Viewpoints on the 2017 American Conference on Theoretical

Jacobson, Bochevarov, Watson, Hughes, Rinaldo, Ehrlich, Steinbrecher, Vaitheeswaran, Philipp, Halls, and Friesner. 2017 13 (11), pp 5780–5797. Abstr...
0 downloads 0 Views 202KB Size
Download PDF
Viewpoint Cite This: J. Phys. Chem. A 2017, 121, 7807-7812

pubs.acs.org/JPCA

Viewpoints on the 2017 American Conference on Theoretical Chemistry

■

INTRODUCTION From July 17th to 21st, theoretical chemists from the United States and abroad gathered at Boston University to take part in the 2017 American Conference on Theoretical Chemistry (ACTC, http://meetatbu.com/actc). The ACTC grew out of a Gordon Research Conference held biennially from 1962 to 1970. The conference in its current form has been held every three years since 1972. The 2017 conference was chaired by Sharon Hammes-Schiffer (Univ. of Illinois at Urbana− Champaign) and vice-chaired by Todd J. Martı ́nez (Stanford Univ). David F. Coker of Boston University served as the Deputy Chair and Local Organizer. More than 30 professors at the forefronts of their respective fields gave talks describing recent research developments. Additionally, over 170 students and postdocs contributed posters detailing their research efforts in theoretical chemistry and its numerous intersecting disciplines. This Viewpoint contains accounts of many of the talks given at the ACTC, delineated by the following concentrations: energy and electron transfer, materials and interfaces, quantum dynamics and spectroscopy, electronic structure, machine learning, and modeling in solution and biological environments. However, most talks could have easily been categorized under two or more of the concentrations listed, emblematic of the large and growing interdisciplinarity in the theoretical chemistry community. The 2017 ACTC was a demonstration of both the depth at which theoretical chemists are understanding fundamental phenomena and the large breadth of topics to which a theoretical chemistry perspective is being applied.

plasmonic nanoparticles may have in light harvesting and sensing applications. David N. Beratan (Duke Univ.) discussed electron transfer in DNA and the difficulties associated with simulating electron transfer in DNA that arise from competing transport mechanisms. The Beratan group has recently described the concept of flickering resonance (FR) electron transfer. The FR electron transfer concept is applicable to general N-site systems and is characterized by the probability of intermediate sites falling within an energy-matching window, leading to an exponentially decaying distance-dependent rate without tunneling [Proc. Natl. Acad. Sci. U.S.A. 2014, 111, 10049. DOI: 10.1073/pnas.1316519111]. As an application, the Beratan group has applied this methodology to G-block DNA singlemolecule junctions to address the question of whether they can establish design principles for engineering nanometer-scale coherence in such systems. They have found that intrablock correlated fluctuations coupled with interblock FR can generate extended coherence [Nat. Chem. 2016, 8, 941. DOI: 10.1038/ nchem.2545]. Moreover, they designed a new DNA sequence as a critical test of their predictions about the origin of the even−odd effect and FR in G-block DNA single-molecule junctions, and experiments by N. J. Tao’s (Arizona State Univ.) group showed excellent agreement with the theoretical predictions. Abraham Nitzan (Univ. of Pennsylvania and Tel Aviv Univ.) spoke about his group’s method for characterizing electron transfer across thermal gradients. This formalism generalizes the standard Marcus theory of electron transfer to encompass situations where donor and acceptor sites are coupled to local environments of different temperatures. Analytical forms for the electron transfer rate between sites of different temperatures were derived, and the corresponding activation energy was shown to be a function of each site’s local temperature [Proc. Natl. Acad. Sci. U.S.A. 2016, 113, 9421. DOI: 10.1073/ pnas.1609141113]. In addition, the Nitzan group has extended this framework to treat metal−molecule interfaces. They have found that, unlike during electron transfer between two molecular sites, there is a thermoelectric effect that can induce preferential directionality for electron transfer in these systems [J. Chem. Phys. 2017, 146, 092305. DOI: 10.1063/1.4971293]. The calculation was also extended to models comprising many modes and many sites characterized by their local temperatures, where thermal transistor effects can appear [Phys. Rev. Lett. 2017, 118, 207201. DOI: 10.1103/PhysRevLett.118.207201]. These studies represent a unified description of charge and heat transfer that provides valuable insight for the design of electronic and thermoelectric devices.

