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p-Diiodobenzene
We studied [1] the two polymorphs (α and β) of the molecular crystal of p-diiodobenzene (p-DIB), which is widely used in drugs, antiseptics and disinfectants. It also finds applications in many other fields, such as electronics, dyes and explosives.
Despite the fact that p-DIB represents a quite simple apolar organic system, its characterization, from the computational point of view, is nontrivial. This is due to the difficulty of common DFT techniques to recover the contribution of dispersive forces to the energy of the system, leading to a wrong description of the crystal structure. This lack affects severely the prediction and the study of all the other electronic properties that can be obtained from the total ground-state energy, e.g. the relative stability between the two polymorphs.
Our results (see figure - PBE-TS/D, PBE0-TS, B3LYP-TS/D) demonstrate that, when properly corrected, DFT calculations successfully predict the relative stability of the α and β phases at zero temperature, in good agreement with accurate Diffusion Monte-Carlo calculations (DMC) [2] which are, however, computationally very expensive. Moreover, we obtained also a good agreement between our fully optimized structures and the experimental ones.
Oxalyl Dihydrazide
This work [3] regards the study of the molecular crystal of oxalyl-dihydrazide (ODH), a compound that is routinely employed in chemical and pharmaceutical industry. The solid ODH differentiates into five polymorphs (α, β, γ, δ, ε) that are governed by inter- and intra- molecular hydrogen bonds. The complex mixture of such interactions with long range dispersive forces makes its computational characterization very challenging. Indeed, a complete experimental energetic profile of this system is still lacking, and it is was investigated by means of periodic dispersion-corrected DFT and local second order Møller–Plesset Perturbation theory (LMP2) calculations.
The empirical dispersion correction schemes proposed by Tkatchenko and Scheffler (TS) [4] and Grimme (D2) [5] have been used in combination with the PBE 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.
PBE-TS, B3LYP-D2 and B3LYP-D3(BJ)+E(3) achieve good predictions of the stability ordering, though the broadness of the energy range is slightly larger than in the case of LMP2. The comparison with previous HMBI [6] calculations and with fully periodic LMP2 [7] calculations reported in this work for the first time, shows that a stability ordering can be defined, even though it spans a larger range of energies.
Structure-dependent dispersion-correction schemes – such as the TS and D3 – lead to results that are reliable, even if relative stabilities of the polymorphs are slightly overestimated with respect to LMP2 results.
The B3LYP-D3(BJ)+E(3)/QZVP methods is promising even if the β polymorph is predicted to be too unstable compared with the LMP2 relative stability. On the contrary, the gCP correction [8], designed to remove the BSSE and devised to compute BSSE-corrected interaction energies, appears to be not suitable for the evaluation of the relative stability of ODH polymorphs. In spite of that, both D3 and gCP deserve to be further investigated in the prediction of molecular crystals polymorphism. The predicted energy ranking that can be deduced from thedifferent methods adopted in the present work appears to be: α > ε > γ ~ δ > β .
References:
[1]: Pedone, A.; Presti, D.; Menziani, M. C., Chem. Phys. Lett. 2012, 541, 12-15.
[2]: K. Hongo, M.A. Watson, R.S. Sànchez-Carrera, T. Iitaka, A. Aspuru-Guzik, J. Phys. Chem. Lett. 2010, 1, 1789.
[3]: D. Presti, A. Pedone, M. C. Menziani, B. Civalleri and L. Maschio, CrystEngComm 2013, 16, 102-109.
[4]: Tkatchenko, A.; Scheffler, M. Phys. Rev. Lett. 2009, 102, 073005.
[5]: Grimme, S. J. Comput. Chem. 2006, 27, 1787-1799.
[6]: S. Wen and G. J. O. Beran, J. Chem. Theory Comput., 2012, 8, 2698-2705.
[7]: a) C. Pisani, L. Maschio, S. Casassa, M. Halo, M. Schütz and D. Usvyat, J. Comput. Chem., 2008, 29, 2113, http://www.cryscor.unito.it ; b) C. Pisani, M. Schütz, S. Casassa, D. Usvyat, L. Maschio, M. Lorenz and A. Erba, Phys. Chem. Chem. Phys., 2012, 14, 7615-7628. ; c) L. Maschio, J. Chem. Theory Comput., 2011, 7, 2818-2830.
[8]: H. Kruse and S. Grimme, J. Chem. Phys., 2012, 136, 154101.
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