Negative heat capacity of small systems in the microcanonical ensemble

We propose a new mechanism to explain the origin of negative heat capacity in small systems. We show how a particular structure of the potential energy surface combined with a microcanonical treatment results in negative heat capacity. Moreover, this effect occurs under true equilibrium conditions. An energy landscape with a sudden and spatially large opening leads to a negative heat capacity. The magnitude of this effect is related to the extent of the opening and the number of particles in the system.