Crystal Polymorphism in Oxalyl Dihydrazide: Is Empirical DFT‑D Accurate Enough?

Crystalline oxalyl dihydrazide has five experimentally known polymorphs whose energetics are governed by subtle balances between intra- and intermolecular interactions, providing a severe challenge for theoretical crystal structure modeling. Previous work has shown that many common density functional methods that neglect van der Waals dispersion cannot correctly describe this system, but it has been argued that empirically dispersion-corrected DFT-D performs much better. Here, we examine these crystals with second-order Møller–Plesset perturbation theory (MP2) and related levels of theory using the fragment-based hybrid many-body interaction method. The energetics prove sensitive to the treatment of electron–electron correlation, the basis set, many-body induction, three-body dispersion, and zero-point contributions. Nevertheless, our best predictions for the polymorph energy ordering based on dispersion-corrected MP2C calculations agree with the available experimental data. In contrast, lower levels of theory, including the common B3LYP-D* and D-PW91 dispersion-corrected density functional approximations, fail to reproduce experimental observations and/or the high-level calculations.