The model of melamine molecules ordering into planar honeycomb and closed packed phases is proposed. To account for the "side-to-side" melamine-melamine molecular interactions, we use the version of the antiferromagnetic Blume-Capel model with some exclusions. The model is solved by Monte Carlo calculations on a triangular lattice, a slightly rescaled version of Au(111) and Ag(111) lattices on which the main experimental data are obtained. The ordered phases are formed when mutual distance between the centers of molecules is within sixth and seventh nearest neighbor distances of rescaled substrate lattice. We obtain the ground state phase diagram with honeycomb and three closed-packed phases and density-temperature phase diagram with three pure phases (gas, honeycomb, and close-packed) and their two-phase coexistences.
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