Multimer Embedding Methods: Molecular Crystals and Beyond

In recent years, advances in method developments have been leading to increasingly reliable predictions of molecular crystals. This has been highlighted by the impressive results in the prediction of possible crystal structures for organic molecules and also organic salts. Much of this progress can be attributed to the increased application of density functional theory including dispersion models using periodic boundary conditions.

We assess the available computational approaches for the revised X23b benchmark set of molecular crystals using thermally-expanded structures which were calculated via the quasi-harmonic approximation. The methods evaluated include density functionals in combination with dispersion models, as well as our own methods which are based on multimer embedding schemes. Utilizing the latter approach, we are able to approximate periodic results of hybrid functionals at a fraction of the computational time needed for the canonical methods. These methods can be evaluated not only for energies, but also for geometries, cell volumes, and vibrational properties. The convergence of the embedding scheme with the multimer order (monomer, dimer, trimer) and distances are discussed, whereas trimer interactions are of surprising importance.

Finally, we discuss the applicability including the success of the embedding scheme at the Cambridge Crystallographic Data Centre 7^th crystal structure prediction blind test of organic molecules.