Browse Articles

Stereoselectivity of cis–trans photoisomerization in ethylene: The significance of symmetry breaking minimum energy conical intersection

This work finds that there are eight distinguished symmetry breaking minimum energy conical intersections (MECIs) between the N/V states of ethylene, in which the first four of them are closer to the reactant configuration than the remaining four. Consequently, the initially excited ethylene has a higher probability to relax to the first four MECIs or their adjacent configurations. Moreover, the ground potential energy surface around the first four MECIs has an asymmetric peaked topology and tilts toward the reactant configuration. Therefore, the cis–trans photoisomerization reaction in ethylene can exhibit the stereoselectivity of returning more back to the reactant rather than forming the product. In addition, the geometry–electronic structure relation and the geometric phase effect associated with a conical intersection were demonstrated by employing the adiabatic-to-diabatic analysis, in which the complete active space self-consistent-field wave function is represented by the equivalent covalent and ionic valence bond functions.

Erratum: “Photoelectron spectrum of isothiocyanic acid, HNCS: Theory and experiment” [J. Chem. Phys. 162, 164310 (2025)]

Hopping mediated transport between finite pools of redox proteins

Transport reactions in biology involve the flow of particles—electrons, ions, or molecules—between reservoirs. We explore how electron transport between finite reservoirs depends on the nature of the reservoirs, including their size, occupancy, and interactions. We compare the transport kinetics produced by narrowband and wideband infinite reservoir models (described earlier) with a finite narrowband reservoir model. The transport between finite reservoirs is found to depend on both the initial charge distribution and the number of carriers present. Whether or not a steady-state transport regime is accessed prior to reaching the equilibrium charge distribution depends on these initial conditions.

Unsupervised tracking of local and collective defects dynamics in metals under deformation

Metals owe their unique mechanical properties to how defects emerge and propagate within their crystal structure under stress. However, the mechanisms leading from the early emerging (local) defects to the amplification of dislocations (collective plastic events) are not easy to track. Here, using tensile-stress atomistic simulations of a copper lattice as a case study, we revisit this classical problem under a new perspective based on local dynamics rather than on purely structural arguments. We use a data-driven approach that allows tracking how local fluctuations emerge and accumulate in the atomic lattice in space and time, anticipating/determining the emergence of local or collective structural defects during deformation. Building solely on the general concepts of local fluctuations and spatiotemporal fluctuation correlations, this approach allows characterizing in a unique way the evolution through the elastic, plastic, and fracture phases, describing metals as complex systems where collective phenomena originate from local dynamical triggering events.

Semiclassical transition state theory through the lens of the restricted partition function

The wide adoption of transition state theory in resolving the rates of molecular processes relies on the simplification from reducing the formal and numerical expense of dynamics by a geometric constraint. Such a reduction is at odds with the uncertainty in localization that the uncertainty principle requires. While many forms of semiclassical transition state theory (SCTST) have been aimed at addressing this challenge, a popular approach has relied on resolving the underlying phase space structure of the exact rate formula to leverage Bohr–Sommerfeld quantization. The Hernandez–Miller SCTST reframed the thermal rate formula into an integral of the so-called restricted partition function (RPF) over the action associated with the reaction. The density-of-state SCTST has reframed the rate formula in terms of the instanton’s density of states (DoS). Here, we show the relationship between the RPF-SCTST and the DoS-SCTST and derive the latter from the former. In this way, we help unify these branches of SCTST and provide a clearer formalism for future advances.

Ultrafast hydrogen-bonding interactions between a photoexcited Cu–anthraquinone donor–acceptor dyad and protic solvents

