In statistical mechanics of continuous systems, a potential for a many-body system is called H-stable (or simply stable) if the potential energy per particle is bounded below by a constant that is independent of the total number of particles.
In many circumstances, if a potential is not H-stable, it is not possible to define a grand canonical partition function in finite volume, because of catastrophic configurations with infinite particles located in a finite space.
Consider a system of particles in positions
; the interaction or potential between a particle in position
is a real, even (possibly unbounded) function.
, The notion of H-stability in quantum mechanics is more subtle.
While in the classical case the kinetic part of the Hamiltonian is not important as it can be zero independently of the position of the particles, in the quantum case the kinetic term plays an important role in the lower bound for the total energy because of the uncertainty principle.
(In fact, stability of matter was the historical reason for introducing such a principle in mechanics.)
The definition of stability is : where E0 is the ground state energy.
The criterion is especially useful in statistical mechanics, where H-stability is necessary to the existence of thermodynamics, i.e. if a system is not H-stable, the thermodynamic limit does not exist.