PNNL

Abstract

The treatment of dissociative states in the calculation of the partition function of a weakly bound system, such as the water dimer, is discussed. For a dissociative system, the number of phase-space configurations that contribute to the total partition function from energies above the dissociation energy depend upon the system volume. For a sufficiently large system volume, entropy from these confiurations will dominate over the energy contribution of the local minimum and contributions from dissociative states will dominate the total partition function. The calculation of the dimer partition function requires limiting the phase space of the cluster or providing a definition of those phase space points that correspond to a dimer. Since there is no unique procedure to constrain the phase space of a dimer, we provide an analysis of the dimer partition function using different constraints. For the water dimer at temperatures in the range 200-500 K, the value of the dimer partition function changes by up to 3 orders of magnitude with variations of the constraint.