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The molecular crystal of oxalyl dihydrazide differentiates into five polymorphs that are governed by inter- and intramolecular hydrogen bonds. The complex mixture of such interactions with long range dispersive forces makes its computational characterization very challenging; thus it represents an ideal benchmark for ab initio methods when striving for a description of polymorphism in molecular crystals. Indeed, a complete experimental energetic profile of this system is still lacking, and it is here investigated by means of periodic dispersion-corrected DFT and Local second order Møller–Plesset Perturbation theory (LMP2) calculations. In this work, the empirical dispersion correction schemes proposed by Tkatchenko and Scheffler (TS) [Tkatchenko et al., Phys. Rev. Lett., 2009, 102, 073005] and Grimme (D2) [Grimme, J. Comput. Chem., 2006, 27, 1787] have been used in combination with the PBE semilocal functional for geometry optimizations. We observed that PBE-TS provides a remarkable improvement in predicting the crystal structure of oxalyl dihydrazide polymorphs with respect to commonly used DFT-D functionals. The relative stabilities of the five forms have then been computed at the PBE-TS/D2, PBE0-D2, B3LYP-D2 and B3LYP-D3(BJ)+gCP level on the PBE-TS hydrogen-optimized geometries and benchmarked against high level periodic LMP2 calculations. PBE-TS, B3LYP-D2 and B3LYP-D3(BJ)+E(3) (that is including three-body corrections) achieve good predictions of the stability ordering, though the broadness of the energy range is slightly larger than in the case of LMP2

Oxalyl dihydrazide polymorphism: a periodic dispersion-corrected DFT and MP2 investigation