Empirical van der Waals corrections to solid-state density functional theory: Iodine and phosphorous containing molecular crystals.

Parameters are derived for a molecular mechanics type dispersive correction to solid-state density functional theory calculations on molecular crystals containing iodine and phosphorous. The molecular C(6) coefficients are derived from photoabsorption differential oscillator strength spectra determined from accurate (e,e) dipole spectra. The cross-over parameters, which ensure correct behavior at short internuclear distances, are obtained by fitting predicted crystal lattice parameters to experimental data. The accuracy of the parameterization is assessed by optimizing the experimental structures of several additional phosphorous and iodine containing molecular crystals and by examining the relative stabilities of the known polymorphs of phosphorous pentoxide and the stabilities of different packings of an iodine containing molecular crystal, 2,9-bis(iodo)anthanthrone, which has been the subject of a crystal structure prediction study. Optimizations of the experimental crystal structures did not lead to significant geometric deviations. The optimized experimental structure of 2,9-bis(iodo)anthanthrone is the lowest energy packing found, indicating a satisfactory description of both energy and structure for these molecular crystals.