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See detailFirst-principles modeling of molecular crystals: structures and stabilities, temperature and pressure
Hoja, Johannes UL; Reilly, Anthony M.; Tkatchenko, Alexandre UL

in Wiley Interdisciplinary Reviews: Computational Molecular Science (2016)

The understanding of the structure, stability, and response properties of molecular crystals at finite temperature and pressure is crucial for the field of crystal engineering and their application. For a ... [more ▼]

The understanding of the structure, stability, and response properties of molecular crystals at finite temperature and pressure is crucial for the field of crystal engineering and their application. For a long time, the field of crystal-structure prediction and modeling of molecular crystals has been dominated by classical mechanistic force-field methods. However, due to increasing computational power and the development of more sophisticated quantum-mechanical approximations, first-principles approaches based on density functional theory can now be applied to practically relevant molecular crystals. The broad transferability of first-principles methods is especially imperative for polymorphic molecular crystals. This review highlights the current status of modeling molecular crystals from first principles. We give an overview of current state-of-the-art approaches and discuss in detail the main challenges and necessary approximations. So far, the main focus in this field has been on calculating stabilities and structures without considering thermal contributions. We discuss techniques that allow one to include thermal effects at a first-principles level in the harmonic or quasi-harmonic approximation, and that are already applicable to realistic systems, or will be in the near future. Furthermore, this review also discusses how to calculate vibrational and elastic properties. Finally, we present a perspective on future uses of first-principles calculations for modeling molecular crystals and summarize the many remaining challenges in this field. [less ▲]

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See detailReport on the sixth blind test of organic crystal structure prediction methods
Reilly, Anthony M.; Cooper, Richard I.; Adjiman, Claire S. et al

in Acta Crystallographica Section B (2016), 72(4), 439--459

The sixth blind test of organic crystal structure prediction (CSP) methods has been held, with five target systems: a small nearly rigid molecule, a polymorphic former drug candidate, a chloride salt ... [more ▼]

The sixth blind test of organic crystal structure prediction (CSP) methods has been held, with five target systems: a small nearly rigid molecule, a polymorphic former drug candidate, a chloride salt hydrate, a co-crystal and a bulky flexible molecule. This blind test has seen substantial growth in the number of participants, with the broad range of prediction methods giving a unique insight into the state of the art in the field. Significant progress has been seen in treating flexible molecules, usage of hierarchical approaches to ranking structures, the application of density-functional approximations, and the establishment of new workflows and `best practices' for performing CSP calculations. All of the targets, apart from a single potentially disordered Z$^\prime$ = 2 polymorph of the drug candidate, were predicted by at least one submission. Despite many remaining challenges, it is clear that CSP methods are becoming more applicable to a wider range of real systems, including salts, hydrates and larger flexible molecules. The results also highlight the potential for CSP calculations to complement and augment experimental studies of organic solid forms. [less ▲]

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See detailvan der Waals dispersion interactions in molecular materials: beyond pairwise additivity
Reilly, Anthony M.; Tkatchenko, Alexandre UL

in Chemical Science (2015), 6(6), 3289-3301

van der Waals (vdW) dispersion interactions are a key ingredient in the structure, stability, and response properties of many molecular materials and essential for us to be able to understand and design ... [more ▼]

van der Waals (vdW) dispersion interactions are a key ingredient in the structure, stability, and response properties of many molecular materials and essential for us to be able to understand and design novel intricate molecular systems. Pairwise-additive models of vdW interactions are ubiquitous, but neglect their true quantum-mechanical many-body nature. In this perspective we focus on recent developments and applications of methods that can capture collective and many-body effects in vdW interactions. Highlighting a number of recent studies in this area, we demonstrate both the need for and usefulness of explicit many-body treatments for obtaining qualitative and quantitative accuracy for modelling molecular materials, with applications presented for small-molecule dimers, supramolecular host-guest complexes, and finally stability and polymorphism in molecular crystals. [less ▲]

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See detailLong-range correlation energy calculated from coupled atomic response functions
Ambrosetti, Alberto; Reilly, Anthony M.; DiStasio, Robert A. Jr et al

in JOURNAL OF CHEMICAL PHYSICS (2014), 140(18),

An accurate determination of the electron correlation energy is an essential prerequisite for describing the structure, stability, and function in a wide variety of systems. Therefore, the development of ... [more ▼]

An accurate determination of the electron correlation energy is an essential prerequisite for describing the structure, stability, and function in a wide variety of systems. Therefore, the development of efficient approaches for the calculation of the correlation energy (and hence the dispersion energy as well) is essential and such methods can be coupled with many density-functional approximations, local methods for the electron correlation energy, and even interatomic force fields. In this work, we build upon the previously developed many-body dispersion (MBD) framework, which is intimately linked to the random-phase approximation for the correlation energy. We separate the correlation energy into short-range contributions that are modeled by semi-local functionals and long-range contributions that are calculated by mapping the complex all-electron problem onto a set of atomic response functions coupled in the dipole approximation. We propose an effective range-separation of the coupling between the atomic response functions that extends the already broad applicability of the MBD method to non-metallic materials with highly anisotropic responses, such as layered nanostructures. Application to a variety of high-quality benchmark datasets illustrates the accuracy and applicability of the improved MBD approach, which offers the prospect of first-principles modeling of large structurally complex systems with an accurate description of the long-range correlation energy. (C) 2014 AIP Publishing LLC. [less ▲]

