The Critical Role of Anharmonicity in Aqueous Ionic Clusters Relevant to Nucleation

Abstract

Calculating accurate free energies of aqueous ionic clusters is important in many areas of chemical physics. A common practice used in ab initio and other methods to determine the cluster free energy is to use a single energetically favorable configuration and then assume the harmonic approximation to the potential energy surface. In the present study, we compare and contrast harmonic free energies to those determined from anharmonic configurational sampling and how both compare to experiment. It is found that anharmonicity plays a critical role in determining accurate free energies for the clusters underlying ion-induced nucleation. Consequently, energies, geometries, and frequencies obtained by using a single or multiple configurations within the harmonic approximation lead to inaccurate aqueous cluster thermodynamics.