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Quantum state-to-state dynamics studies of the C(3P) + OH(X2Π) → CO(a3Π) + H(2S) reaction based on a new HCO(12A″) potential energy surface
Using the multi-reference configuration interaction method with the aug-cc-pV(Q/5)Z basis set, 7762 ab initio energy points were computed and used to construct an analytical potential energy surface (PES) for HCO(12A″). The fitted PES has an overall root-mean-square deviation of 0.833 kcal/mol. Based on this PES, the geometry, energy, and harmonic frequencies of stationary points were analyzed in detail, showing good agreement with other theoretical data. Subsequently, quantum time-dependent wave packet (TDWP) and quasi-classical trajectory (QCT) calculations were performed on this new PES to study the reaction C(3P) + OH(X2Π) → CO(a3Π) + H(2S). The reaction probabilities, integral cross sections, differential cross sections, product rot-vibrational distributions, and rate constants were obtained. TDWP results show a rich resonance structure, and the QCT calculations provide a qualitatively correct description of the reaction cross section. Discrepancies between the dynamical information derived from the two methods indicate pronounced quantum effects in this reaction. The reaction is predominantly governed by a complex-forming mechanism. Although increasing collision energy shortens the complex lifetime, the indirect mechanism remains operative across the studied energy range. This study provides fundamental insights into the microscopic reaction mechanisms and dynamics of carbon chemistry in interstellar environments and provides theoretical support for future experimental results.
Scaling transferable coarse-graining with mean force matching
Coarse-grained molecular dynamics often sacrifices accuracy and transferability for computational efficiency, but the use of machine-learned potentials helps coarse-grained models attain performance on par with atomistic molecular dynamics. Nevertheless, developing representations of the coarse-grained potential energy surface faces severe scaling challenges due to the extreme data demands of widely used “bottom-up” coarse-graining objectives. In this work, we show that mean force matching, a strategy for training thermodynamically consistent coarse-grained models, requires 50× fewer training samples, but obtains better accuracy in the potential of mean force for unseen proteins compared to other commonly used objectives. By systematically removing noise from the objective function, we demonstrate that it is possible to scale machine learning architectures for coarse-graining, enabling highly accurate and transferable models. We show the advantages of mean force matching both theoretically and through exhaustive benchmarking using thermodynamic consistency as the primary metric of accuracy.
Coherent biexciton transport in the presence of exciton–exciton annihilation in molecular aggregates
Biexciton dynamics in molecular aggregates provides a sensitive probe of the interplay between quantum coherence, band structure, and dissipation under strong excitation conditions. We present a theoretical framework for biexciton dynamics in molecular aggregates that explicitly treats populations and coherences across excitation manifolds within a reduced density-matrix formalism. By extending kinetic descriptions beyond the weak-coupling limit, the approach captures the influence of exciton delocalization and exciton–exciton annihilation while remaining computationally tractable within a Markovian description of environmental relaxation. Using this framework, we investigate how the spatial profile and momentum composition of the initial biexciton state govern fluorescence decay and transport. Incoherent initial conditions lead to strongly non-exponential relaxation and time-dependent diffusion driven by nonlinear population kinetics. In contrast, coherently prepared biexciton states exhibit pronounced early-time coherent transport, whose character depends sensitively on whether the initial state is prepared as a standing-wave or traveling-wave superposition of single-exciton modes. Despite nearly identical emission dynamics for J and H aggregates, biexciton transport properties differ markedly due to band structure-dependent interference effects. Our results demonstrate that biexciton dynamics remains strongly influenced by initial-state coherence and momentum composition. In addition to the initial-state preparation, the coherent-to-incoherent crossover and the diffusive spreading of the exciton density are sensitive to internal conversion processes such as exciton fusion and the decay to the first excited state. The present work establishes initial-state preparation as a key control parameter for many-exciton transport in excitonic systems and provides a general framework for interpreting nonlinear optical experiments beyond population-based descriptions.