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See detailRole of Dispersion Interactions in the Polymorphism and Entropic Stabilization of the Aspirin Crystal
Reilly, Anthony M.; Tkatchenko, Alexandre UL

in PHYSICAL REVIEW LETTERS (2014), 113(5),

Aspirin has been used and studied for over a century but has only recently been shown to have an additional polymorphic form, known as form II. Since the two observed solid forms of aspirin are degenerate ... [more ▼]

Aspirin has been used and studied for over a century but has only recently been shown to have an additional polymorphic form, known as form II. Since the two observed solid forms of aspirin are degenerate in terms of lattice energy, kinetic effects have been suggested to determine the metastability of the less abundant form II. Here first-principles calculations provide an alternative explanation based on free-energy differences at room temperature. The explicit consideration of many-body van der Waals interactions in the free energy demonstrates that the stability of the most abundant form of aspirin is due to a subtle coupling between collective electronic fluctuations and quantized lattice vibrations. In addition, a systematic analysis of the elastic properties of the two forms of aspirin rules out mechanical instability of form II as making it metastable. [less ▲]

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See detailUnderstanding the role of vibrations, exact exchange, and many-body van der Waals interactions in the cohesive properties of molecular crystals
Reilly, Anthony M.; Tkatchenko, Alexandre UL

in Journal of Chemical Physics (2013), 139(2),

The development and application of computational methods for studying molecular crystals, particularly density-functional theory (DFT), is a large and ever-growing field, driven by their numerous ... [more ▼]

The development and application of computational methods for studying molecular crystals, particularly density-functional theory (DFT), is a large and ever-growing field, driven by their numerous applications. Here we expand on our recent study of the importance of many-body van der Waals interactions in molecular crystals [A. M. Reilly and A. Tkatchenko, J. Phys. Chem. Lett. 4, 1028 (2013)], with a larger database of 23 molecular crystals. Particular attention has been paid to the role of the vibrational contributions that are required to compare experiment sublimation enthalpies with calculated lattice energies, employing both phonon calculations and experimental heat-capacity data to provide harmonic and anharmonic estimates of the vibrational contributions. Exact exchange, which is rarely considered in DFT studies of molecular crystals, is shown to have a significant contribution to lattice energies, systematically improving agreement between theory and experiment. When the vibrational and exact-exchange contributions are coupled with a many-body approach to dispersion, DFT yields a mean absolute error (3.92 kJ/mol) within the coveted "chemical accuracy" target (4.2 kJ/mol). The role of many-body dispersion for structures has also been investigated for a subset of the database, showing good performance compared to X-ray and neutron diffraction crystal structures. The results show that the approach employed here can reach the demanding accuracy of crystal-structure prediction and organic material design with minimal empiricism. © 2013 AIP Publishing LLC. [less ▲]

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See detailElectrodynamic response and stability of molecular crystals
Schatschneider, Bohdan; Liang, Jian-Jie; Reilly, Anthony M. et al

in PHYSICAL REVIEW B (2013), 87(6),

We show that electrodynamic dipolar interactions, responsible for long-range fluctuations in matter, play a significant role in the stability of molecular crystals. Density functional theory calculations ... [more ▼]

We show that electrodynamic dipolar interactions, responsible for long-range fluctuations in matter, play a significant role in the stability of molecular crystals. Density functional theory calculations with van der Waals interactions determined from a semilocal ``atom-in-a-molecule'' model result in a large overestimation of the dielectric constants and sublimation enthalpies for polyacene crystals from naphthalene to pentacene, whereas an accurate treatment of nonlocal electrodynamic response leads to an agreement with the measured values for both quantities. Our findings suggest that collective response effects play a substantial role not only for optical excitations, but also for cohesive properties of noncovalently bound molecular crystals. DOI: 10.1103/PhysRevB.87.060104 [less ▲]

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See detailSeamless and Accurate Modeling of Organic Molecular Materials
Reilly, Anthony M.; Tkatchenko, Alexandre UL

in Journal of Physical Chemistry Letters (2013), 4(6), 1028-1033

The near endless possibilities for assembling molecular materials has long posed a difficult challenge for theory. All crystal-structure prediction methods acknowledge the crucial contribution of van der ... [more ▼]

The near endless possibilities for assembling molecular materials has long posed a difficult challenge for theory. All crystal-structure prediction methods acknowledge the crucial contribution of van der Waals or dispersion interactions, but few go beyond a pairwise additive description of dispersion, ignoring its many-body nature. Here we use two databases to show how a many-body approach to dispersion can seamlessly model both solid and gas-phase interactions within the coveted ``chemical accuracy'' benchmark, while the underlying pairwise approach fails for solid-state interactions due to the absence of many-body polarization and energy contributions. Our results show that recently developed methods that treat the truly collective nature of dispersion interactions are able to reach the accuracy required for predicting molecular materials, when coupled with nonempirical density functionals. [less ▲]

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See detailMany-Body Dispersion Interactions in Molecular Crystal Polymorphism
Marom, Noa; DiStasio, Jr; Atalla, Viktor et al

in ANGEWANDTE CHEMIE-INTERNATIONAL EDITION (2013), 52(26), 6629-6632

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