Two-state model for critical points and the negative slope of the melting curve

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