■

ENERGY AND ELECTRON TRANSFER Theoretical analysis of the microscopic mechanisms that facilitate energy and electron transfer processes are crucial to the design of electronic devices. The 2017 ACTC featured several lectures that highlighted recent advances in the development and implementation of methods geared toward furthering progress on this front. George C. Schatz (Northwestern Univ.) presented his group’s recently developed realtime electrodynamics approach toward the study of plasmoncoupled resonance energy transfer in the spirit of Förster theory. This formalism, termed the time-domain electrodynamics resonance energy transfer (TD-ED-RET) method, is capable of computing energy transfer rates in the presence of media with space-dependent, frequency-dependent, or complex dielectric functions [J. Phys. Chem. Lett. 2017, 8, 2357. DOI: 10.1021/acs.jpclett.7b00526]. As an example, they have studied the plasmonic enhancement factor (PEF) of exciton transport rates in the presence of gold nanostructures. They have found that the PEF associated with gold nanospheres can reach ∼1 × 102, while the PEF associated with gold nanorods can approach ∼1 × 106 [J. Chem. Phys. 2017, 146, 064109. DOI: 10.1063/ 1.4975815]. These studies point toward the utility that © 2017 American Chemical Society

Published: October 19, 2017 7807

DOI: 10.1021/acs.jpca.7b09624 J. Phys. Chem. A 2017, 121, 7807−7812

Viewpoint

The Journal of Physical Chemistry A

■

MATERIALS AND INTERFACES Modeling and simulation of novel materials and their reactive interfaces are vital to understanding heterogeneous catalysis and emerging technologies for alternative energy, sensing, and other applications. Many ACTC speakers demonstrated how the theory community is increasingly using its expertise to address these challenging systems. Emily A. Carter (Princeton Univ.) opened her talk by emphasizing the theoretical chemistry community’s responsibility to improve the quality of life on Earth by way of advancing sustainable energy technologies. The Carter group’s efforts in DOI:ng so have included understanding and optimizing materials for dye-sensitized solar cells (DSSCs) and photovoltaics. Alloys of NiO are one class of candidate materials for DSSCs. The Carter group has predicted that alloying NiO with Li2O enhances hole injection and transport to the back electrode [Phys. Chem. Chem. Phys. 2015, 17, 18098. DOI: 10.1039/C5CP03429A]. For transparent conducting layers in photovoltaics, alloying NiO with zinc or magnesium oxides can break the anisotropy of carrier transport in the pure material without reducing the desirably large band gap [J. Appl. Phys. 2015, 118, 185102. DOI: 10.1063/ 1.4935478]. Professor Carter finished her talk by presenting an embedded correlated wave function theory [Acc. Chem. Res. 2014, 47, 2768. DOI: 10.1021/ar500086h] study of plasmoninduced hot electron transfer for breaking dihydrogen and dinitrogen bonds [Nano Lett. 2013, 13, 240. DOI: 10.1021/ nl303940z; J. Am. Chem. Soc. 2017, 139, 4390. DOI: 10.1021/ jacs.6b12301]. Benjamin G. Levine (Michigan State Univ.) discussed his group’s efforts developing and applying methods to understand nonradiative recombination, the conversion of electronic energy to heat, with specific application to semiconducting Si nanoclusters. This phenomenon is of great interest, because nonradiative recombination often inhibits the efficiencies of photovoltaic cells, photocatalysts, and other semiconductor devices. It has long been known that pathways to efficient nonradiative recombination may be introduced by defects in materials, but theorists and experimentalists alike have struggled to connect such pathways with specific defect structures. The Levine group has addressed this problem by developing and applying a wave function-based electronic structure method, configuration interaction singles natural orbital complete active space configuration interaction, that is both size-intensive [J. Chem. Phys. 2015, 142, 024102. DOI: 10.1063/1.4905124] and has been significantly accelerated on graphics processing unit hardware [J. Chem. Theory Comput. 2015, 11, 4708. DOI: 10.1021/acs.jctc.5b00634]. These methods, used in a nonadiabatic molecular dynamics framework, have enabled the identification of conical intersections associated with defects characteristic of the oxidation of Si nanoparticles [Nano Lett. 2015, 15, 6247. DOI: 10.1021/ acs.nanolett.5b02848]. Andrew M. Rappe (Univ. of Pennsylvania) discussed his group’s work in understanding and optimizing the bulk photovoltaic effect (BPVE) in light-harvesting materials. The BPVE is a phenomenon in which single crystals of noncentrosymmetric systems demonstrate a photocurrent and voltage. The Rappe group has largely attributed the BPVE to shift current, which arises from the second-order perturbation theory expansion for current, and has demonstrated that a firstprinciples calculation of the shift current in BaTiO3 is in strong

