Understanding the Role of Dispersion Interactions in the Mechanical Properties of Molecular Crystals

• Applicant: Professor Dr. Alexandre Tkatchenko
• Subject Area: Theoretical Chemistry: Molecules, Materials, Surfaces ; Theoretical Chemistry: Electronic Structure, Dynamics, Simulation ; Theoretical Condensed Matter Physics
• Term: from 2015 to 2018
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 271390424

• Final Report Year: 2018

Final Report Abstract

This project focused on understanding and highlighting the importance of van der Waals (vdW) dispersion interactions for molecular crystals. Molecular crystals are solids composed of molecular moieties, which are held together by non-covalent interactions and are used for instance as pharmaceuticals or organic semiconductors. An understanding of the structures, stabilities, and response properties of molecular crystals would provide important insight for crystal engineering and drug design. Many properties of molecular crystals—including mechanical properties— are highly structure and temperature dependent, which makes the modeling of these properties rather challenging. Accurate lattice energies of molecular crystals can be obtained by utilizing vdW-inclusive density functional theory (DFT), especially when paired with a sophisticated model for dispersion interactions such as the many-body dispersion (MBD) model. We have shown for an ammonia crystal that accounting for long-range dispersion interactions is crucial for the description of the bulk modulus. In order to compare our calculations to experimental values at a temperature of 194 K, we have considered the thermal expansion of the crystal within the quasi-harmonic approximation. In this case the bulk modulus calculated with a DFT+MBD approach agrees within 1.5 % with the experimental value. In contrast, a DFT approach lacking long-range dispersion interactions leads to an error of about 66 %. Furthermore, dispersion interactions also improve the description of the anisotropy of the elastic constants. Since response properties of molecular crystals can also highly depend on the crystal-packing arrangement of the involved molecules (polymorphs), an accurate description of the relative polymorphs stabilities is crucial. Therefore, we have developed a hierarchical approach based on vdW-inclusive DFT intended for the final stages of a crystal structure prediction procedure. With this approach we are able to obtain excellent relative stability rankings for a quite diverse set of molecular crystals. We show that many-body dispersion interactions and vibrational free energies are crucial for obtaining accurate results. However, computationally cheaper but still accurate methods would be needed to make such an approach broadly applicable. Therefore, we have combined the atomic-pairwise Tkatchenko-Scheffler dispersion model as well as the MBD model with the density functional tight binding approach, which is computationally much more efficient than DFT. We have shown that such an approach provides an overall good description of molecular crystal structures and could therefore be utilized for a structural pre-screening. A proper re-parametrization of the used parameter sets in the future could potentially enable accurate calculations of mechanical and vibrational properties using tight binding.

Publications

• Acta Crystallogr. Sect. B: Struct. Sci. 72, 439 (2016)
  A. M. Reilly, ..., C. R. Groom
  
• Wiley Interdiscip. Rev. Comput. Mol. Sci. 7, 1294 (2017)
  J. Hoja, A. M. Reilly, and A. Tkatchenko
  
• Faraday Discuss. 211, 253 (2018)
  J. Hoja and A. Tkatchenko
  
• Phys. Chem. Lett. 9, 399 (2018)
  M. Mortazavi, J. G. Brandenburg, R. J. Maurer, and A. Tkatchenko, J.
  
• Sci. Adv. 5, eaau3338 (2019)
  J. Hoja, H.-Y. Ko, M. A. Neumann, R. Car, R. A. DiStasio Jr., and A. Tkatchenko