Browse Articles

Three-slab model for the dielectric permittivity of a lipid bilayer

A model for the tensorial dielectric permittivity of phospholipid membranes is presented here. The four-nanometer-thick membrane is treated as a composite made up of three dielectric slabs: one for each of the two phospholipid head-group regions and one for the entire domain spanned by the lipid tails. Equal and opposite bound surface charge densities surround each head-group slab and account for the membrane dipole potential. Three-slab model parameters are obtained from molecular dynamics simulations and capture both the zero-field electric potential and the membrane response to applied electric fields. The tail region is well-approximated as having vacuum permittivity, while the head-group region is highly anisotropic due to the configurations of molecular dipoles. For the bilayers studied, the out-of-plane permittivity of the head-group region is 10–15 times that of the vacuum, while the in-plane permittivity is an order of magnitude larger. Membrane responses to applied electric fields up to 30 millivolts per nanometer are found to be in the linear regime. The model overcomes a fundamental limitation of microscopic theories—where the out-of-plane permittivity lacks a meaningful continuum interpretation in the head-group region due to large gradients in the local electric field—by averaging over slab widths, thereby introducing new length scales. Our approach can be extended to characterize interfacial systems with similar microscopic permittivities.

Vibrational carbon character analysis of astronomical PAHs

Emission features of astronomical spectra typically attributed to aromatic and aliphatic portions of polycyclic aromatic hydrocarbons (PAHs) should not be so clearly binned as previously ascribed. Vibrational carbon character analysis (VCCA) herein shows that the 3.4 μm aromatic infrared emission band (AIB), typically labeled as “aliphatic,” possesses more than 45% non-aliphatic (i.e., aromatic) character for the test case of indene, the smallest astronomically detected PAH. Significant non-aliphatic character is seen as well for the 6.85 and 7.25 μm emission bands of indene, which are also considered to be markers of aliphaticity. VCCA predicts similar behavior for all three emission bands in the case of 2-ethynyltoluene, the proposed interstellar precursor of indene. Although the typical aromatic emission bands (i.e., 3.3 and 6.2 μm bands) exhibit slight aliphatic character for both indene and 2-ethynyltoluene, these bands are primarily aromatic. While these are only two example molecules, if significant character mixing occurs for the emission bands of most potential AIB carriers, current aliphatic/aromatic AIB attributions must be reconsidered.

Assessing the influence of <i>d</i> -orbital radius on formation of localized photogenerated states in corundum metal oxides

Photogenerated polarons are fundamental to the photophysics of transition metal oxide semiconductors. It is, therefore, imperative to understand the mechanisms by which polarons form upon photoexcitation of transition metal oxides to realize their potential in photoapplications. Hematite (α-Fe2O3) is known to form photoexcited small polarons, which limit its performance as a photoelectrocatalyst for water oxidation. Here, we report a systematic comparison of the electronic, optical, and vibrational properties of hematite to those of other metal oxides in the corundum crystal family that elucidates the impact of d-orbital radius on carrier–phonon coupling. Three corundum metal oxides - α-Al2O3 (no d-electrons), α-Fe2O3 (3d), and α-Rh2O3 (4d) - are analyzed with a combined approach of resonance Raman spectroscopy, thermal difference optical spectroscopy, and computational modeling of electronic and vibrational states. We find that the Raman spectrum of α-Al2O3 does not change as the Raman excitation is varied across the visible region, as there is no optical absorption. In contrast, both α-Fe2O3 and α-Rh2O3 exhibit strong coupling of phonons to optical transitions at the onset of absorption, which is evidence of excitation into a polaronic state. Closely comparing the optical polaronic properties of α-Fe2O3 and α-Rh2O3, we establish that increased lattice covalency in α-Rh2O3 arising from the increased radial extension of the 4d orbitals influences which phonon modes mediate photogenerated polaron formation.