agreement with experiments [Phys. Rev. Lett. 2012, 109, 116601. DOI: 10.1103/PhysRevLett.109.116601]. Professor Rappe then detailed computational materials design strategies for maximizing the BPVE. Doping and solid solutions of materials have been shown to reduce band gaps into the visible range [Nature 2013, 503, 509. DOI: 10.1038/nature12622], and materials with delocalized band edge states furthermore enhance the BPVE [Nat. Commun. 2016, 7, 10419. DOI: 10.1038/ncomms10419]. In addition, the Rappe group has predicted a large BPVE in polar, non-ferroelectric materials, such as chalcogenides [J. Chem. Phys. 2014, 141, 204704. DOI: 10.1063/1.4901433; Phys. Rev. B 2016, 93, 195210. DOI: 10.1103/PhysRevB.93.195210]. Anastassia N. Alexandrova (UCLA) demonstrated how her group is understanding the relationship between structure and catalytic activity of subnano surface-supported Pt clusters. While such clusters are active heterogeneous catalysts for the high-temperature conversion of hydrocarbons, they are highly fluxional and prone to interconversion between isomers. To describe this structural fluxionality, the Alexandrova group constructs isomerization intermediates by a combined interpolation and partial force-field optimization approach. These methodologies along with molecular dynamics identified numerous thermally accessible isomers for multiple cluster sizes and ethylene coverages [ACS Catal. 2017, 7, 3322. DOI: 10.1021/acscatal.7b00409]. In addition, Professor Alexandrova described how her group has understood how doping these clusters with boron prevents a catalyst degradation process known as coking [J. Am. Chem. Soc. 2017, 139, 11568. DOI: 10.1021/jacs.7b05894].

■

QUANTUM DYNAMICS AND SPECTROSCOPY The development and implementation of accurate, yet computationally economical, quantum dynamics methods is of paramount importance to enhancing our understanding of nonequilibrium quantum mechanical processes. Joseph E. Subotnik (Univ. of Pennsylvania) presented his group’s recent developments in simulating nonadiabatic dynamics with a manifold of electronic states. As a model problem, the Subotnik group has studied vibrational relaxation in an idealized molecule colliding with a metal surface. To study this problem, they have developed a semiclassical dynamics method that interpolates between contrasting perturbation theories (i.e., in the diabatic vs adiabatic limit) [J. Chem. Theory Comput. 2016, 12, 4178−4183. DOI: 10.1021/acs.jctc.6b00533]. This method, termed the broadened classical master equation (BCME), was shown to produce outgoing vibrational energy level distributions in good agreement with the contrasting perturbative methods within their valid parameter regimes. The BCME represents an important step toward the development of efficient quantum dynamics methods that are reliable in parameter regimes, where standard perturbative methods are known to break down. Thomas E. Markland (Stanford Univ.) shared his group’s results for the computation of IR and Raman spectra of aqueous systems using ab initio molecular dynamics simulations incorporating nuclear quantum effects [J. Phys. Chem. Lett. 2017, 8, 1545−1551. DOI: 10.1021/acs.jpclett.7b00391]. Using ring polymer molecular dynamics simulations, the Markland group has produced linear vibrational spectra in excellent agreement with experimental results for HCl aqueous solutions across a wide range of concentrations. They have shown that the vibrational spectra and properties of its time 7808

