Browse Articles

Orbital angular momentum textures and currents in a discrete helix: Equilibrium and linear response

Recently, nonequilibrium orbital angular momentum in low-dimensional systems has attracted renewed attention. Here, we introduce a minimal three-orbital tight-binding model for a single helical chain and show that chirality alone generates a momentum-dependent orbital-angular-momentum texture through Slater–Koster hybridization in the local basis (pr, pϕ, pz), without requiring atomic spin–orbit coupling. In the single-helix geometry, the radial orbital texture vanishes identically, while the azimuthal and longitudinal components remain finite and arise from the odd-in-momentum (pz, pr) and (pr, pϕ) sectors. As a result, the equilibrium average orbital texture vanishes by parity, although persistent-like orbital angular momentum currents may still exist and imply chirality-dependent end magnetization in a finite helix. Under an applied longitudinal electric field, the system develops a finite orbital Edelstein response, whereas the projected longitudinal orbital conductivity vanishes in the linear regime by parity. When spin degrees of freedom are included, the orbital texture acts as a source of spin polarization through orbital-to-spin transduction. Instead of being limited by the weak, natural spin–orbit coupling of individual atoms, the overall spin response is driven by the much stronger interactions of overlapping molecular orbitals, making it a stronger candidate for spin injection than the conventional spin Edelstein mechanism. These results identify chirality as the minimal microscopic ingredient for generating orbital angular momentum response in one-dimensional systems and support an orbital route to spin selectivity in chiral conductors.

Revisiting the electronic structure of PtO: Cryogenic photoelectron spectroscopy and relativistic insights

We investigate the electronic structure of PtO via cryogenic anion photoelectron spectroscopy combined with relativistic multireference calculations, clarifying long-standing ambiguities in the assignment of its low-lying electronic manifold. Vibrationally resolved spectra covering the low-lying states are obtained via 193 nm photodetachment and 400 nm velocity map imaging (VMI) of PtO− anions cooled to 13 K. The adiabatic and vertical detachment energies are measured to be 2.16 and 2.29 eV, respectively, in good agreement with relativistic SOC-NEVPT2 predictions within 0.12 eV. High-resolution VMI reveals a fine-structure splitting of 1008 ± 30 cm−1 in the X3Σ− ground state, quantitatively reproduced by SOC-NEVPT2 as a 959 cm−1 second-order spin–orbit splitting. The two components display nearly identical vibrational structure, confirmed by Franck–Condon simulations. At higher binding energy, an intense σ-detachment feature is observed at 3.27 eV, providing a decisive spectroscopic anchor for reassessing the electronic manifold. Together with theoretical results, this feature establishes that the band at 2.73 eV corresponds to the A1Δ singlet state rather than to a widely split 3Π-state manifold, as previously proposed. These results establish a revised, experimentally benchmarked electronic structure of PtO and demonstrate that cryogenic cooling combined with high-energy photodetachment is essential for resolving spin-orbit-coupled manifolds in heavy transition-metal oxides.

Bioelectrical regionalization of multicellular aggregates by microRNAs

We theoretically explore how microRNAs (miRNAs) can modulate the expression of ion channel proteins and the resulting cell membrane potentials in multicellular aggregates. To this end, we simulate the spatiotemporal regionalizations that characterize experimentally instructive bioelectrical states. Instead of focusing on a particular biological system, we consider a simple but illustrative biophysical model for the interplay between genetics and bioelectricity at the single-cell level. At the multicellular level, the assumed intercellular connectivity can establish spatiotemporal patterns of coupled bioelectrical and transcriptional states. The resulting distributed control exerts a significant influence on protein expression in multicellular aggregates, establishing instructive maps for development and regeneration. The system dynamics are simulated in spatially inhomogeneous aggregates under different miRNA production rates and intercellular connectivity. The results allow a qualitative understanding of the context-dependent effects observed when signaling biomolecules are spatially regionalized and transferred through intercellular junctions under the influence of multicellular electrical potentials.

