frustration parameter $R = J_2 /|J_1|$. We find that the cluster-size has a considerable effect on the existence and location of a tricritical point in the phase diagram at which the phase transition changes from the second order to the first one. 
A. Bob\'ak, T. Lu\v{c}ivjansk\'y, M. \v{Z}ukovi\v{c}, M. Borovsk\'y,   T. Balcerzak
cond-mat.stat-mech