DOI: 10.1021/acs.jpca.7b09624 J. Phys. Chem. A 2017, 121, 7807−7812

Viewpoint

The Journal of Physical Chemistry A

but breaks spin symmetry. He demonstrated that merging CCSD and projected Hartree−Fock (PHF) can overcome these failings. The Scuseria group has developed two methods for combining CCSD and PHF: symmetry adapted, where PHF is cast as a nonexponential polynomial of particle-hole excitations from a restricted Hartree−Fock (RHF) reference and combined with CCSD, and symmetry broken, where the broken symmetry of UCCSD is restored by projection. In the symmetry adapted formalism, the Scuseria group has demonstrated that SUHF and CCSD can be merged to give spin-projected variational CCSD, which correctly models N2 dissociation but does not perform quite as well as UCCSD for the Hubbard model [J. Chem. Phys. 2016, 145, 111102. DOI: 10.1063/1.4963082; J. Chem. Phys. 2017, 146, 184105. DOI: 10.1063/1.4983065]. They also recently developed projected UCCSD (PUCCSD), a method based on the symmetry-broken formalism [J. Chem. Phys. 2017, 147, 064111. DOI: 10.1063/ 1.4991020]. PUCCSD outperforms UCCSD for H4, N2, and Hubbard model systems while maintaining the correct symmetry, and further testing of PUCCSD on more complex molecular systems is ongoing. Laura Gagliardi (Univ. of Minnesota, Twin Cities) showcased her group’s ongoing development of the generalized active space self-consistent field (GASSCF) method and its utilization together with multiconfigurational pair-density functional theory (MC-PDFT), developed by her group in collaboration with the Truhlar group, to capture both static and dynamic correlation in multireference systems. The Gagliardi group has shown that MC-PDFT can capture bond and excitation energies with complete active space perturbation theory (CASPT2) accuracy for complete active space selfconsistent field (CASSCF) cost [J. Chem. Theory Comput. 2015, 11, 82. DOI: 10.1021/ct5008235; J. Phys. Chem. Lett. 2016, 7, 586. DOI: 10.1021/acs.jpclett.5b02773; J. Chem. Phys. 2017, 146, 034101. DOI: 10.1063/1.4973709]. They also demonstrated that the separated-pair (SP) approximation, a technique reminiscent of generalized valence bond-restricted pairing (GVB-RP) [Int. J. Quantum Chem. 1999, 73, 1. DOI: 10.1002/(SICI)1097-461X(1999)73:13.0.CO;2-0], can be used to systematically select and partition GAS active spaces [Chem. Sci. 2016, 7, 2399. DOI: 10.1039/C5SC03321G; J. Chem. Theory Comput. 2017, 13, 616. DOI: 10.1021/acs.jctc.6b01102]. The group recently applied MC-PDFT with GASSCF using several frontier orbital partitions to calculate the singlet−triplet gaps for the polyacene series through dodecacene, showing excellent agreement with both experimental and other theoretical values [Chem. Sci. 2017, 8, 2741. DOI: 10.1039/C6SC05036K]. Current research directions, including the development of MC-PDFT analytical gradients and the benchmark study of uranium-based molecular magnets, were also presented. Francesco A. Evangelista (Emory Univ.) presented his group’s work on two electronic structure methods designed to tackle systems with significant multireference character and a manifold of chemically relevant excited states. The Evangelista group has developed adaptive configuration interaction (ACI), a selected CI method that excludes determinants based on a cumulative error estimation. They applied ACI to N 2 dissociation to demonstrate the approach converges to within the specified tolerance of the correct full CI result [J. Chem. Phys. 2016, 144, 161106. DOI: 10.1063/1.4948308]. The Evangelista group has made recent progress on their driven similarity renormalization group (DSRG) technique, which is a

evolution can be related to a simple collective coordinate that describes the asymmetry of a proton’s solvation. This work represents a great advancement in understanding the relationship between vibrational spectroscopy features and proton defect structures in complex environments. David F. Coker (Boston Univ.) highlighted the importance of going beyond Hamiltonians that are parametrized by ensemble-averaged experiments to gain a deeper understanding of the role that instantaneous configurations of light harvesting complexes (LHCs) play in constructing highly efficient excitation energy transfer pathways. The Coker group has recently developed a first-principles method for characterizing Frenkel-exciton model Hamiltonians. The Coker group has demonstrated the robustness of this method in an application to the phycobiliprotein LHCs from cryptophyte algae, highlighting the role that strongly coupled vibrational degrees of freedom play in facilitating ultrafast excitation energy transfer processes. Their ensemble of instantaneous Hamiltonians has been used to generate approximate linear spectroscopic signals for these systems [J. Am. Chem. Soc. 2017, 139, 7803. DOI: 10.1021/jacs.7b01780]. Additionally, the Coker group has recently developed a nonperturbative method for computing nonlinear optical spectroscopy in non-Markovian open quantum systems that is based on a semiclassical path integral propagator. This method makes no inherent assumptions about the form of intrasystem or system-bath couplings and was demonstrated through proof-of-concept calculations.