Singlet fission spin dynamics from molecular structure: A modular computational pipeline

Singlet fission (SF), which has applications in areas ranging from solar energy to quantum information, relies critically on transitions within a multi-spin manifold. These transitions are driven by fluctuations in the spin–spin exchange interaction, which have been linked to changes in nuclear geometry or exciton migration. While simple calculations have supported this mechanism, to date, limited efforts have been made to model realistic fluctuations, which are informed by the actual structure and properties of physical materials. In this paper, we develop a modular computational pipeline for calculating SF spin dynamics by way of electronic structural calculations, molecular dynamics, and numerical models of spin dynamics. The outputs of this pipeline aid in the interpretation of measured spin dynamics and allow us to place constraints on geometric fluctuations, which are consistent with these observations.

The rocky path of DFT into chemistry—Discussions at a symposium and reflections on a circular journey in honor of Axel Becke 1953–2025

We summarize a recent symposium on density functional theory for electronic structure in chemical applications and honor a key contributor, Axel Becke. The theory of electronic structure is entirely quantum mechanical and, in its original wave function form, runs into mathematical and numerical problems due to high spatial dimensionality, as each electron adds three new coordinates. The approximate reduction of dimensionality in the form of density functional theory (DFT) was first proposed in the mid-1920s but took 40 years to be verified as possible in principle. The development of increasingly accurate functionals, expressing the electronic energy in terms of the density of the electrons, was initially driven by the physics community, but after a series of innovations, including (i) going from local to non-local exchange–correlation energy functionals and (ii) partial retention of exact exchange in terms of wave functions, there was a breakthrough in the early 1990s, and leading quantum chemists accepted DFT. The resulting improved ability to solve chemical problems by computation was rewarded with a Nobel Prize to Walter Kohn and John Pople in 1998. Given the many controversies surrounding the introduction of DFT into chemistry, a symposium was held at the Royal Swedish Academy of Sciences on November 7 and 8, 2024, to look back at the barriers and breakthroughs and identify the key contributors and their advancements. Axel Becke played a major role in bringing DFT into chemistry. We record the unique breadth and depth of the discussions at the symposium and the role of Becke, whose life ended a year thereafter.

Δ-learning for transferable machine learning interatomic potentials

Machine-learning interatomic potentials (MLIPs) trained by directly learning the total interatomic interaction energies can suffer from limited transferability, unphysical behavior beyond a finite cutoff, and large errors for out-of-distribution geometries such as transition states and uncommon conformers. We evaluate Δ-learning (delta-learning) as a remedy by training an ANI-style high-dimensional neural network (HDNNP) potential as a correction to predict PBE0/aug-cc-pVTZ energies from a third-order tight-binding density functional theory (DFTB3) baseline model. On a held-out test set derived from the modified ANI-1x training set, the Δ-learning model (named ANIDFTB-Δ) achieves a mean absolute error (MAE) of 0.82 kcal/mol, while the HDNNP model (ANIPBE0-Direct) has an MAE of 2.01 kcal/mol. On selected GMTKN55 benchmarks, the Δ model systematically improves relative energies for conformers and tautomers and avoids catastrophic outliers on challenging structures. The good physical description of DFTB3 significantly reduces the error in proton-transfer transition states in the PX13 benchmark and intermolecular interactions in the DES370K dataset in comparison with reference target PBE0. The long-range electrostatics in DFTB3 also partially correct the long-range behavior of local descriptor MLIPs outside of their predetermined cutoff, reducing the MAE from 0.098 to 0.019 kcal/mol. The computational cost of this method is incrementally more expensive than DFTB3 alone, making it practical for extensive simulations of moderately sized systems, although the DFTB3 step makes the Δ model much more costly than a simple HDNNP alone.

