Computer Simulation for the Melting Curve Maximum Phenomenon: Two-Species Soft-Core Model

Computer simulations by a Monte Carlo method are carried out for the two-species soft-core model to demonstrate an example of the melting curve maximum phenomena observed in many substances. In this model, atoms are assumed to have one excited internal level in which they have an effective radius smaller than that in the ground level. The atoms interact with each other through a soft-core potential with the inverse power 12. The effective radius of the soft core depends on the radii of relevant atoms. The density dependences of pressure and of free energy are obtained for the fluid and the single occupancy branches, respectively. The phase diagram in the temperature vs pressure plane is derived, showing a melting curve maximum, at which the melting transition occurs accompanying an entropy discontinuity without any volume change. The entropy discontinuity at melting is found to increase with increasing pressure around the melting curve maximum point. The obtained radial distribution functions have neither an extra shoulder nor a subsidiary peak even in the mixing region of the two species.