An electrostatic correction for improved crystal density predictions of energetic ionic compounds

Abstract

In recent years, there has been considerable interest in predicting the crystal densities of both molecular and ionic energetic compounds using the computed volumes Vm of the isolated gas phase molecules or ions. The surfaces enclosing the volumes are taken to be the 0.001 au (electrons/bohr3) contours of the molecules’ and ions’ electronic densities. For molecular solids, it is known that the ratio M/Vm (M = molecular mass) gives densities that are overall reasonably good, although they can be markedly improved by introduction of an electrostatic interaction correction term. For ionic solids, the subject of this paper, the ratio M/Vm (M = formula unit mass) is not nearly as effective; Vm tends to be significantly larger than the effective volumes of the ions in the crystal, leading to underestimated densities, with an average absolute error of 0.089 g/cm3. The correction term that improves molecular crystal densities is not applicable in the case of ionic solids; however we show, for a database of 25 compounds plus five test cases, that an average absolute error of 0.033 g/cm3 can be achieved by combining M/Vm with terms involving the average positive and negative potentials and areas on the cationic and anionic surfaces. The root-mean-square error is 0.040 g/cm3.