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Viewpoints on the 2016 Theory and Applications of Computational Chemistry Conference
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excited-state and band-edge positions, charge-transport mechanisms, surface redox reactions, etc. As an illustrative example, she described their successful prediction of the surface reconstruction and the nature of the semiconductor−electrolyte interface on GaP when it catalyzes the production of CH3OH by CO2 reduction [Chem. Mater. 2016, 28, 5799]. She also discussed their assessment of acidities and hydricities of different intermediates at the interface. Their future work will elucidate the full catalytic mechanism on a variety of semiconductor surfaces. David Baker (University of Washington, www.bakerlab.org) presented his group’s recent progress in the understanding of protein structures using state-of-the-art computational schemes. Their research can be divided into three parts: the structural prediction of proteins for given amino acid sequences, the fixedbackbone designs of amino acid sequences for given protein structures, and the de novo design of novel proteins with selected functionality from undetermined amino acid sequences. These studies were performed based on quasi-Newton optimization, as the sizes of the proteins forbid quantum mechanical methods. Intramolecular and intermolecular interactions were parametrized on small molecules and the protein database used in the Rosetta 2015 force field. These approaches have been proven to be exceptionally accurate by numerous examples, including but not limited to unbound two-dimensional protein arrays [Science 2015, 348, 1365] and twocomponent self-assembling icosahedra [Science 2016, 353, 389]. Creatively, they transferred a major portion of their computational load to the distributed computing projects Rosetta@ home [boinc.bakerlab.org] and FoldIt [fold.it]. He concluded the presentation by discussing several unresolved problems, including analysis of the structural changes of proteins during biochemical reactions, assessment of the limited accuracy of the force field, and explicit treatment of the hydrogen bonds. Sharon Hammes-Schiffer (University of Illinois at Urbana− Champaign, hammes-schiffer-group.org) detailed her group’s recent advances in understanding proton-coupled electrontransfer (PCET) reactions, which are observed in a wide range of biological and chemical redox processes. In their earlier studies, they developed the PCET theory that formulates the rate constant expressions in terms of the activation free energy barrier, the reorganization energy, and the vibronic coupling between the reactant and product vibronic states (which in a certain limit is proportional to the overlap of the reactant and product proton vibrational wave functions). This theory incorporates proton donor−acceptor motion as well as a quantum mechanical description of the electrons and transferring proton [J. Chem. Phys. 1999, 111, 4672; J. Chem. Phys. 2000, 113, 2385; J. Chem. Phys. 2015, 122, 014505]. One successful application of their PCET model is its accurate reproduction of the relative rate constants for C−H bond
heoretical and computational chemists from all over the world witnessed and participated in the exciting occurrence of the fourth quadrennial Theory and Applications of Computational Chemistry (TACC) Conference held on the campus of University of Washington in Seattle, Washington, from August 28 to September 2, 2016 [www.tacc2016.org]. Following the tradition established by the first three TACC conferences, the 2016 conference highlighted interdisciplinary topics in which cutting-edge computational chemistry theory is developed and applied. A special focus this year is the methodological developments that connect the mesoscopic or macroscopic properties with the quantum chemical properties at an atomic or molecular level. The speakers consisted of over 150 world-renowned scientists invited from institutions located in 26 different countries and regions. The conference was organized by leading theoretical and computational chemists working in the United States, including Michel Dupuis (University at Buffalo), Xiaosong Li (University of Washington), Aurora Clark (Washington State University), Thomas Dunning, Jr. (University of Washington/Northwest Institute for Advanced Computing/Pacific Northwest National Laboratory), Karol Kowalski (Pacific Northwest National Laboratory), Anne McCoy (University of Washington), and Niranjan Govind (Pacific Northwest National Laboratory). Michel Dupuis (University at Buffalo) delivered the opening remarks for the conference, in which he challenged the speakers to spell out the single most impactful development of the past few years that affected their recent research and will push frontiers in their future research. The conference was scheduled with six common sessions of keynote and plenary speeches and 45 additional parallel sessions of invited and contributed talks, as well as three poster sessions that presented over 250 posters. The parallel sessions featured a special symposium given in honor of the 80th birthday of Ernest Davidson (University of Washington), who is one of the founding fathers of quantum chemistry. The oral and poster presentations at the conference covered a broad range of methodology and application topics, including but not limited to