Quantum and classical equilibrium properties for exactly solvable models of weakly interacting systems

Abstract

We investigate the importance of quantum-mechanical effects for equilibrium thermodynamics and the structure of weakly interacting systems. Two inclusion complexes with soft guest–host interaction potentials (endohedral ³He buckminsterfullerene, ³He@C₆₀, and H₂ molecule inside (H₂O)₂₀ cage) are examined by solving the Schrödinger equation for the nuclear motion of the guest. We demonstrate that quantum corrections are highly sensitive to the shape of the interaction potential. Anharmonic potential energy surfaces, exhibiting multiple, degenerate minima, magnify quantum contributions. Commonly used harmonic corrections are therefore unreliable for soft interaction potentials. We also show that quantum corrections to equilibrium constants and thermally averaged structural parameters may become significant at temperatures close to ambient. In the recently discovered hydrogen clathrate hydrate, quantum effects likely result in a ∼45 K decrease of the decomposition temperature at atmospheric pressure.