Phase diagram of a two-dimensional fluid with internal quantum states

A model molecular fluid with two-dimensional classical degrees of freedom and two internal quantum states is studied by path integral Monte Carlo simulations. The temperature-density phase diagram is obtained to a high degree of precision using finite-size scaling techniques and local structure analysis methods. We locate the phase boundaries of gas, liquid, square and hexagonal lattice solid phases together with a preferred internal quantum state resulting in tricritical and triple points. The simulation results for the liquid-solid transitions are compared with recent density functional theory calculations.