The rational design of solar energy catalysts requires a mechanistic understanding of the ultrafast interactions with the solvent environment. We have designed a new Cu(I)–anthraquinone framework (CuEthyneAnQ) to serve as a model for studying hydrogen-bonding effects in charge accumulating photocatalysts. Herein, we report the ground and excited-state characterization of CuEthyneAnQ by electrochemical and ultrafast optical transient absorption (OTA) spectroscopy measurements. Significant stabilization of the AnQ-centered reductions due to hydrogen-bonding was observed by electrochemical measurements in protic solvent mixtures. Analysis of the excited-state photophysics with OTA reveals electron transfer occurring in tens of picoseconds after metal-to-ligand charge transfer excitation, resulting in the charge-separated state of Cu(II)EthyneAnQ·–. Charge recombination occurs in 4 ns in aprotic solvent and extends to 19 ns in protic solvent. In order to examine the influence of hydrogen-bonding on the electron-transfer dynamics, we performed OTA measurements on CuEthyneAnQ in varying aprotic:protic solvent mixtures. We observe three effects that depend on the concentration of the protic solvent: (1) after charge separation, a diffusion-limited hydrogen-bond forms with the reduced AnQ·–; (2) the slowdown in charge recombination with protic solvent addition is due to hydrogen-bond stabilization in accordance with Marcus theory; and (3) a spectral shift occurs in the charge-separated state due to an increasing number of hydrogen-bond interactions. Our results are supported by time-dependent density functional theory calculations with explicit solvent hydrogen-bonding interactions. These insights underscore the potential of Cu-based donor–acceptor complexes and mixed-solvent systems to offer valuable guidelines for the design of more efficient photocatalytic systems.

The high throughput construction and analysis of bilayers of tetrahedra

A method for generating significant numbers of network configurations is developed appropriate to bilayers of systems such as SiO2, GeO2, and aluminosilicates. The presence of a mirror plane allows the bilayer structures to exactly map onto a two-dimensional network of three-coordinate nodes (equivalent to a percolating network of rings). A bond switching algorithm is employed to generate a range of disordered (amorphous) network topologies (characterized by the ring size distribution and the nearest-neighbor connectivities, as measured by the Aboav–Weaire Law and assortativity). Bilayer configurations are generated from these networks, and energy minimizations are performed using a hierarchy of potential models: a purely harmonic potential, a harmonic potential with an inter-tetrahedral repulsive term, a rigid-ion model, and a polarizable-ion model. The harmonic potential shows a flexibility window whose extent depends on the spatial extent of the inter-tetrahedral repulsive term. The window becomes less well-defined for higher level (rigid-ion and polarizable-ion) models. Distortions of the bilayer networks are characterized with reference to both the ideal (hexagonal) crystal and the amorphous networks. In addition, a “pore evaporation” algorithm is developed and used to generate a range of potential zeolitic bilayer networks. These networks, which are ordered but contain significant numbers of non-hexagonal rings, provide a useful contrast to the disordered networks.

Time-resolved nonlinear microspectroscopy with Gaussian beams: Photon echo and spatially encoded coherence

We extend our theoretical framework for time-resolved nonlinear microspectroscopy [M. Cho, J. Chem. Phys. 162, 124201 (2025)] to coherent four-wave-mixing spectroscopy using paraxial Laguerre–Gaussian (LG) beams. Unlike pump–probe or transient absorption techniques, photon echo is highly sensitive to the spatial phase structure of LG beams. This sensitivity arises because coherence evolution in inhomogeneously broadened absorbers depends on the radial and azimuthal indices of the LG modes used in the write-and-read processes within the photon echo configuration. Recent advances in spatial light modulators, metasurfaces, and ultrafast laser techniques have significantly improved spatial and temporal control over quantum materials. These innovations enable new approaches to studying heterogeneous systems, developing multidimensional microspectroscopy, and exploring alternative quantum information storage methods. In this work, we investigate photon echo signals generated by LG beams and derive analytical expressions for rephasing and non-rephasing photon echoes. Our results reveal how beam parameters influence nonlinear spatiotemporal responses, capturing spatial variations in pulse amplitudes, phases, and inhomogeneity-induced dephasing and rephasing. We show that customized ultrafast pulses and structured spatial light fields can enhance the spatial separation of photon echo signals and increase the density of stored quantum information. This work advances nonlinear molecular spectroscopy and quantum information science by leveraging structured light fields and ultrafast optics.