Time-resolved ultrabroadband far-to-mid-infrared spectroscopy directly reveals doorway-mediated vibrational energy flow in an energetic crystal (β-HMX)
Understanding how vibrational energy flows from molecular vibrations to lattice phonons is critical for controlling the sensitivity of energetic molecular crystals. The intervening low-frequency doorway modes play a key role in this process, yet their ultrafast dynamics remain largely unexplored due to the lack of experimental access. Here, we overcome this limitation using time-resolved ultrabroadband far-to-mid-infrared spectroscopy, which allows us to directly monitor vibrational energy transfer (VET) involving doorway and lattice modes across the full 100–1700 cm−1 range in crystalline β-HMX—a spectral window inaccessible to conventional mid-infrared techniques. This broad range reveals a complete three-stage energy flow pathway: sub-picosecond vibrational energy redistribution among high-frequency modes, a 2–25 ps multi-stage VET network (vibrational → doorway → lattice), and final ∼140 ps thermalization. Doorway-mode-mediated pathways govern VET, with an efficient route from NO2 symmetric stretch (ν20) to short-lived doorway modes (ν8–ν10, N–NO2 in-plane bend) and then to the lattice. These results provide the first direct experimental validation of the doorway mechanism in an energetic molecular crystal and demonstrate the unique power of ultrabroadband infrared spectroscopy for revealing previously hidden vibrational channels.
Structural definition of water activity reveals near-ideal thermodynamic behavior in electrolyte solutions
Chemical activity is central to thermodynamics and, through the chemical potential, governs phase equilibria and transport. While activity appears formally in thermodynamics, it is typically accessed empirically through the activity coefficient or indirectly via free-energy differences. Here, we show that electrolyte solutions traditionally classified as strongly non-ideal exhibit near-ideal behavior when activity is expressed in terms of the appropriate microscopic species. Using molecular dynamics simulations, we define water activity as the quotient of unbound solvent molecules (e.g., free waters) to the sum of unbound solvent molecules and solvent–solute clusters, both identified directly from solvation structure. Applied to NaCl solutions, this framework quantitatively reproduces experimental activities over the entire solvation range without free-energy calculations or fitting parameters, which is the first time to our knowledge. These results establish a structural foundation for chemical activity and suggest that apparent non-ideality arises from an incomplete molecular description of mole fraction rather than a breakdown of ideal thermodynamics.
Modeling crossflow filtration: Effect of shear on particle-enriched polarization and cake layers
Crossflow filtration is a pressure-driven separation and enrichment process of colloidal particles, where a feed dispersion is continuously pumped through a hollow cylindrical membrane channel permeable to the solvent only. During the filtration process, particles are advected to and accumulate at the membrane, forming a fluid-like concentration polarization (CP) layer and a solid-like cake layer. Based on an accurate semi-analytic method developed in this paper, we determine spatially resolved flow and particle concentration profiles in the transition regime between ultrafiltration and microfiltration, where the effects of shear flow compete with Brownian particle motion. The results are presented for model dispersions of colloidal hard spheres, wherein we account for the shear-rate and concentration dependence of collective diffusion and viscosity, and for a thin, permeable filter cake. In particular, a non-monotonic shear-rate dependence of the axial flow across the CP layer is found. We show that the shear-rate dependence of particle transport properties gives insight into the origin of a long-standing apparent paradox related to the critical permeate flux characterizing the onset of cake formation.
Anomalous phase behaviors near the multiphase coexistence point in 1-alkyl-3-methylimidazolium ionic liquids
The complex phase behaviors of ionic liquids near the multiphase coexistence point (MCP) were examined using synchrotron small- and wide-angle x-ray scattering. The cations utilized in this study were 1-alkyl-3-methylimidazolium [Cnmim]+, where n represents the alkyl chain length, and the anions were Cl−, Br−, I−, and [NO3]−. In proximity to the alkyl chain length corresponding to the MCP (nMCP), crystal polymorphism or multiple phase transition pathways were induced under low-temperature (LT) or high-pressure (HP) conditions. Within the [C8mim][X] series, [C8mim]Cl and [C8mim]Br were crystallized under HP conditions. The multiphase coexistence in [C10mim]Br and [C10mim][NO3] was sensitively dependent on the heating rate and was explained by the generalized Gibbs phase rule. With increasing deviation from nMCP, the LT and HP phase behaviors of [Cnmim][X] considerably changed.
Microscopic origin of droplet line tension
The size dependence of the equilibrium contact angle of sessile droplets, commonly termed line tension, lies beyond classic Young’s law. Here, we identify a fundamental contribution to line tension arising from body gravity effects and surface-pressure effects within an adsorption layer. This mechanism resolves the multiscale behavior of droplets from nanometric to millimetric sizes, for which the apparent line tension changes sign and spans several orders of magnitude, consistent with existing experiments and simulations. The sign and magnitude are governed by surface wettability, the surface composition in the adsorption layer, and droplet size. Our results provide a unified physical interpretation of the experimentally observed variability in both the sign and magnitude of line tensions.