■

ELECTRONIC STRUCTURE Several talks of the 2017 ACTC centered on the development of novel electronic structure theories, with a focus on capturing electron correlation inexpensively, and the application of these methods to previously infeasible systems. Garnet Kin-Lic Chan (California Institute of Technology) emphasized that manyelectron quantum chemistry is often not exponentially hard for physical systems in practice and discussed his group’s work on understanding the electronic structure of nitrogenase and hightemperature superconducting (SC) materials. The Chan group has applied density matrix renormalization group (DMRG) to show that [4Fe-4S] clusters have many low-lying electronic states [Nat. Chem. 2014, 6, 927. DOI: 10.1038/nchem.2041]. More recently, they have connected broken symmetry meanfield guesses to initial DMRG wave functions, allowing them to navigate the dense manifold of electronic states [J. Chem. Theory Comput. 2017, 13, 2681. DOI: 10.1021/ acs.jctc.7b00270]. This technique is allowing them to extend their work to more complex clusters, including the [3Fe-1Mo4S] Holm synthetic cluster and the [8Fe-7S] P cluster [Li and Chan, in preparation]. The Chan group has also recently utilized their density matrix embedding theory (DMET) to calculate the phase diagram of a two-dimensional (2D) Hubbard model, which serves as a model system for a hightemperature superconductor [Phys. Rev. Lett. 2012, 109, 186404. DOI: 10.1103/PhysRevLett.109.186404; Phys. Rev. B 2016, 93, 035126. DOI: 10.1103/PhysRevB.93.035126]. They observe crystallized charge stripes forming in the underdoped region, and SC pairing occurs when the stripes are compressed, supporting the fluctuating stripes mechanism of SC [ArXiv 2017, 1701.00054]. Gustavo E. Scuseria (Rice Univ.) highlighted the failure of single-reference symmetry adapted coupled-cluster (CC) methods in regions of strong correlation, where CCSD breaks down upon dissociation and UCCSD yields the correct energy 7809

DOI: 10.1021/acs.jpca.7b09624 J. Phys. Chem. A 2017, 121, 7807−7812

Viewpoint

The Journal of Physical Chemistry A

3394. DOI: 10.1002/adfm.201600079]. More recently, they demonstrated that ligand nucleophilicity can be used to tune indium phosphide (InP) kinetics, improving the synthesizability of InP as a nontoxic alternative to CdSe for quantum dots [Chem. Mater. 2017, 29, 3632. DOI: 10.1021/acs.chemmater.7b00472]. The Kulik group has curated a small set of geometry-independent descriptors for inorganic compounds, allowing them to use neural nets to perform accurate highthroughput screening for spin-state ordering, sensitivity to Hartree−Fock exchange, and spin-state specific bond lengths [Chem. Sci. 2017, 8, 5137. DOI: 10.1039/C7SC01247K]. Professor Kulik showed preliminary results using a genetic algorithm paired with their neural net to discover new non-Fe spin crossover species and discussed a heuristic for evaluating the fitness of the neural net to previously unexplored chemical space. As several speakers demonstrated that machine learning shows great promise for quantum chemistry, there is also a continued need to pursue efficient computational implementations that can generate large quantities of high-quality data. In ACTC tradition, vice-chair Todd J. Martı ́nez (Stanford Univ.) gave the closing talk of the conference on the use of graphics processing unit (GPU) accelerated J and K matrix builds, drawing a comparison between efficient J/K kernels in electronic structures to black-box matrix multiplication kernels (i.e., DGEMM) in state-of-the-art numerical linear algebra packages. The Martı ́nez group has recently developed J/K build implementations of the COSMO polarizable continuum solvation model [J. Chem. Theory Comput. 2015, 11, 3131. DOI: 10.1021/acs.jctc.5b00370] and the state-averaged CASSCF (SA-CASSCF) method [J. Chem. Phys. 2017, 146, 174113. DOI: 10.1063/1.4979844]. As an application, they have used ab initio multiple spawning (AIMS) to study the photoinduced ring opening in provitamin D3, the largest nonadiabatic dynamics simulation at the SA-CASSCF/AIMS level of theory to date [J. Phys. Chem. Lett. 2016, 7, 2444. DOI: 10.1021/acs.jpclett.6b00970]. The failure of single reference methods, including linear response theories (e.g., TD-DFT) and equations-of-motion (EOM) methods, to properly describe conical intersections was also strongly emphasized [Mol. Phys. 2006, 104, 1039. DOI: 10.1080/00268970500417762; J. Chem. Phys. 2007, 127, 044105. DOI: 10.1063/1.2755681; ArXiv 2017, 1708.01252].