Concentrated aqueous lithium chloride solution dynamics: The role of chemical exchange on anisotropy and vibrational population relaxations

Ultrafast polarization-selective pump–probe experiments, conducted on the OD stretch of dilute HOD, are reported for LiCl/H2O solutions ranging from 1–24 to 1–128 (ion pairs–water molecules), 2.3–0.4 m. The results are compared to prior and revised experiments on 1–4 to 1–16 concentrations, 13.9–3.5 m. Vibrational population relaxation and anisotropy decays were measured for hydroxyls hydrogen-bonded to chlorides (HBCs). In contrast to higher salt concentrations, at ≤∼1–32 (1.7 m salt), the HBC population relaxation times and anisotropy decays are concentration independent. 1–32 marks a transition from high concentrations of ion pairs, clusters, and ion networks to concentrations of ion pairs low enough not to affect observable molecular level dynamics. At a concentration of approximately 1–32 and lower salt concentrations, chemical exchange is responsible for HBC anisotropy decay and plays a role in population relaxation. Wavelength-dependent population relaxation was used to obtain lifetime amplitude spectra (LAS), which show distinct species that are not observable with FT-IR. At very high salt concentrations, e.g., 1–6, there are no “pure” water regions, and the LAS has two bands: HBCs and hydroxyls of water oxygens solvating Li+. At lower salt concentrations, there is also a “pure” water band in the LAS. The HBC band shape is concentration independent from 1–4 to 1–128.

1H-detected measurement of spin relaxation constants in ultrawide solid-state NMR line shapes via progressive saturation of the proton reservoir

Nuclear magnetic resonance relaxation time constants provide valuable insights into the dynamic processes and structure of a system. However, determination of these relaxation parameters is often challenging for low-γ nuclides, which exhibit low sensitivity and are typically subject to large anisotropic interactions. Here, we introduce two new pulse sequences, PROSPR-T1 and PROSPR-T2, for measuring longitudinal and transverse relaxation time constants in solids. These sequences are modifications of the PROgressive Saturation of the Proton Reservoir (PROSPR) experiment, developed to facilitate the detection of insensitive nuclides. Following an initial analysis of the adiabatic requirements of the PROSPR experiment, the performance of PROSPR to measure T1/T2′ is thoroughly evaluated on a range of spins, including 119Sn (I = 1/2), 35Cl (I = 3/2), and 14N (I = 1). Experimental results demonstrate that optimized versions of PROSPR can be up to an order of magnitude more sensitive than their original implementation. These new methods offer accurate relaxation time constants, even for spectra spanning over 1 MHz, without demanding frequency stepping throughout the powder pattern.

Solvation governs cation transference in glyme-based lithium battery electrolytes

The efficacy of electrochemical systems is governed by the cation transference number, which represents the fraction of current carried by the working ion. Energy is wasted when field-induced motion also drives anions and solvent molecules, decreasing the transference number to near-zero. We present a systematic study of cation transference in a series of electrolytes: tetraglyme (TG), octaglyme (OG), and poly(ethylene oxide) (PEO) mixed with lithium bis(trifluoromethanesulfonyl)imide. In all three electrolytes, starting from the dilute salt concentration limit, the experimentally measured cation transference number decreases with increasing concentration, reaching a minimum between −0.1 and −0.2, before rising back to positive values. Explicit measurements of field-induced species’ velocities by electrophoretic nuclear magnetic resonance indicate that negative cation transference numbers in TG and OG electrolytes are dictated by solvation interactions with minimal contribution from anion-cation interactions. Simulation-based solvation structures indicate that OG serves as a bridge between TG and PEO. Multi-charge positive clusters, which are negligible in TG, become increasingly important at higher chain lengths (OG and PEO). As migrating cations drag their solvation shells, this solvation-induced motion is amplified in glyme electrolytes because of covalent interactions between solvating glyme molecules and free glyme molecules.

