Reliable prediction of properties of polymorphic molecular crystals, in particular of organic compounds, has been an unsolved problem for a long time. While the methodology of crystal structure predictions (CSPs) improved substantially in the past few years, the reliability is still low. One of the main reasons is that empirical force fields (EFFs) are mostly used during the initial stages of ranking hundreds of thousands of plausible crystal structures. Such force fields are known to be valid only for systems and conditions close to used in fitting them. Force fields developed by fitting to results of ab initio calculations (AIFFs) do not suffer of such problems. However, since AIFFs use a more involved functional form than EFFs, they could not be used in early stages of CSPs since the CSP codes accepted only very simple functional forms. We have extended one of such codes and are now able to perform with essentially negligible time the T = 0 K crystal lattice optimizations of tens of thousands of crystal structures with AIFFs fitted to interaction energies calculated using symmetry-adapted perturbation theory (SAPT). On all crystals tested so far, this approach ranks the experimental polymorph in the top 5 consistently. Thus, it appears to be the first approach ever that reaches such a high scoring efficiency. With the search set limited to only 5 polymorphs, other methods such molecular dynamics with polarizable force fields or periodic DFT+D can be used to perform calculations on these 5 polymorphs, resulting in the experimental polymorph ranked as number 1. Another application of SAPT force fields which will be discussed are predictions of structure and energetics of hydrates compared to hydrogen peroxide solvates. This work demonstrated that it is possible to replace water by hydrogen peroxide in most hydrates, leading to crystals that are denser and thus have better detonation properties.

Efficient and reliable crystal structure predictions using first-principles force fields from symmetry-adapted perturbation theory

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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