Quantum kernel-based delta (Δ)-learning for correcting a semiempirical method in the prediction of reaction barrier heights

A quantum machine learning based Δ-learning framework is presented for the prediction of reaction barrier heights of organic molecules. Corrections learned by quantum kernel-based models are applied to mitigate the underlying errors of the semiempirical PM7 method, improving its predictive accuracy toward results obtained at the ωB97X-D3/def2-TZVP level of theory. Both fidelity and projected quantum kernels are implemented using seven distinct encoding circuits and passed to three classical regression algorithms, namely, support vector regression, kernel ridge regression, and Gaussian process regression, yielding a total of 840 benchmarked model configurations across circuit widths of up to 9 qubits and circuit depths of up to five layer repetitions. The encoding circuits are additionally characterized in terms of their expressibility and entangling capability. The top-performing models, constructed using the 9-qubit YZ-CX encoding circuit, predict reaction barrier heights with a mean absolute error ranging between 4.95 and 5.03 kcal/mol, underscoring an improvement of ∼54% over the uncorrected PM7 baseline.

Unveiling chiral electron–photon correlation effects in circularly polarized optical devices

Strong coupling with circularly polarized vacuum fluctuations offers a viable route to manipulate molecular chirality. While experiments are advancing toward the realization of chiral cavities, a mean-field theoretical framework for describing electron–photon interaction in this platform has been missing. Here, we present a mean-field theory that can be systematically improved to capture the chiral correlation effects responsible for the enantioselective power of chiral light. We use strong coupling Møller–Plesset perturbation theory for accessing the excitation manifold of electrons and chiral virtual photons. We apply the developed methods to selected chiral systems and show that the mean-field theory captures cavity frequency dispersion but fails to describe the chiral discrimination arising from coupled electron–photon excitations.

Mixed quantum–classical simulations of intramolecular singlet fission

Intramolecular singlet fission is a multistate nonadiabatic process in which vibronic couplings govern ultrafast triplet-pair formation and subsequent relaxation. We benchmark mixed quantum–classical approaches—Ehrenfest dynamics, linearized semiclassical spin-mapping (spin-LSC), and the map-ping approach to surface hopping (MASH) – against numerically exact multilayer multicon-figuration time-dependent Hartree (ML-MCTDH) calculations for models of phenylene-linked pentacene dimers. While Ehrenfest dynamics exhibits pronounced mean-field artifacts and incorrect long-time populations, spin-LSC improves short-time transfer but suffers from transient negative populations and back-transfer effects. MASH provides the most consistent agreement across timescales and molecular variants and remains free of unphysical populations. We further analyze the role of zero-point motion in the initial vibrational distribution and show that it affects the accuracy of different mapping-based approaches in distinct ways. Our results demonstrate that spin-mapping surface-hopping methods offer a robust route to extend singlet-fission simulations beyond the regime accessible to numerically exact quantum dynamics.

Electronic strong coupling of gas-phase molecular iodine

Molecular polaritons, hybrid light–matter states formed from the strong coupling of molecular transitions and discrete photonic modes, are a compelling platform for optical control of chemical reactivity. Despite the origins of the field of polaritonics in atomic gases, strong coupling of molecular gases remains underexplored. The pristine, solvent-free gas-phase environment may prove ideal for gaining mechanistic understanding of molecular behavior under strong light–matter coupling. In this work, we achieve electronic strong coupling of the B–X, ν1 = 0 → 32, J = 53 → 52 and B–X, ν1 = 0 → 34, J = 103 → 102 rovibronic transitions of gas-phase iodine (I2) lying near 532.2 nm. We access a range of coupling strengths and detuning conditions with fine control over molecular number density and cavity length stabilization. This effort represents the first demonstration of electronic polaritons in a molecular gas and opens a new platform for polariton photochemistry and photophysics.

Simple analytical models of spectral focusing stimulated Raman scattering microscopy

Stimulated Raman Scattering (SRS) microscopy is a powerful, rapid label-free, and chemical-specific imaging modality. While the underlying physics is well understood and modeled numerically, we show that, with simple approximations, one can derive equations which enable rapid simulations. Here, we develop an analytical model for spectral focusing SRS microscopy, validate it against numerical simulations, and apply it to the analysis of several experimental implementations. Of use to experimentalists, our model permits rapid simulation of advanced modulation schemes for background (non-Raman) signal removal in SRS microscopy. We compare and discuss their relative advantages and drawbacks.

