  We present a thermodynamic model which explains the presence of a negative
slope in the melt curve, as observed in systems as diverse as the alkali metals
and molecular hydrogen at high pressure. We assume that components of the
system can be in one of two well defined states - one associated with low
energy, the other with low volume.
  The model exhibits a number of measurable features which are also observed in
these systems and are therefore expected to be associated with all negative
Clapeyron-slope systems: first order phase transitions, thermodynamic anomalies
along Widom lines.
  The melt curve maximum is a feature of the model, but appears well below the
pressures where the change in state occurs in the solid: the solid-solid
transition is related to the melt line minimum. An example of the model fitted
to the electride transition in potassium is discussed.