Topological insights into dense frictional suspension rheology: Third-order loops drive discontinuous shear thickening

Dense suspensions exhibit a significant change in viscosity under external deformation, a phenomenon known as shear thickening. Recent studies have identified a stress-induced transition from lubricated, unconstrained interactions to frictional contacts, which play a crucial role in shear thickening. This work investigates the rheological behavior and frictional contact network evolution during continuous and discontinuous shear thickening (DST) in two-dimensional simulations. We find that at low stress, during weak thickening, the frictional contact network is composed of quasilinear chains along the compression axis. With increasing stress, the unweighted contact network becomes more isotropic and forms loop-like structures. We show that third-order loops within the frictional contact network are key to the DST. Our findings revealed a strong correlation between the number of third-order loops and the viscosity of the suspension. Notably, this relationship remains independent of the packing fraction, applied stress, and interparticle friction, highlighting the fundamental role of the mesoscale network topology in governing macroscopic rheology and connecting to the microscopic physics.

Erratum: “Quantum corrections to the kinetic energy and the <i>ab initio</i>-based prediction of the thermodynamic properties and vapor–liquid equilibria of hydrogen” [J. Chem. Phys. 162, 124502 (2025)]

Ultrafast dynamics of transient exciton state in methylammonium lead iodide perovskite at room temperature

Metal-halide perovskites are promising materials for photovoltaics and other optoelectronic applications. Understanding their photophysical properties, especially the energy landscape and dynamics of photoexcited carriers, is essential. This work focuses on studying ultrafast dynamics of photoexcited carriers in a methylammonium lead iodide (MAPI) perovskite thin film at room temperature. We first performed steady-state spectroscopic measurements to characterize the sample and then implemented optical 2D coherent spectroscopy in the non-collinear geometry to probe the ultrafast dynamics of photoexcited carriers. A series of rephasing one-quantum 2D spectra at different waiting times revealed that a transient exciton resonance coexists with the free-carrier resonance for hundreds of femtoseconds before the excitons dissociate into free carriers. The free-carrier resonance persists for a significantly longer time, up to at least 1 ns, and exhibits the signs of spectral diffusion. We then characterized the spectral diffusion by calculating the frequency–frequency correlation function. The acquired 2D spectra revealed unique transient dynamics in MAPI perovskites that might be difficult to probe with one-dimensional techniques.

Top–down optimization of aqueous electrolyte force fields to model chemical potentials and solubilities

Electrolyte solutions are vital to the development of technologies such as batteries and carbon sequestration methods. Accurate but efficient simulation models are crucial in guiding the development of such technologies. In this work, we investigate how the inclusion of atomic polarizability, by means of Drude oscillators, affects the ability of efficient, classical force fields to model the temperature dependence of the aqueous solubility and activity coefficients of alkali-halide salts. To achieve this, we propose a new method to efficiently and accurately compute derivatives of the salt chemical potential with respect to force field parameters, enabling gradient-based fitting directly to chemical potential data. Using this method, we attempt to refine polarizable models to better predict solid–solution solubility limits as functions of temperature. We find that while solubility predictions can be improved, polarizable models are incapable of reproducing the slope of the solid–solution coexistence line for NaCl. This implies that classical polarizability (with constant atomic charges) alone is an insufficient description of system many-body interactions, and “first-principles” descriptions of the energy landscape are necessary to achieve true predictive power in electrolyte modeling.

Low-frequency Raman spectra of amyloid fibrils

We report on how low-frequency Raman measurements can be used as a facile tool to investigate the supramolecular structure of amyloid fibrils. We investigate the low-frequency Raman spectra (&amp;lt;500 cm−1) of six different amyloid fibrils exhibiting parallel β-sheet structures prepared from amyloid-β1–40, amylin, amyloid-β25–35, and amylin20–29 peptides. We propose band assignments using a combination of semi-empirical tight-binding calculations and insights gleaned from previously published studies on model polypeptides in β-sheet conformations. We discuss how low-frequency Raman modes can be used to probe the interactions, packing, and ordering of strands and side chains within fibril β-sheets to gain insights into their supramolecular structures.