A theory of electronic structure for coarse-grained resolutions

We present a stochastic diabatic projection method for constructing electronic structure models that operate at coarse-grained (CG) molecular resolutions. By minimizing the relative entropy of joint electronic-nuclear distributions between the fine-grained (FG) and CG resolutions, we derive a stochastic effective Hamiltonian that decomposes CG configuration dependent electronic structure into deterministic means and residual fluctuations. This framework bridges downfolding and molecular coarse-graining theory, enabling the systematic reproduction of electronic structure renormalized over eliminated nuclear and electronic degrees of freedom.

Cross-linking reaction kinetics of cubic hydrogen silsesquioxane radicals: A theoretical study

In this work, high-level density functional theory calculations were employed to investigate the cross-linking reactions of hydrogen silsesquioxane (HSQ) radicals. We provide a comprehensive elucidation of the whole reaction process, spanning from hydroxylation of hydrogen silsesquioxane radical by water to dehydration condensation of hydroxylated HSQ molecules, by the characterization of key species, energy barriers, and reaction energies. Furthermore, the potential effect of ambient humidity on the cross-linking process was assessed by involving an extra water molecule in both the hydroxylation and dehydration condensation reaction steps. We found that the participation of an extra water molecule can reduce the transition-state energies of both reactions relative to the corresponding reactants by hydrogen-bonds. By incorporating the water concentration at a specific humidity into the kinetic calculations, we demonstrated that the extra water molecules do not catalyze the hydroxylation of HSQ radicals; however, the water molecules facilitate the dehydration condensation reaction, potentially increasing the rate constants by up to a factor of three under high-humidity conditions, as compared to the anhydrous reaction. This study provides a theoretical explanation for that the presence of water promotes HSQ cross-linking, suggesting that a sufficiently high humidity could be beneficial in practical production processes.

<i>Ab initio</i> strong-pump strong-probe doorway-window simulation protocol

We have developed an on-the-fly methodology for the simulation of strong-field pump-probe (PP) spectra using semiclassical trajectory methods. This methodology generalizes the weak-field semiclassical doorway-window (DW) simulation protocol to pump and probe pulses of any intensity. The strong-field DW functions are defined by explicit analytical formulas which are fully determined by the molecular transition dipole moments and electronic energies evaluated along the semiclassical trajectories, so that the weak-field and strong-field PP signals can be simulated at roughly the same computational cost. The developed methodology has been applied for the simulation of strong-field PP spectra of pyrazine. The obtained results vividly demonstrate the advantage of using strong pump and (notably) probe pulses for increasing the information content of PP measurements. In particular, the strong probe pulse significantly enhances the weak transitions originating from the lowest excited electronic states of pyrazine, thereby rendering the nonadiabatic wavepacket dynamics in these weakly allowed states visible.

Quantum scattering of NO A2Σ+ + Ne: Benchmarking the potential energy surface of the collision complex

A new van der Waals potential energy surface (PES) for the NO A2Σ+ + Ne collision complex is reported. This system provides a great test for electronic structure by directly contrasting exact quantum scattering simulations on generated potential energy surfaces with the experiment. The need for a new PES for this system is apparent due to discrepancies in calculated differential cross sections for inelastic rotational scattering on older PESs compared to recent experimental data. We thus perform a detailed benchmarking of the effects of electron correlation, basis set size and type, core correlation, non-Born–Oppenheimer effects, and relativistic effects. Following this, we then construct a new PES and compare time-independent quantum scattering on this new potential to two older ones. Agreement with the experiment is improved on the new PES but still not exact at all scattering energies.