Harmonic-oscillator-referenced ring-polymer molecular dynamics. II. Theoretical foundations of reference-modified dynamics and Gaussian reconstruction

The Paper I [H. Wang, J. Chem. Phys. 165, ■ (2026)] introduced harmonic-oscillator-referenced ring-polymer molecular dynamics (HO-RPMD) as a practical framework for approximately computing linear and nonlinear real-time quantum correlation functions. The present study develops the corresponding theoretical foundation in greater detail. The method is organized around three coordinated ingredients: exact harmonic-reference sampling in imaginary time, centroid-anchored real-time propagation, and Gaussian reconstruction of nonlinear observables from an anchored linear kernel together with sampled static information. We show that these elements are structurally linked. Once the quantum Boltzmann operator is represented by a harmonic reference kernel rather than by the primitive free-particle Trotter factorization, the centroid component of the real-time dynamical generator cannot generally be left unchanged without shifting the harmonic oscillation frequency. Likewise, beyond linear centroid observables, a direct nonlinear ring-polymer correlation function is no longer naturally protected from contamination by the internal fluctuation modes, and an explicit reconstruction step is therefore needed. We formulate these points at the operator, path integral, normal mode, and observable reconstruction levels and derive the finite-P reference distribution, the centroid-anchoring condition, and the Gaussian reconstruction logic. The resulting theory clarifies the relation of HO-RPMD to standard RPMD, centroid molecular dynamics, symmetrized/Wigner-type correlation-function formulations, and Gaussian moment-reconstruction ideas. It also makes clear that, beyond the harmonic limit, the natural practical nonlinear target of the present framework is the canonical hybrid correlation function, while nonlinear Kubo quantities remain primarily useful as harmonic benchmarks.

Publisher’s Note: “Reentrant phase transition in pH-responsive microgel suspensions” [J. Chem. Phys. 165, 024903 (2026)]

Living helices in fluctuating polymer chains: Cooperative nucleation and dynamics

Helical segments in polymer chains are often transient, finite, and dynamically evolving, yet their origin and stability remain incompletely understood. Here, we develop a minimal coarse-grained statistical-mechanical theory that explains how such “living helices” emerge in fluctuating polymer systems. Using a three-state model with cooperative interactions, we show that helix formation proceeds through a multistep nucleation mechanism. An initial constrained pre-nucleus forms first, followed by cooperative stabilization that promotes the growth of finite helical segments. The resulting free-energy landscape naturally favors marginally stable helices whose size is determined by a competition between cooperative gains and nonlinear penalties arising from stiffness, torsional strain, and solvent fluctuations. By formulating the dynamics as a stochastic process in segment size, we derive analytical expressions for both formation times and lifetimes within a mean first-passage framework. For representative parameters relevant to flexible polymers and peptide segments, the theory predicts characteristic timescales in the nanosecond to sub-microsecond range. These results provide a unified physical picture of “living helices” as finite, mobile, and fluctuating excitations and identify cooperativity and fluctuations as the key determinants of transient secondary structure in polymeric systems.

Publisher’s Note: “A microscopic theory of small-droplet adhesion on solid surfaces” [J. Chem. Phys. 165, 024707 (2026)]

Interfacial thermal conductance between a polyethylene glycol polymer chain and water: A molecular dynamics study

Understanding interfacial heat transfer between polymers and water is crucial for the design of biomaterials, drug delivery platforms, and nano-fluidic systems. In this study, we employed all-atom molecular dynamics (MD) simulations to quantify the interfacial thermal conductance between an infinitely diluted polyethylene glycol (PEG) 36-mer chain and explicit water over the temperature range of 280–350 K. To compare the conformational behavior of the PEG chain, we examined its radius of gyration and observed a temperature-dependent chain collapse consistent with previous coarse-grained models. By employing a transient non-equilibrium MD approach, we imposed temperature difference across the interface and analyzed the energy relaxation behavior to compute heat transfer across the polymer–water interfaces. Moreover, we investigate the impact of polymer conformation on heat transfer by considering modulations of the Lennard-Jones polymer/solvent interactions, different from the original PEG–water interactions. Our results demonstrate that both temperature and Lennard-Jones interfacial interaction strength influence interfacial thermal conductance, with temperature playing the dominant role. Structural factors such as chain conformation and interfacial area were found to mediate the effect of interfacial interaction. Additional analysis of the vibrational density of states and the mean square displacement reveal that vibrational coupling has minimal impact on thermal conductance across interfaces, whereas increased water thermal motion enhances energy transfer. These findings highlight the structural and dynamical origins of interfacial thermal conductance and provide atomistic insights into the tuning of interfacial heat transport in molecular systems through temperature and solvent interactions.