Cluster perturbation theory. XII. Parallel implementation of variational excitation energy series for the coupled cluster singles and doubles model

An efficient implementation of the variational cluster perturbation excitation energy series through fifth order is described. The series has the coupled cluster singles excitation energies as zeroth order and targets the coupled cluster singles and doubles (CCSD) excitation energies. The implementation utilizes the resolution of the identity approximation for the two-electron integrals. The perturbation series uses the 2n + 1/2n + 2 rules to eliminate all parameters higher than second order. The computational efficiency is illustrated by performing excitation energy calculations on a single node for systems with up to 1000 basis functions and comparing with CCSD calculations. Using the hybrid parallel open multiprocessing/message-passing interface implementation with graphics processing unit offloading, fifth-order cluster perturbation excitation energies for systems with 1750 basis functions can be calculated in 8 h using 20 nodes on the Frontier supercomputer at the Oak Ridge National Laboratory. A benchmark of 131 excitation energies for a diverse set of molecules is presented. For the fourth and fifth order models, a few prominent outliers are present. We demonstrate how these cases may be identified using a simple diagnostic, with the remaining values being indistinguishable from CCSD excitation energies in practice.

Cascade at local yield strain for silica and metallic glass

We report observations of unusual first plastic events in silica and metallic glasses in the shear startup regime at applied strain two orders of magnitude smaller than yield strain. The (non-affine) particle displacement fields during these events have complex real space structure with multiple disconnected cores of high displacement appearing at the same applied strain under athermal quasistatic simple shear deformation and identified by using a “cell based cluster analysis” method. By monitoring the stress relaxation during the first plastic event by Langevin dynamics simulation, we directly show the cascade nature of these events. Thus, these first plastic events are reminiscent of avalanches in the post-yielding steady state, but unlike the steady state avalanches, we show that these events are not system spanning. To understand the nature of these events, we tune three factors that are known to affect brittleness of a glass. These are (i) sample preparation history, (ii) inter-particle interactions, and (iii) rigidity of the background matrix applying a “soft matrix” probe recently developed by some of us. In each case, we show that such first plastic events are more probable in more ductile glasses. Our observations are consistent with the picture that more ductile materials are softer, implying that understanding the role of softness may be a promising route to develop microscopic quantifiers of brittleness and thus clarifying the physical origin of brittle-to-ductile transition.

Microwave-free nuclear spin hyperpolarization through photo-CIDNP in static and rotating solids

This study advances the theoretical foundation of photo-chemically induced dynamic nuclear polarization (photo-CIDNP)—a powerful mechanism for enhancing nuclear spin sensitivity without microwave irradiation. Using an operator-based effective Hamiltonian approach, we derive precise resonance matching conditions and identify key dipolar scaling factors governing the photo-CIDNP Hamiltonian under both static and magic-angle spinning conditions. Our analytical formulation of coherent evolution of photoexcited singlet state exhibits strong agreement with numerical simulations, reinforcing the validity of our theoretical framework. By unraveling the intricate interplay of spin parameters in the radical-pair mechanism, our findings provide critical insights into optimizing photo-CIDNP efficiency and guiding the rational design of tailored molecular systems. The ability to develop highly efficient photo-CIDNP sensitizers marks a crucial step toward harnessing hyperpolarized nuclear magnetic resonance and magnetic resonance imaging, paving the way for next-generation advancements in biomedical imaging and materials science.

Energy dissipation in ensembles of catalytic Janus particles

The conversion of chemical energy into mechanical energy, which drives the motion of active particles, inherently involves energy dissipation. Dissipation plays a crucial role in transport efficiency, structure formation in self-organizing systems, and the thermodynamic properties of active particle suspensions. In this work, we present a thermodynamic analysis that derives the energy dissipation of coupled irreversible processes occurring in particles, substrate, and solvent. Dissipation in chemical reactions is examined under conditions where the reaction flux follows a nonlinear dependence on affinity, as described by the law of mass action. Our approach considers concentration-dependent reaction rates, in contrast to some previous descriptions of active particles, which assume a constant reaction rate and, consequently, a constant active velocity of the particles. We analyze entropy production for both cases, highlighting significant discrepancies and demonstrating that assuming a constant active velocity overlooks key thermodynamic contributions. Our framework provides a more accurate and self-consistent characterization of entropy production, capturing the inherent nonlinearities of active particle dynamics.