Infrared spectrum of HCl–O2 dimer and tentative assignment of HCl–(O2)3 tetramer
Spectra of the weakly bound dimer HCl–O2 were studied in the mid-infrared region of the HCl fundamental band (≈2890 cm−1) using a tunable optical parametric oscillator source to probe a pulsed supersonic slit jet expansion. This is the first reported gas-phase spectrum of HCl–O2 in any region. Over 180 H35Cl–O2 and H37Cl–O2 rotational transitions were assigned and interpreted in terms of four sub-states: a Σ ground state, a Π state at about 2.3 cm−1, another Σ state at about 3.3 cm−1, and another Π state at an unknown energy (possibly 1–2 cm−1). For each isotopologue, rotational energies for 75 levels in the ground and excited vibrational states [v(H–Cl) = 0 and 1] were derived directly from the spectra. Efforts to interpret the spectra using an existing theory of weakly bound dimers containing O2 were not very successful, probably due to the presence of large amplitude intermolecular motions. Spectra of two larger clusters, likely each containing one HCl and multiple O2 molecules, were also assigned. One of these spectra, is well simulated by a symmetric top molecule with B = 0.028 cm−1 and is likely due to the tetramer HCl–(O2)3. Although the other spectrum is not understood at all, its dependence on O2 concentration and apparent B-value (≈0.045 cm−1) suggest that it could be due to the trimer HCl–(O2)2.
Cooperative elastic mechanism of activated structural relaxation in glassy liquids
The dramatic slowdown of dynamics in deeply supercooled liquids remains a central problem in condensed matter physics, chemistry, and materials science. Within the inherent-structure framework, structural relaxation may be viewed as activated transitions between basins of the potential energy landscape via high-energy bottleneck configurations. The intermediate state can be modeled as a transient entropy droplet, representing a locally reconfigured region embedded in an amorphous solid, whose formation is driven by configurational entropy and opposed by elastic mismatch with the surrounding matrix. In this study, we develop an elasticity-based theory for the activation free energy associated with the formation of such a droplet. By incorporating intrinsic elastic heterogeneity and non-affine strain redistribution, we show that the mismatch energy is reduced through cooperative pathways involving softer regions of the material. This leads to an effective interfacial penalty that scales as R3/2 with droplet size, in contrast to the conventional R2 scaling expected for a homogeneous medium. The resulting free-energy balance yields activation barriers inversely proportional to the configurational entropy, thereby recovering the Adam–Gibbs relation for structural relaxation. While the same scaling is invoked in random first-order transition theory, it is derived here from elastic heterogeneity and non-affine deformation, providing a complementary mechanical interpretation of entropy-controlled dynamics in glassy systems.
Uniform distributions in nonuniform systems: Wall potentials generating constant density profiles in classical density functional theory
We study the inverse problem of classical density functional theory for inhomogeneous fluids: finding the wall potential that produces a constant equilibrium density profile, i.e., a perfectly flat density distribution in the accessible region adjacent to a substrate. Within Rosenfeld’s fundamental measure theory, we solve this problem for a one-component fluid in planar, spherical, and cylindrical geometries, considering both a hard-sphere fluid and a fluid with an additional truncated Lennard-Jones attraction treated at the mean-field level. Explicit analytical expressions are obtained for planar walls, while spherical walls also admit an analytical treatment in a more cumbersome form. The cylindrical case is treated numerically. The construction provides an explicit microscopic realization of structure-cancelling wall fields related to flat-profile conditions that occur under special matching conditions in interfacial theories of wetting and drying. The theory also yields a compact collection of formulas for weighted densities and one-body direct correlation functions in the three fundamental geometries, providing useful reference expressions for density-functional implementations. The resulting analytic wall potentials are validated in independent density functional calculations, which confirm that the prescribed flat profiles are recovered within numerical accuracy.
Exploring nanographene for single molecule imaging at cryogenic temperatures
Imaging single fluorescent molecules at cryogenic temperatures can increase photon budgets by suppressing photobleaching and non-radiative loss. Combined with rapid-freezing vitrification, it enables correlative cryo-fluorescence and electron microscopy. Obtaining a wide set of fluorophores with suitable blinking characteristics at cryogenic temperatures has remained a challenge. Nanographenes are self-blinking fluorophores that could fill this gap, yet their low-temperature intermittency remains largely unquantified. Here, we characterize the blinking and photon output of single dibenzo[hi,st]ovalene (DBOV-azide) nanographene fluorophores on glass from 91 to 293 K over excitation irradiances of 1.2–5.9 kW cm−2. On/off fluorescence states are extracted from wide-field images using a generalized likelihood ratio test, with simulation-based validation to reduce false detections under high-background conditions. We find that DBOV-azide blinks at all temperatures, with mean on-times that decrease with increasing temperature (longer at 91 K than at room temperature) and that further shorten with increasing irradiance at 91 K. For short time scales (t < 1.5 s), on- and off-time distributions follow power laws with exponents −1.6 to −1.2 that show no systematic dependence on temperature or irradiance. The mean on/off ratio drops by ∼10× from 91 to 293 K, indicating a worse duty cycle at lower temperature. Photon output increases with irradiance and is higher at cryogenic temperatures, with an order-of-magnitude increase in photons per on-event compared to room temperature.