method developments of electronic structure theory, computer-aided molecular discovery and design, simplified models for biological and biomimetic complex systems, and interfacial reactions in multiphase materials. This report summarizes a number of selected lectures in detail; most of them are keynote and plenary presentations. These choices reflect solely the impressions made on the authors by the presentations at the conference. Emily Carter (Princeton University, carter.princeton.edu) introduced the broad field of sustainable energy research and reported her group’s contributions to this field based on their quantum-mechanics-based simulation “toolkits”. Representative tools include the locally correlated and embedded correlated wave function theory and the orbital-free density functional theory. She discussed the wide applications of these methods to species and processes that are involved in multiphase complex systems such as photoelectrocatalysts, to understand their © 2016 American Chemical Society
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is important for a wide variety of properties, including drug bioavailability. Furthermore, XDM has been used to predict enantiomeric excess in packed chiral molecules [Angew. Chem. Int. Ed. 2014, 53, 7879]. Alán Aspuru-Guzik (Harvard University, aspuru.chem. harvard.edu) introduced his ambitious research in accelerating the discovery of new molecules using artificial intelligence. In particular, his group has developed screening methodologies and tools for the enormous chemical space for synthesizable molecules with desired functionality and aims to provide answers for complicated chemical design with computer programs. Their protocol begins with the creation of an initial library of molecular fragments based on their diversity, accessibility, efficiency, and stability. It is followed by a stepwise selection of promising candidates among all possible assemblies of these fragments using high-throughput quantum chemistry and machine learning. In the end, they improve the behavior of this process based on the feedback from peer theoreticians and experimentalists using a platform named Molecular Space Shuttle. Along with the overview for technical details in the methodology, he introduced a wide array of applications of their protocol in predicting the new organic candidates for materials in interdisciplinary fields. For example, they were able to screen over quinone derivatives and selected the best electrolyte in organic−inorganic aqueous flow batteries based on their redox potentials [Nature 2014, 505, 195]. They also managed to predict the existence of long-lasting blue organic OLEDs based on materials that allow thermally activated delayed fluorescence (TADF), which was successfully fabricated by experiments [Nat. Mater. 2016, 15, 1120]. The heated discussions afterward raised two future directions for this research. One is the limited size of the training set from experimentally available molecules, which, Aspuru-Guzik assures, can be overcome by more investigation into training techniques ranging from simple linear regressions to Gaussian processes or Kernel-Ridge regressions. The other is the balance between the small singlet−triplet energy gap and large oscillator strengths, which is an ongoing research goal for the OLED project. Giulia Galli (University of Chicago, galligroup.uchicago.edu) presented the recent progress of her group in developing and applying first-principle molecular dynamics (MD) and manybody perturbation theory (in particular GW) to gain insights into the spectroscopic and transport properties of heterogeneous materials for energy conversion problems. She covered heterogeneous materials used for water photocatalysis [Nat. Comm. 2015, 6, 8769], solar cells [Appl. Phys. Rev. 2016, 3, 040807], and aqueous solutions [J. Am. Chem. Soc. 2016, 138, 6912]. To characterize and predict the properties of these heterogeneous materials, they created structural models from first principles and performed ab initio MD to generate trajectories, which were then utilized to perform large-scale electronic structure calculations based on hybrid DFT or GW. Regarding methodological developments, she focused on their recent development of efficient algorithms and the WEST code [www.west-code.org], to perform GW calculations without the explicit evaluation of virtual electronic orbitals [J. Chem. Theory Comput. 2015, 11, 2680]. She also presented results for solids and molecules obtained with recently developed global dielectric-dependent hybrid (DDH) functionals [Phys. Rev. B 2016, 93, 235106; Phys. Rev. X 2016, 6, 041002]. Weitao Yang (Duke University, chem.duke.edu/labs/yang) detailed his group’s efforts in improving DFT by extending