The transfer tensor method: An analytical study case

The transfer tensor method (TTM) is a versatile tool for analyzing and propagating general open quantum systems. It captures in a compact manner all memory effects in a non-Markovian system through a straightforward transformation of a set of dynamical maps. Transfer tensors (TTs) provide the exact convolutional propagator associated with a given time discretization over the past evolution of an open quantum system. Here we show that, for any finite time discretization, the memory kernel of the Nakajima–Zwanzig equation deviates from the exact TTs, although both converge in the continuous-time limit, as expected. We examine this behavior in the context of an analytically solvable model: a two-level atom resonant with a lossy cavity in the Jaynes–Cummings limit. The atomic dynamics separate into two decoupled subspaces—the coherence and the population difference. We derive exact expressions for the dynamical map, the TTs, and the memory kernel governing the coherence, and we relate them to their counterparts for the population difference. As a function of the ratio between the cavity loss rate and the atom-cavity coupling strength, we identify regions of enhanced non-Markovianity in which the dynamics can be described as fully Markovian for certain TTM time-step choices.

On intermolecular interactions in the Hamiltonian used in polaritonic chemistry

Experiments have shown that strong coupling between molecular excitations and a mode of a Fabry–Pérot cavity can significantly alter molecular properties, such as reaction rates and equilibrium constants. However, in spite of the large body of theoretical work, the mechanism behind this change is still not well understood. In order to make progress, we first take a step back and investigate the appropriateness of the Hamiltonian that most recent studies are based on. In particular, we investigate the dipole self-energy, which can be divided into self-terms and cross-terms. While the self-terms are an indispensable part of the Hamiltonian, the cross terms—which have received attention as they seem to mediate distance-independent interactions between all molecules in the cavity—are known to, under certain conditions, cancel exactly with the usually neglected intermolecular Coulombic interactions. In this study, we revisit how this cancellation comes about in free space and in a perfect cavity, clarifying that it can only be found when looking beyond the single-mode approximation and taking the full continuum of light modes into account. We also provide numerical evidence suggesting that this cancellation may extend to the case of an imperfect cavity and show how the situation changes for a more realistic cavity in the framework of macroscopic quantum electrodynamics. Finally, we discuss the implications of this cancellation for the single-mode Hamiltonian.

q-vSZPs: A polarized, adaptive minimal Gaussian basis set for the elements <i>Z</i> = 1–86 designed for efficient mean-field electronic structure calculations

We present q-vSZPs, a polarized, environment-adaptive minimal Gaussian basis set parameterized for elements Z = 1–86 (excluding lanthanides) and designed for efficient mean-field quantum-mechanical calculations on large molecular systems. Building on the previously introduced q-vSZP basis set, the number of primitive functions is substantially reduced while retaining the key feature of charge- and coordination-number-dependent contraction coefficients, which enable environment-dependent “breathing” of the atomic orbitals. The effective atomic charges required for the basis set setup are obtained from the bond-capacity electronegativity equilibration model (EEQBC), which accurately reproduces density functional theory (DFT)-level Hirshfeld charges at negligible computational cost. Comprehensive benchmarking on the GMTKN55 database demonstrates that q-vSZPs incurs only a moderate loss in accuracy relative to q-vSZP (WTMAD-2 increase of 2.9 kcal⋅mol−1), while substantially outperforming conventional non-adaptive minimal basis sets (MBs). For non-covalent interactions, the performance even surpasses that of the double-ζ def2-SVP basis set, owing to a substantially lower basis set superposition error as a consequence of the molecular optimization strategy. A non-adaptive variant using averaged charges and coordination numbers (qavg-vSZPs) is additionally provided as a drop-in minimal basis for any quantum-chemical program. For a representative 4108-atom system, q-vSZPs achieves a twofold reduction in both SCF wall time and memory consumption for DFT calculations relative to q-vSZP, placing it in the efficiency range of other MBs. The utility of q-vSZPs is further demonstrated through a mixed-basis application for the computation of the Raman spectrum of adenine in water.