solution to the intruder state problem in multireference calculations. They have improved the efficiency of their second-order implementation (DSRG-MRPT2) [J. Chem. Theory Comput. 2015, 11, 2097. DOI: 10.1021/ acs.jctc.5b00134; J. Chem. Phys. 2016, 144, 204111. DOI: 10.1063/1.4951684] and developed third-order [J. Chem. Phys. 2017, 146, 124132. DOI: 10.1063/1.4979016] and nonperturbative [J. Chem. Phys. 2016, 144, 164114. DOI: 10.1063/1.4947218] implementations, which are significantly more accurate than DSRG-MRPT2 in certain diatomic dissociations. Preliminary results for singlet−triplet gaps using ACI with a DSRG-MRPT2 correction for the polyacene series through hexacene show excellent agreement with experiment values, and preliminary figures showing the avoided crossing in LiF and the NH3 S0/S1 conical intersection were presented to showcase the ability of state-averaged DSRG to correctly capture excited states.

■

MACHINE LEARNING AND IMPLEMENTATION The rise of machine learning techniques, particularly neural networks, in quantum chemistry was apparent at the 2017 ACTC. Alán Aspuru-Guzik (Harvard Univ.) started the first session of the conference with an overview of discriminative and generative machine learning techniques and his group’s work in applying machine learning to molecular design. The Aspuru-Guzik group has developed a differentiable analogue to circular fingerprints that can be trained through standard neural net techniques, allowing them to predict properties such as solubility, toxicity, and photovoltaic efficiency with higher accuracy than with circular fingerprints alone [NIPS 2015, 28, 2215]. The group, along with industry partners, has applied neural nets trained on time-dependent density functional theory (TD-DFT) vertical excitation energies to highthroughput screening of novel organic light-emitting diode (OLED) compounds, culminating in the realization of a device with an external quantum efficiency of 22% [Nat. Mater. 2016, 15, 1120. DOI: 10.1038/nmat4717]. They recently introduced a variational autoencoder to convert SMILES strings to and from a chemical latent space, allowing for optimization and interpolation between different molecules. This is a potential solution to the pervasive inverse design problem in chemistry [ArXiv 2017, 1610.02415]. The Aspuru-Guzik group also applied the concept of generative adversarial networks (GAN) to chemistry, utilizing their Objective-Reinforced GAN (ORGAN) method to generate new molecular candidates with target features such as druglikeness, synthesizability, and solubility from a database of small organic molecules [ArXiv 2017, 1705.10843]. Heather J. Kulik (Massachusetts Institute of Technology) focused on her group’s efforts in providing automated structure generation and machine learning tools for inorganic chemistry through their molSimplify package [J. Comput. Chem. 2016, 37, 2106. DOI: 10.1002/jcc.24437; Ind. Eng. Chem. Res. 2017, 56, 4898. DOI: 10.1021/acs.iecr.7b00808]. The Kulik group has developed an automated procedure for generating initial inorganic complexes based on coordinating or mutating ligands around a metal center that outperforms universal force field (UFF). As an application, they designed several ligand environments that increased the selectivity of a model of electrochemically active ferrocene polymers to formate over perchlorate while still allowing the formate to be released by the reduced ferrocene [Chem. Mater. 2016, 28, 6207. DOI: 10.1021/acs.chemmater.6b02378; Adv. Funct. Mater. 2016, 26,

■

MODELING IN SOLUTION AND BIOLOGICAL ENVIRONMENTS A number of ACTC speakers demonstrated how theoretical chemists develop an understanding of chemical and biological phenomena at multiple scales in length and time. Depending on the nature of the problem, theoretical chemists employ methods based on quantum mechanics, classical molecular dynamics, coarse-graining, and continuum electrostatics to study equilibria and dynamics in various solvents, proteins, membranes, and related environments. Victor S. Batista (Yale Univ.) spoke about his group’s investigations into natural and artificial photosynthesis, in particular, mechanistic studies of the water-splitting reaction of the oxygen-evolving complex (OEC) of photosystem II (PSII). The Batista group uses a hybrid quantum mechanics/molecular mechanics (QM/MM) approach to simulate the oxomanganese complexes and their surrounding environments that are central to performing water splitting in nature. Among the group’s many recent advances in this field are the joint computational7810