Identifiability limits and deep-learning-assisted reconstruction of rotational density matrices for symmetric-top molecules

Recovering the rotational density matrix of a molecular ensemble from time-resolved angular distributions is central to understanding ultrafast rotational dynamics, yet the inverse problem is severely underdetermined. We analyze the forward operator that maps the density matrix of laser-aligned symmetric-top molecules to the angular distribution retrieved in pump–probe experiments and demonstrate through singular value decomposition that 74%–88% of the real density matrix unknowns lie in the null space for maximum angular momentum quantum numbers Jmax = 2–5. This rank deficiency is intrinsic to the measurement geometry and imposes a linear lower bound on reconstruction error: the minimum-norm least-squares (pseudoinverse) solution sets all null-space components to zero, establishing the best achievable error for any linear, unbiased estimator. Nonlinear constraints—positive semidefiniteness, trace conservation, and block symmetries—partially recover null-space information, but the residual error grows with Jmax, reaching 15%–44% for Jmax = 5. We present a two-stage pipeline in which a convolutional neural network trained on simulated data provides a warm start for the fast iterative shrinkage-thresholding algorithm. For both CF3I and CH3Cl across Jmax = 2–6, this approach reduces the Frobenius reconstruction error by 80%–99% relative to optimization from thermal equilibrium, maintaining stable errors of 0.5%–1.1% as Jmax increases. The pipeline is robust to data noise down to a 20 dB signal-to-noise ratio and operates ten times faster than the baseline and outperforms the maximum-entropy approach by a factor of 15–39×. The classical iterative quantum tomography algorithm becomes numerically unstable for Jmax ≥ 4, whereas the proposed method converges reliably at all tested truncation levels.

Tuning glass-forming dynamics by modifying hydrogen bonding: From polyalcohols to van der Waals liquids

This work investigates how modifications of molecular structure influence the dynamics of hydrogen-bonded glass-formers, with particular focus on glycerol and sorbitol. By systematically eliminating hydrogen bonding through acetylation—yielding glycerol triacetate and sorbitol hexaacetate—and by comparing with previous studies involving pressure, nanoconfinement, and hyperquenching, we assess the role of hydrogen bonds in governing structural and secondary relaxation processes. Broadband dielectric spectroscopy reveals that the removal of hydroxyl groups leads to a substantial reduction in dielectric strength and notable changes in the structural α-relaxation, including variations in glass transition temperature, fragility, and the non-exponentiality parameter. These changes differ qualitatively between glycerol and sorbitol, indicating that the impact of hydrogen bonding depends sensitively on molecular architecture. In contrast, the acetylated systems behave as van der Waals glass-formers and conform to established correlations between dielectric strength and relaxation shape. Analysis of secondary relaxations shows that the Johari–Goldstein β-relaxation persists across all modifications, with its characteristic timescale remaining closely linked to the primitive relaxation time predicted by the coupling model. External perturbations such as high pressure and hyperquenching modify intermolecular coupling and relaxation dispersion without significantly affecting dielectric strength, while nanoconfinement reduces cooperativity and brings the α-relaxation close to the primitive limit. Overall, the results demonstrate that hydrogen bonding strongly influences intermolecular coupling and dynamic heterogeneity, but the fundamental relation between primary and secondary relaxations remains robust.

Self-assembly: From blueprints to breakthroughs

Intermolecular dynamics of deep eutectic solvents probed via dynamic optical Kerr effect spectroscopy

Deep eutectic solvents (DESs) are room-temperature liquid-state mixtures composed of two or more components that exhibit a significant depression in their melting points compared with those of their pure components. Owing to their facile preparation and broad range of component combinations, DESs are gaining attraction in materials chemistry and engineering. The intermolecular interactions in DESs are often more diverse than those in ionic liquids or hydrogen-bonding liquids, because they include different kinds of species, which are key to the strong eutectic effect. Thus, DESs are an attractive research target in chemical physics and physical chemistry from both dynamic and static perspectives. This article focuses on the intermolecular dynamics of DESs, particularly those explored using dynamic optical Kerr effect spectroscopy (OKES) and other spectroscopic techniques. OKES is a robust spectroscopic technique for observing molecular dynamics in the low-frequency (or terahertz to subterahertz) region, as it captures both intermolecular vibrations and collective orientational dynamics. Notably, this technique enables detecting dynamics without the need for a probe molecule. This article summarizes key findings from existing research, including characteristic vibrational modes and relaxation processes, as well as their dependence on DES constituents and the effects of water and temperature. Finally, potential directions for future research on DES dynamics are discussed. We hope that this Perspective will help not only to understand the current research situation in DES dynamics, but also to drive further progress and development of the DES and its related research area.

