Probing the Accuracy of First-Principles Modeling of Molecular Crystals: Calculation of Sublimation Pressures

Abstract

An insight into current possibilities of obtaining the sublimation pressures for molecular crystals from first principles is presented. Due to their extreme sensitivity to any computational uncertainties, sublimation pressures are the strictest possible representation of first-principles data on the cohesive properties of molecular crystals, emphasizing the significance of any computational uncertainties of cohesive energies, sublimation enthalpies, or sublimation entropies which might seem acceptable from a purely energetic point of view. The sublimation pressure was computed for 20 selected molecular crystals by combining the calculated static cohesive energy, vibrational contributions to thermodynamic properties in the crystalline phase, and ideal-gas thermodynamic properties required to obtain the sublimation enthalpy and entropy as a function of temperature. The calculated sublimation pressures were compared to reference experimentally based values developed in this work. By an analysis of the uncertainties on the basis of a comparison to experimental sublimation pressures and both enthalpic and entropic contributions, the uncertainty limits for prediction of sublimation pressure based on first-principles approaches are discussed and estimated. As the sublimation pressure depends exponentially on both enthalpic and entropic contributions, the current accuracy of first-principles calculations allows its prediction typically within a factor of 10. This can still be viewed as a success, given typical uncertainties in experimentally determined sublimation thermodynamic properties, especially when extremely low volatility compounds are considered.