DOI: 10.1021/acs.jpca.7b09624 J. Phys. Chem. A 2017, 121, 7807−7812

Viewpoint

The Journal of Physical Chemistry A

that points in phase-space along the path in imaginary time are treated simply as two interacting quasiparticles. In this picture, one particle acts as a classical particle that feels the potential energy function, while the other particle imparts the quantum effects to the system via a special coupling to the classical particle. Moreover, the quantum particle can be the only particle seen by the observer [J. Chem. Phys. 2015, 143, 094104. DOI: 10.1063/1.4929790]. Qiang Cui (Univ. of Wisconsin-Madison) highlighted his group’s efforts in understanding mechanochemical coupling as it pertains to large-scale transitions in proteins. One problem Professor Cui described was the open/closed transition in adenylate kinase, which consists of semirigid domains connected by hinge residues, coupled with ligand binding. Atomistic simulations were able to detail the multiple pathways and time scales, domain stabilities, and transition kinetics involved in the open/closed transition that coarse-grained studies could not capture [J. Mol. Biol. 2010, 400, 618. DOI: 10.1016/j.jmb.2010.05.015]. In addition, the Cui group has worked to understand the time scales of ATP hydrolysis in myosin, particularly in terms of conformational transitions beyond the active site, using a mixed quantum mechanical (density-functional tight-binding)-molecular mechanical methodology [Chem. Rev. 2016, 116, 5301. DOI: 10.1021/ acs.chemrev.5b00584]. Simulations of ATP hydrolysis show how reaction free energy barrier can change with the protein conformation, including important contributions from water reorganization [Biochemistry 2017, 56, 1482. DOI: 10.1021/ acs.biochem.7b00016]. Lyudmila V. Slipchenko (Purdue Univ.) highlighted her group’s recent developments in polarizable embedding using the effective fragment potential (EFP) method. EFP is a perturbative approach on the interactions between fragments representative of solvent or other environmental motifs and a solvated complex typically treated at a quantum mechanical level. EFP is parametrized by ab initio calculations and often outperforms some DFT functionals and classical force fields in terms of interaction energies [J. Chem. Theory Comput. 2012, 8, 2835. DOI: 10.1021/ct200673a]. The Slipchenko group has extended the EFP approach to biological systems such that a macromolecule is treated as a combination of the fragments for which EFP is parametrized. This approach has described protein effects on photochemical events [J. Phys. Chem. B 2016, 120, 6562. DOI: 10.1021/acs.jpcb.6b04166]. Lastly, Professor Slipchenko described ongoing efforts to formalize and apply a full embedding in the quantum mechanical-EFP framework [J. Chem. Phys. 2012, 136, 244107. DOI: 10.1063/1.4729535], including dispersion and exchange-repulsion interactions between subsystems.