From fluctuating entropic neck to Rosenfeld–Adam–Gibbs crossover dynamics in supercooled liquids

The pronounced dynamical slowdown in supercooled liquids is accompanied by a sequence of crossovers, most notably from collision-dominated transport at high temperatures to activated structural relaxation at low temperatures, as reflected in the transition from Rosenfeld excess-entropy scaling to Adam–Gibbs behavior. We develop a unified theoretical framework based on a memory-function formalism combined with a configuration-space extension of Zwanzig’s entropic-neck picture. Starting from coupled slow coordinates that describe intra-basin motion and inter-basin escape, we derive a reduced description in which transport occurs through entropically constrained pathways connecting metastable basins in the inherent-structure landscape. In contrast to simplified parallel-channel pictures, the intrabasin coordinate and the neck (transition) coordinate are dynamically coupled, leading to a nonseparable memory kernel with both local (collisional) and collective (barrier-crossing) contributions. At high temperatures, fluctuations sample broad regions of configuration space, the effective neck is wide, and transport is governed by local structural entropy, recovering Rosenfeld scaling. Upon supercooling, progressive constriction of accessible pathways generates an entropic bottleneck whose statistics are controlled by configurational entropy, yielding Adam–Gibbs-type activated dynamics. The formalism provides a semi-microscopic interpretation of the crossover, clarifies its relation to the mode-coupling temperature, and naturally extends to nonequilibrium conditions, where the same entropic-neck mechanism governs the evolution of fictive temperature and its connection to fragility.

Using a low-storage contour-integral eigensolver for nonsymmetric matrices with collocation to compute vibrational spectra

Collocation is advantageous for computing vibrational spectra because it obviates the need for quadrature. However, to use collocation, one must solve a non-Hermitian matrix eigenvalue problem. Simple, accurate Lanczos algorithms that require storing only a few vectors can be used only for symmetric matrices. Established eigensolvers for nonsymmetric problems require storing (and orthogonalizing) many vectors. For large matrices, they require a lot of memory. We propose a low-storage eigensolver combining the block Sakurai–Sugiura method and the multi-shift quasi-minimal residual linear solver. It requires storing only a few vectors. The method has many of the advantages of the well-known Cullum–Willoughby Lanczos approach for symmetric matrices.

GTAttn-XC: Physically constrained attention for nonlocal density functionals

Machine-learning-based nonlocal density functional approximations have demonstrated substantial potential in advancing the applicability of electronic density functional theory. However, most existing approaches rely on handcrafted local or nonlocal descriptors, which limits their scalability in modeling long-range electronic responses. In this work, we propose a novel exchange–correlation functional approximation model—GTAttn-XC, which introduces a learnable attention mechanism to enable unsupervised modeling of long-range electronic responses. By coupling a multiscale real-space grid graph representation with attention operators, the proposed method achieves a unified description of local accuracy and nonlocal interactions, while avoiding the computational overhead associated with explicit high-order correlation terms. Evaluations on multiple benchmark datasets, including MGCDB84, demonstrate that the model consistently delivers high accuracy across a range of tasks, such as weak interactions, reaction energies, barrier heights, and thermochemical energies. These results establish a new technical pathway toward high-accuracy nonlocal exchange–correlation approximations.

Beyond the gradient expansion approximation: A generalized gradient expansion for exchange

The gradient expansion approximation (GEA) exchange in density functional theory is derived from the long-wavelength response of jellium and yields an analytic expansion in even powers of the reduced density gradient s. This structure reflects the perturbative q→0 limit rather than an exact constraint. Relaxing analyticity in s leads naturally to a generalized gradient expansion (GGE) based on a Puiseux series containing both integer and fractional powers. The exchange-hole structure of inhomogeneous jellium indicates that finite-wavevector singularities associated with the Kohn-anomaly at the Fermi surface generate non-analytic contributions, including a leading s3/2 behavior. Non-analytic, non-polynomial, and mixed structures are already intrinsic to widely used generalized gradient approximation (GGA) exchange functionals. With natural extensions to correlation, the GGE provides a minimal and rigorous extension of the GEA, enabling systematic construction of new GGA exchange–correlation functionals.