activation in the soybean lipoxygenase enzyme and a physical explanation for the surprisingly high kinetic isotope effect (KIE) between hydrogen and deuterium. Their theory shows strong evidence for nonadiabatic behavior in this PCET process and indicates that the large KIE is caused by small proton vibrational wave function overlaps [J. Am. Chem. Soc. 2004, 126, 5763; J. Am. Chem. Soc. 2007, 129, 187]. They have also extended this theory to electrochemical PCET [J. Phys. Chem. C 2008, 112, 12386]. In this area, their work suggests routes to design and improve biomimetic catalysts for energy conversion, in part by improving ligand flexibility toward the catalytic sites [J. Phys. Chem. Lett. 2013, 4, 542]. Their studies also include the development of theoretical methods to describe photoinduced PCET [J. Phys. Chem. B 2016, 120, 2407]. Jean-Luc Brédas (King Abdullah University of Science and Technology, ctcam.kaust.edu.sa) introduced his group’s research in the field of organic electronics and focused on the characterization of hybrid organic-conducting oxide interfaces. In the first part of his presentation, he provided an overview of the current industrial frontier of applications such as organic light-emitting diodes (OLEDs), organic field-effect transistors (OFETs), and organic photovoltaic (OPV) cells, exploiting efficient, flexible, and stable organic semiconductors with extended π-conjugation. In the second half, he described their understanding of how the work function of a metal or a conducting oxide can be tuned through deposition of a selfassembled organic monolayer (SAM). Such a modification can turn a stable, originally high-work-function metal oxide electrode into a low-work-function electrode that can efficiently inject or collect electrons [Science 2012, 336, 327]. Using density functional theory (DFT), they screened for appropriate SAMs that modulate the Fermi energy of the metal oxide and can lower the charge injection/collection barriers. The modification of the work function can be decomposed, at the DFT level, into three contributions coming from the interfacial dipole moment, the electrostatic potential step across the SAM, and the surface geometry relaxation [Acc. Chem. Res. 2008, 41, 721]. As an example, the work function of indium tin oxide can be modified by over 1 eV via deposition of a trifluorobenzyl phosphonic acid SAM; in that case, both the substituent (via its charge density and its influence on the molecular tilt angle on the surface) and the functional group binding to the surface (via the interfacial charge redistribution) significantly affect the work function [Adv. Mat. 2009, 21, 4496]. Furthermore, he discussed the critical role of surface defects (such as O vacancies and Zn interstitial defects) in determining the surface charge transfer at the organic−zinc oxide (ZnO) interface [Adv. Mat. 2014, 26, 4711]. Their research provides systematic guidance for tuning the work function at organic-conducting oxide interfaces via surface modification and defect control, which is critical for applications. The Davidson’s symposium featured Erin Johnson’s speech (Dalhousie University, schooner.chem.dal.ca/wiki/Johnson_ Group_Wiki), in which she presented her group’s research on dispersion corrections in DFT and, in particular, their importance to predicting the packing arrangement (polymorphism) of molecular solids. Developed in conjunction with Axel Becke (Dalhousie University), the exchange-hole dipole moment theory (XDM) [J. Chem. Phys. 2007, 127, 154108] was used to energetically rank polymorphs of various molecules to determine how they pack experimentally, with far greater predictive success than traditional approaches based on force fields [J. Chem. Phys. 2012, 137, 054103]. The packing structure 8486
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particular, these chromophores can adopt relative orientations favorable to efficient singlet fission through room-temperature fluctuations [J. Mater. Chem. A 2016, 4, 10500]. Toru Shiozaki (Northwestern University, www.shiozaki.northwestern.edu) showcased work that developed accurate multireference wave function methods for calculating the magnetic properties of strongly correlated molecules, especially the hyperfine coupling constant and the zero-field splitting in heavy-metal-containing systems [J. Chem. Theory Comput. 2016, 12, 4347]. To this end, his group achieved the first realization of analytical nuclear gradients for the CASPT2 approach [J. Chem. Theory Comput. 2016, 12, 3781; J. Chem. Phys. 2013, 142, 051103], and in addition they improved the efficiencies of CASSCF, CASPT2, and MRCI with the four-component Dirac Hamiltonian so that they can be applied to sizable systems [J. Chem. Theory Comput. 2015, 11, 4733; J. Chem. Phys. 2015, 142, 044112; J. Chem. Phys. 2013, 138, 204113]. For junior researchers (graduate students, postdocs, and early career faculties) including the authors, attending the interdisciplinary speeches presented in the 2016 TACC conference, broadened their knowledge across different fields of theoretical and computational chemistry. We were offered the opportunity to explore beyond our own research fields and generate innovative ideas about future research topics. The conference also promoted meaningful communications and collaborations between different fields of theoretical and computational chemistry. The organizers and attendees gratefully acknowledge the following sponsors: University of Washington, Washington State University, Pacific Northwest National Laboratory, University at Buffalo, U.S. Department of Energy, Gaussian Inc, NVIDIA Corporation, Hewlett-Packard Enterprise Development LP, Software for Chemistry & Materials, COSMOlogic GmbH & Co. KG, The Journal of Chemical Physics, and The Journal of Physical Chemistry. Without their financial and moral support the exciting conference and banquet dinner would not have been viable. Finally, the authors would like to thank all speakers whose presentations are covered in this report for their careful reading and helpful suggestions.