experimental characterization of ammonia binding to the second coordination sphere of the OEC in PSII [Biochemistry 2016, 55, 4432. DOI: 10.1021/acs.biochem.6b00543] and an elucidation of the rate-limiting oxygen−oxygen bond formation that occurs at the S3 state of the OEC [Biochemistry 2016, 55, 981. DOI: 10.1021/acs.biochem.6b00041]. Professor Batista also highlighted his group’s mechanistic study of a biomimetic oxomanganese complex for artificial photosynthesis that elucidated the role of proton-coupled electron transfer mediated by carboxylate groups during oxygen evolution [ACS Catal. 2015, 5, 2317. DOI: 10.1021/acscatal.5b00054]. Given the ubiquity of chemical processes in aqueous environments, theorists have made it a priority to perform tractable simulations of water that demonstrate chemical accuracy. Francesco Paesani (Univ. of California, San Diego) presented his group’s many-body molecular dynamics approach toward modeling aqueous chemistry. The Paesani group has developed many-body potential energy functions (PEFs) for water [J. Chem. Theory Comput. 2013, 9, 1103. DOI: 10.1021/ ct300913g; J. Chem. Theory Comput. 2014, 10, 1599. DOI: 10.1021/ct500079y; J. Chem. Theory Comput. 2014, 10, 2906. DOI: 10.1021/ct5004115] and ion−water systems [J. Chem. Theory Comput. 2016, 12, 2698. DOI: 10.1021/ acs.jctc.6b00302; J. Chem. Phys. 2017, 147, 161715. DOI: 10.1063/1.4993213], which exhibit chemical accuracy at the coupled-cluster level from the gas to the condensed phase. These many-body PEFs include explicit two- and three-body interactions, with all higher-order terms being represented through classical many-body induction. Professor Paesani highlighted that his group’s many-body approach outperforms, in terms of accuracy, existing DFT models of liquid water [Chem. Rev. 2016, 116, 7501. DOI: 10.1021/acs.chemrev.5b00644] and ice [J. Chem. Theory Comput. 2017, 13, 1778. DOI: 10.1021/acs.jctc.6b01248]. By explicitly treating nuclear quantum effects within the path-integral formalism, the Paesani group has helped decipher the vibrational spectra of water in different environments, from small clusters in the gas phase [J. Am. Chem. Soc. 2017, 139, 7082. DOI: 10.1021/ jacs.7b03143] to bulk water [J. Chem. Theory Comput. 2015, 11, 1145. DOI: 10.1021/ct501131j], the air/water interface [J. Am. Chem. Soc. 2016, 138, 3912. DOI: 10.1021/jacs.6b00893], and ice [J. Phys. Chem. Lett. 2017, 8, 2579. DOI: 10.1021/ acs.jpclett.7b01106]. Gregory A. Voth (Univ. of Chicago) introduced broadly the concept of coarse-graining in biomolecular and other condensed-phase simulation as a means of understanding phenomena across many length and time scales. Professor Voth described his group’s ultracoarse-graining (UCG) approach, in which coarse-grained particles have internal states [J. Chem. Theory Comput. 2013, 9, 2466. DOI: 10.1021/ct4000444]. Such internal states are dynamic, modulate the interactions between different particles, and can be representative of structural, chemical, or other changes to the coarse-grained motif. The Voth group has recently performed a UCG simulation of the dynamic assembly of an entire HIV capsid [Nat. Comm. 2016, 7, 11568. DOI: 10.1038/ncomms11568]. Professor Voth furthermore demonstrated that, in the limit of UCG state dynamics in which the states change frequently, the dynamics resemble an adiabatic or mean-field picture in that there exists a rapid local equilibrium [J. Chem. Theory Comput. 2017, 13, 1010. DOI: 10.1021/acs.jctc.6b01081]. Lastly, he spoke about describing quantum statistical mechanics from a coarse-graining of the Feynman path-integral formalism, such

■

CONCLUSION The ACTC persists as an important gathering of the theoretical chemistry community as well as showcase for novel ideas, methods, and applications across a number of intersecting disciplines. This is perhaps best exemplified by the large degree to which students and postdocs participate in the ACTC and interact with the speakers and one another, strengthening the sense of scientific community between attendees at every career stage. These interactions as well as the increasing academic diversity of the field made for a dynamic and highly edifying conference. The meeting concluded with an announcement of the next ACTC, in 2020, which will be chaired by the 2017 vice-chair, Todd Martı ́nez. 7811

DOI: 10.1021/acs.jpca.7b09624 J. Phys. Chem. A 2017, 121, 7807−7812

Viewpoint

The Journal of Physical Chemistry A We wish to thank the sponsors of the ACTC for facilitating this instructive meeting: Department of Energy Basic Energy Sciences (DE-SC0017658), American Chemical Society Publications (Chemical Reviews, The Journal of Physical Chemistry, and Journal of Chemical Theory and Computation), and AIP Publishing (Journal of Chemical Physics). In addition, we are deeply grateful to the faculty and staff at Boston University for seamlessly coordinating this large gathering. We furthermore would like to thank the professors whose talks were highlighted herein for their help in crafting this manuscript and interpreting complex research ideas as concisely as possible. Lastly, we thank and commend the organizers, Professors Hammes-Schiffer, Martı ́nez, and Coker, for orchestrating this highly successful iteration of the ACTC and look forward excitedly to 2020.

Zachary K. Goldsmith*,† Justin Provazza*,‡ Stefan Seritan*,§,∥ †

■

Department of Chemistry, University of Illinois at Urbana−Champaign, Urbana, Illinois 61801, United States ‡ Department of Chemistry, Boston University, Boston, Massachusetts 02215, United States § Department of Chemistry and the PULSE Institute, Stanford University, Stanford, California 94305, United States ∥ SLAC National Accelerator Laboratory, Menlo Park, California 94025, United States

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. (Z.K.G.) *E-mail: [email protected]. (J.P.) *E-mail: [email protected]. (S.S.) Notes

The authors declare no competing financial interest.

7812

DOI: 10.1021/acs.jpca.7b09624 J. Phys. Chem. A 2017, 121, 7807−7812