Intrinsic Raman scattering activity of OH and OD oscillators in liquid H2O, D2O, and HOD: Roles of zero-point energy and vibrational coupling
Liquid water is a strongly associated and vibrationally delocalized system in which nuclear quantum effects (NQEs), particularly zero-point energy and vibrational anharmonicity, influence the structure and dynamics. Here, we examine the roles of NQE and vibrational coupling on the intrinsic Raman scattering activity of the OH (σH) and OD (σD) oscillators in liquid H2O, D2O, and HOD using polarized Raman spectroscopy. For neat H2O and D2O, σHisoσDiso = 1.35 ± 0.02, and on isotopic dilution, it decreases to 0.9 ± 0.08. This arises from a symmetric 20% drop in σHiso and 20% rise in σDiso, revealing quantitative intensity redistribution via OH-OD vibrational coupling in isotopically diluted water. In other words, even in HOD, the OH and OD stretch modes are not fully decoupled vibrationally. The corresponding ratio for the anisotropic component is σHanisoσDaniso = 1.3 ± 0.05, and it remains unchanged upon isotopic dilution. The experimental σHanisoσDaniso = 1.3 ± 0.05 closely matches the quantum harmonic oscillator predicted ratio of 1.37 corresponding to comparable polarizability derivatives in H2O and D2O. Thus, ∼30% higher scattering activity of OH primarily arises from reduced mass dependent zero-point vibrational amplitude rather than anharmonicity-driven changes in polarizability.
The entropic barrier around the conical intersection seam
Conical intersections (CIs) are seen as the main mediators of nonadiabatic transitions; yet, mixed quantum–classical (MQC) simulations that treat nuclei as classical point particles rarely, if ever, sample geometries with exactly degenerate electronic energies. Here, we show that this behavior arises from a fundamental statistical–mechanical constraint on classical nuclear motion. Using a linear vibronic coupling model, we derive the free energy along the adiabatic energy gap and demonstrate analytically that as the gap approaches zero, the free energy diverges around the CI seam. Molecular dynamics simulations of the methaniminium cation on the S1 surface confirm this prediction: trajectories can approach regions with small adiabatic gaps, but never reach the CI seam, even if the CI corresponds to a region of lowest potential energy. These results clarify why MQC methods successfully capture nonadiabatic behavior without sampling exact degeneracies and agree with recent findings that classical trajectories can sense the presence of CIs without visiting them.
Metallic hydrogen confined by graphene
The metallization of hydrogen is a prerequisite for the emergence of superconducting properties, along with those of hydrides. Chemical precompression has enabled a class of high-transition-temperature superconductors based on hydrogen-rich hydrides, whereas direct metallization of molecular hydrides remains challenging. In this study, we demonstrate the metallization of hydrogen confined within fixed two-dimensional graphene channels, which provide a nanoscale environment for effective compression. The confined hydrogen forms compressed molecular hydrides whose intermolecular H–H separations are significantly reduced compared to those in ambient molecular hydrogen. Electronic structure calculations reveal that all the hydrogens in both stacks exhibit metallic properties. The Fermi surface of both stacks exhibits a peculiar nesting pattern, suggesting enhanced electron–phonon coupling. These results identify graphene-confined hydrogen (graphene–H6) systems as a promising platform for achieving metallic hydrogen and potentially low-pressure, high-Tc superconductors.
Spectroscopic identification and structural characterization of Ag2H2− by mass-selected photoelectron velocity-map imaging
Ag2H2− is identified and characterized by photoelectron velocity-map imaging and quantum chemical calculations. High-resolution mass selection unambiguously confirms the formation of Ag2H2−, yielding a vibrationally resolved spectrum with a distinct progression assigned to the Ag–H stretching mode. Comparison between experiment and theory shows that Ag2H2− adopts a distorted Y-shaped geometry with one bridging H and one terminal H, while for neutral Ag2H2, this geometry is a local minimum. Further bonding analysis of the corresponding neutral counterpart reveals that the unusually low Ag–H stretching frequency originates from synergistic donation and back-donation between the two AgH subunits.
