Dispersion Interactions in High-Density Molecular Crystals

Dispersion interactions are ubiquitous quantum mechanical phenomena arising from correlated electron density fluctuations in molecules and materials. As a key component of non-bonded interactions, dispersion forces play a critical role in determining the structure and stability of molecular crystals. Due to the relative intermolecular separation in high-density molecular crystals, an accurate description of these non-bonded interactions requires the inclusion of terms beyond the asymptotic induced-dipole--induced-dipole ($C_6/R^6$) contribution. In this work, we have developed a first principles based approach within the framework of Density Functional Theory (i.e., that only depends on the charge density $n(\mathbf{r})$) for capturing the higher-order induced multipolar contributions to the correlation energy. As a first application of this method, we have investigated the structure and stability of the high-density ice molecular crystal polymorphs at the ice VI---ice VII---ice VIII triple point (278K, 2.1GPa) using \textit{ab-initio} molecular dynamics in the isobaric-isothermal ($NpT$) ensemble.