4 research outputs found

    Accidental Degeneracy in Crystalline Aspirin: New Insights from High-Level ab Initio Calculations

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    We perform the first high-level <i>ab initio</i> calculations (MP2) on crystalline aspirin using a newly developed fragment-based QM/MM method. Contrary to earlier density functional theory predictions, the two polymorphs are virtually degenerate, which is consistent with experimentally observed intergrowth structures. This near-degeneracy arises “accidentally” from a competition between intramolecular relaxation (form I) and intermolecular hydrogen bonding (form II)

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

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    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

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

    No full text
    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

    The Impact of Carrier Delocalization and Interfacial Electric Field Fluctuations on Organic Photovoltaics

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    Organic photovoltaic (OPV) devices hold a great deal of promise for the emerging solar market. However, to unlock this promise, it is necessary to understand how OPV devices generate free charges. Here, we analyze the energetics and charge delocalization of the interfacial charges in poly­(<i>p</i>-phenylenevinylene) (PPV)/[6,6]-phenyl-C<sub>61</sub>-butyric acid methyl ester (PCBM) and poly­(3-hexylthiophene-2,5-diyl) (P3HT)/PCBM devices. We find that, in the PPV system, the interface does not produce molecular disorder, but an interfacial electric field is formed upon the inclusion of environmental polarization that promotes charge separation. In contrast, the P3HT system shows a significant driving force for charge separation due to interfacial disorder confining the hole. However, this feature is overpowered by the polarization of the electronic environment, which generates a field that inhibits charge separation. In the two systems studied herein, electrostatic effects dominate charge separation, overpowering interfacially induced disorder. This suggests that, when balancing polymeric order with electrostratic effects, the latter should take priority
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