Solid-solid and solid-fluid equilibria of the most popular models of methanol obtained by computer simulation.

The ability of the most popular models of methanol (H1, OPLS, L2, and L1) for the prediction of the solid-solid and the solid-fluid equilibria was analyzed in detail in this work by using molecular simulation. The three solid phases (α, β, and γ) detected experimentally as being thermodynamically stable, as well as the fluid phase, were considered for the calculations. It turns out that all the models provide similar results. The α, γ, and fluid phases were found to be thermodynamically stable for a certain range of temperatures and pressures, whereas the β phase was always metastable. The coexistence curves (α-fluid, α-γ, γ-fluid) corresponding to all the models took the same shape except for some slight differences about their locations. From a qualitative point of view, it can be considered that the four models give a reasonable prediction of the phase diagram of methanol. However, there are important quantitative discrepancies. The melting points fell in the interval 214-223 K, whereas the γ phase was predicted to be stable at pressures above 12 × 10(4) bar. These results are quite different in relation to the experiments since the melting point of methanol is 175.6 K and the γ phase is stable at 3.5 × 10(4) bar at room temperature. In addition, the values of the melting enthalpy obtained by the different models are very similar but about 50% higher than the experimental value. Therefore, it is clear that there is room for improvement. Reducing the stability of the α phase with respect to the other phases seems to be a necessary condition to construct an improved potential.