Unbiased exploration of the transition region in ice nucleation using the <i>NpH</i> ensemble

In this work, we employ the isenthalpic–isobaric (NpH) ensemble to sample the transition region of ice nucleation without any external bias, thereby avoiding potentially artificial memory effects introduced by projections onto collective variables. Within this framework, we identify relevant degrees of freedom that expose the intrinsically non-Markovian nature of the largest nucleus size, indicating that it is not sufficient on its own to describe nucleation dynamics. The NpH ensemble leads to long-lived nuclei through the coupling between latent heat release or absorption and temperature fluctuations. As a result, nuclei persist over extended timescales and undergo a slow internal evolution, which we refer to as aging. A signature of this behavior is the emergence of hysteresis in the largest cluster size–temperature plane. To quantify these effects, we perform a structural analysis based on a high-dimensional set of descriptors, which we project onto a low-dimensional latent space using a neural network-based autoencoder. This approach reveals the existence of structurally distinct classes of nuclei with similar sizes and temperatures. Finally, we compare the nucleus sizes obtained with this approach with those from previous studies employing different methodologies, finding good agreement.

Para–ortho H2 conversion in collisions with NO: A surprising mechanism

Molecular hydrogen exists in two kinds, para-H2 and ortho-H2, which act as distinct molecules in many physical and chemical processes. Their interconversion is extremely slow but can be catalyzed by collisions with paramagnetic molecules such as O2 and NO. Temperature-dependent rate coefficients for para–ortho H2 conversion in gas-phase collisions with O2, NO, and NO2 were measured in 1933 in a beautiful series of experiments by Farkas and Sachsse [Z. Phys. Chem. B 23, 1–18 (1933)]. A complete and quantitative theoretical study of the conversion rate coefficients in collisions with O2 was published recently [X. Zhang et al., Nat. Sci. 1, e10002 (2021)]. The present paper describes a similar, although more difficult, study of para–ortho H2 conversion in collisions with NO. Not only does it yield conversion rate coefficients in good agreement with the experimental data, but it also explains the surprising finding that collisions with NO are substantially more effective in converting para-H2 into ortho-H2 than collisions with O2. It shows, moreover, that the effectivity of NO is caused by a completely unexpected mechanism: coupling with the transient magnetic dipole of NO generated by the transition of its excited spin–orbit state to the ground state in collisions in which para-H2 near-resonantly converts into ortho-H2.

Toward viable H2 storage in Ca decorated low-dimensional materials with insights from reference quantum Monte Carlo

Hydrogen technology is set to be a key energy alternative for mitigating pollution and reducing CO2 emissions. However, the current storage mechanism of hydrogen molecules in carbon fiber tanks detracts from the fuel economy of hydrogen in mobile applications, necessitating the development of alternative storage mechanisms. Adsorbing hydrogen in its molecular form (H2) under typical operating conditions of proton exchange membranes can potentially meet storage requirements. However, H2 is the smallest molecule with only two electrons and, therefore, it has very limited propensity to physisorb in a material within the binding energy window of −0.2 to −0.4 eV that is suitable for storage. Calcium atom decorators on graphene have previously shown promise for tunable H2 binding, but the system is thermodynamically unstable toward the formation of calcium hydride. Moreover, the absolute adsorption of H2 is challenging to predict accurately and is typically overestimated with van der Waals inclusive density functional approximations. In this work, we perform state-of-the-art fixed-node diffusion Monte Carlo alongside a selection of density functional approximations for two strategies of anchoring Ca: (i) Ca on boron-doped graphene and (ii) Ca inside carbon nanotubes. We predict reliable Ca and H2 binding energies and establish that Ca is anchored inside carbon nanotubes and on boron-doped graphene, while boosting the H2 adsorption energy. Importantly, the H2 adsorption energy is found to be improved by the anchoring strategies, with the energy inside a Ca decorated carbon nanotube reaching the viable storage window. The reference DMC binding energies provide much-needed benchmarks for developing data-driven methods and guiding experiment in the systematic design of hydrogen storage materials.