Multiconfigurational Gaussian wavepacket simulations of exciton diffusion in semiconducting polymer chains: Efficient finite-temperature simulations with Langevin driving
First-principles quantum-dynamical simulations of photoinduced exciton dynamics are carried out using the variational two-layer Gaussian-based multiconfiguration time-dependent Hartree (2L-GMCTDH) method combined with stochastic Langevin dynamics. Analogously to earlier reference calculations [Binder and Burghardt, Faraday Discuss. 221, 406 (2020)], a generalized Frenkel–Holstein Hamiltonian is constructed for a 20-site oligothiophene chain as a minimal model for intra-chain exciton migration in poly-(3-hexylthiophene) (P3HT). Here, exciton quasi-particles undergo polaronic trapping due to local high-frequency modes, while transport is induced by thermal driving due to ring-torsional modes. It is shown that the 2L-GMCTDH simulations provide a highly flexible and efficient framework where the use of multiple explicit local reservoirs can be replaced with multiple Langevin thermostats. The computation of temperature-dependent exciton diffusion coefficients is illustrated, along with the dependence on static disorder.
Coherent nonlinear optical probes for cavity-dressed vibrational mode mixing: Multidimensional double-quantum coherence and photon-echo spectroscopy
Cavity dressing of molecular vibrational dynamics expands the role of characteristic vibrations as spectroscopic markers of underlying ultrafast dynamics. Interacting vibrational modes exhibit pronounced excited state delocalization due to the interaction with the cavity mode, which is reflected in the ultrafast dynamics. We characterize the ultrafast dynamics of cavity-dressed characteristic vibrations, namely vibrational polaritons, in the presence of dissipation and collective vibrational nonlinearity. Specifically, we present two complementary three-pulse coherent multidimensional spectroscopic techniques for monitoring the dynamics of cavity-dressed one- and two-quantum vibrational excitations. A microscopic theory that includes low- and high-energy phonon modes consistently describes the dissipative dynamics, including population relaxation and dephasing. The cavity coupling strengths are comparable to the vibrational mode couplings, suggesting the possibility of controlling intermolecular vibrational energy redistribution. This simulation framework is extendable to various cases of cavity-controlled nonlinear spectroscopies of dissipative molecular systems, including large ensembles.
Scalar machine learning of tensorial quantities—Born effective charges from monopole models
Predicting tensorial properties with machine learning models typically requires carefully designed tensorial descriptors. In this work, we introduce an alternative strategy for learning tensorial quantities based on scalar descriptors. We apply this approach to the Born effective charge tensor, showing that scalar (monopole) kernel models can successfully capture its tensorial nature by exploiting the definition of the Born effective charge tensor as the derivative of the polarization with respect to atomic displacements. We compare this method with tensorial (dipole) kernel models, as established in our previous work, in which the tensorial structure of the Born effective charge is encoded directly in the kernel and obtained via its derivative. Both approaches are then used for charge partitioning, enabling the separation of monopole and dipole contributions. Finally, we demonstrate the effectiveness of the framework by computing finite-temperature infrared spectra for a range of complex materials.
Exact tunneling splittings from path-integral hybrid Monte Carlo with enveloping bridging potentials
A path-integral hybrid Monte Carlo approach with enveloping bridging potentials (PIHMC-EBP) is proposed for calculating numerically exact tunneling splittings in molecular systems. The central idea is to construct an approximately barrierless bridging potential that smoothly connects symmetry-related regions of ring-polymer phase space, enabling direct sampling of the free-energy profile from which the relevant splittings are obtained. Two tailored nonlocal updates are designed to enhance the sampling of slow collective motions. Compared with path-integral molecular dynamics using thermodynamic integration, PIHMC-EBP obviates the need for quadrature and time step convergence checks, thereby substantially reducing the manual effort required to analyze the results. Applications to malonaldehyde (and its deuterated isotopologue) and the HCl dimer using state-of-the-art potential energy surfaces (PESs) provide the most precise tunneling splittings reported to date for both systems, while simultaneously reducing the overall computational cost by several times and three orders of magnitude, respectively. Finally, application to the water dimer yields the first numerically exact path-integral calculations of the ground-state tunneling splittings on three different PESs, all obtained simultaneously by reweighting a single set of trajectories.