conventional density functionals with local scaling corrections and pairing fluctuations. In the first half of the presentation, he discussed how traditional DFT fails to predict molecular properties such as the dissociation energy of H2+ due to the delocalization error (in local density approximation (LDA) or generalized gradient approximation (GGA)), the localization error (in Hartree−Fock), or the static correlation error [J. Chem. Phys. 1998, 109, 2604]. In particular, the energy functionals do not exhibit correct linear behaviors in terms of the fractional charges and fractional spins. To eliminate the delocalization error, they developed a local scaling correction scheme that imposes the Perdew−Parr−Levy−Balduz (PPLB) linearity condition, which greatly improves the description of the global potential energy surface for H2+ as well as other molecular properties [Phys. Rev. Lett. 2015, 114, 053001; Science 2008, 321, 792]. The second part of his presentation featured their development of the particle−particle random phase approximation (pp-RPA) rooted in the second-order many-body perturbation theory. Borrowed from nuclear physics, pp-RPA describes the ground-state exchange-correlation energy in terms of pairing matrix linear fluctuations [Phys. Rev. A 2013, 88, 030501]. It is able to accurately predict the ground-state correlation energy as well as single, double, Rydberg, and charge-transfer electron excitations and yield better performance than the conventional particle−hole random phase approximation (ph-RPA) when it is used in time-dependent DFT calculations [J. Chem. Phys. 2013, 139, 224105; J. Phys. Chem. A 2015, 119, 4923; Proc. Natl. Acad. Sci. 2016, 113, E5098]. Todd Martı ́nez (Stanford University, mtzweb.stanford.edu) presented work on expanding the use of nonadiabatic dynamics, in particular, the development of methods to overcome the bottlenecks in the application to large systems. Advances in exploiting element and rank sparsity, as well as simulation acceleration on graphical processing units (GPUs), were shown. For extremely large systems, he presented a coarse graining approach called the ab initio exciton model [Acc. Chem. Res. 2014, 47, 2857] that allows unprecedented simulations of ab initio exciton dynamics on the light harvesting complex II (LH2) and similar large proteins. Insights from these simulations should give other researchers guidance toward biomimetic compounds to advance solar cell technology. Besides the keynote and plenary presentations offered by senior theoretical and computational chemists mentioned above, this conference also recognized research contributed by early career faculty members. For example, Lee-Ping Wang (University of California, Davis, www.lpwchem.org) introduced a translation-rotation-internal coordinate (TRIC) system for simplifying geometry optimization calculations, in which molecular translations are described collectively as the centroid position and molecular orientations are represented with the exponential map parametrization of quaternions. This methodology proved superior to existing approaches in various systems such as water clusters and organic donor−acceptor systems [J. Chem. Phys. 2016, 144, 214108]. Tim Kowalczyk (Western Washington University, cse.wwu.edu/chemistry/kowalct2) showed work investigating photoinduced charge separation and singlet fission in two-dimensional covalent organic frameworks (COF). His group has discovered that organic donor and acceptor chromophores such as acenes, when embedded within the framework as the “linkers”, can largely preserve their electronic character in the COF environment. In
Tiecheng Zhou† Joshua J. Goings‡ Zhou Lin*,¶ †
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Department of Chemistry and Materials Science and Engineering Program, Washington State University, Pullman, Washington 99164, United States ‡ Department of Chemistry, University of Washington, Seattle, Washington 98195, United States ¶ Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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