How a solid can be turned into a gas without passing through a first-order phase transformation

Recent calculations of the absolute free energy of a solid using molecular dynamics and Monte Carlo simulated data employ a ''thermodynamic'' path which is numerically indistinguishable from a path which connects a solid and an ideal gas. No first-order phase transition is observed on this path. We present an interpretation of this type of path, we identify a particular path along which the numerical uncertainty in calculating the free energy is minimized, and we report numerical studies on a Lennard-Jones system which support the surprising proposition that there is such a singularity-free path.