Exploring flat multi-minima potential energy surface by matrix isolation IR spectroscopy: Revisiting methanol–H2S complex

An extensive potential energy surface search of the methanol–H2S binary complex reveals four distinct conformers (two each of S–H⋯O and O–H⋯S H-bonded) within a small energy window (binding energies within −2.9 to −3.3 kcal mol−1). The four conformers are interconvertible via rotation along the C–O bond of methanol and the S–H bond of H2S, with very small rotational barriers (&amp;lt;0.5 kcal mol−1). All four stable methanol–H2S complexes have been experimentally identified for the first time in cold and solid argon and nitrogen matrices via spectral signatures in νO−H and νS−H regions, while an earlier study in a molecular beam could identify only the global minimum structure. To further investigate matrix effects, quantum chemical calculations were carried out for the methanol–H2S complex embedded within an argon environment modeled as a face-centered cubic lattice, mimicking the solid matrix conditions. The rotational barriers were found to increase appreciably inside the model (&amp;gt;1.5 kcal mol−1). Rate constants for conformational interconversions revealed that populations in local minima rapidly transfer to the global minimum at low temperatures, while the same become prohibitively slow inside the model. This prediction aligns well with the trapping of hitherto unidentified local minima conformers in a matrix environment.

Validating orbital-optimized linearized coupled-cluster theory for high-accuracy thermochemistry and spectroscopy of prototypical π-systems: Ethylene and its ions

High-level ab initio estimates for the equilibrium structure (rCC = 1.330 79 ± 0.000 31 Å, rCH = 1.080 75 ± 0.000 21 Å, and αCCH = 121.423° ± 0.023°), the adiabatic ionization energy (10.52 eV), and the 0 K proton affinity (673.2 kJ/mol) of ethylene were obtained via focal-point extrapolation of coupled cluster results to the basis set limit. This methodology also provided accurate equilibrium structures for the ethylene radical cation (ionized ethylene) and the ethenium cation (bridged protonated ethylene). An additive correction scheme allowed for the prediction of the vibrational spectrum of ethylene with a root-mean-square error of 2.4 cm−1. A systematic evaluation of popular wavefunction-based methods revealed that the recently implemented orbital-optimized linearized coupled cluster (OLCCD) method predicts the geometry and properties (the ionization potential, the proton affinity, and the fundamental frequencies) of ethylene significantly better than the orbital-optimized Møller–Plesset (OMP2 and OMP3) and the standard coupled cluster with singles and doubles methods. Notably, where coupled-cluster with single, double, and perturbative triple excitations analytic Hessians suffer from wavefunction instability issues along several normal modes, OLCCD yields superior vibrational anharmonicities, making it a robust alternative for challenging π-systems.

Infrared spectroscopy of H3+ and Hn+ ions in helium nanodroplets

This work reports the infrared laser spectroscopic study of the H3+ and Hn+ (n = 5, 6, 7, 9) ions in helium droplets. The ions were produced upon electron impact ionization of helium droplets and charge transfer to small hydrogen clusters formed within the He droplets. The spectrum of H3+ shows a well-resolved rotational structure, indicating a small (∼8%) decrease in the rotational constants as compared to the gas-phase values. This small change is rationalized in terms of the small anisotropy of the H3+ interaction potential with He atoms. However, it was found that the interaction with the helium environment has a profound effect on the spectrum of the fluxional H5+ ions. Whereas the previously measured gas-phase spectrum has well-separated bands, the spectrum of H5+ in helium droplets shows a broad spectrum extending over 2000 cm−1 containing some maxima with frequencies close to those of the gas phase. The bands of H7+ and H9+ due to vibrations of their H2 moieties were also observed near 4000 cm−1. The structure of these ions could be treated as an H3+ core solvated in H2 molecules and He atoms. The spectrum of larger Hn+ clusters is also reported.