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Wavelength-dependent intramolecular singlet fission through an excimer-like intermediate
Singlet Fission (SF) into two triplets offers exciting avenues for high-efficiency photovoltaics and optically initializable qubits. While the chemical space of SF chromophores is ever-expanding, the underlying mechanistic details of electronic-nuclear motions accompanying SF are often glossed over. Rigid SF dimers with well-defined orientations are helpful to decipher such details. Here, using polarization-controlled white-light two-dimensional and pump–probe spectroscopies, we investigate a new class of contorted naphthalenediimide dimers, recently reported to have a favorable intramolecular SF (iSF) pathway. 2D cross-peaks directly identify the two Davydov components of the dimer. 2D maps reveal that excitation of either Davydov component leads to an intermediate state, which is generated within our instrument response. This intermediate proceeds to form a relaxed TT1 state whose formation kinetics is strongly dependent on which Davydov component is excited. We also find that the intermediate formation and relaxation are vibronically coherent with enhanced quantum beats only in the TT1 photoproduct, suggesting that intermolecular twisting and ruffling coordinates are strongly displaced upon TT1 formation. Polarization anisotropy directly tracks electronic motion during these steps and curiously reveals minimal electronic reorientation during TT1 formation. A likely hypothesis for this observation is that significantly mixed singlet–triplet electronic character is maintained throughout the nuclear evolution away from the Franck–Condon geometry toward relaxed TT1 without any reduction in the singlet electronic character. Such a mixing can introduce triplet annihilation channels and can therefore prevent the formation of long-lived high-spin triplets. The synthetic design of iSF dimers should aim to minimize this electronic mixing.
A simple fourth order propagator based on the Magnus expansion in the Liouville space: Application to a Λ-system and assessment of the rotating wave approximation
A simple fourth-order propagator [Ture and Jang, J. Phys. Chem. A 128, 2871 (2024)] based on the Magnus expansion is extended to the Liouville space for both closed-system and Lindbladian open-system quantum dynamics. For both dynamics, commutator free versions of fourth-order propagators are provided as well. These propagators are then applied to the dynamics of a driven Λ-system, where Lindblad terms represent the effect of a photonic bath. For both dynamics, the accuracy of the rotating wave approximation (RWA) for the matter–radiation interaction is assessed. We confirmed reasonable performance of RWA for weak and resonant fields. However, small errors appear for moderate fields and substantial errors can be found for strong fields where coherent population trapping can still be expected. We also found that the presence of bath for open-system quantum dynamics consistently reduces the errors of the RWA. These results provide quantitative information on how the RWA breaks down beyond weak field or for non-resonant cases. Major results are benchmarked against results of our sixth-order ME-based propagator. We also provide numerical comparison of our algorithms with other fourth-order algorithms for the Λ-system. These confirm reasonable performance of our simple propagators and the improvement gained through commutator-free expressions.
Does correlated orbital theory improve PBE-like functionals?
Correlated Orbital Theory (COT) provides an exact one-particle framework by imposing rigorous physical constraints on Kohn–Sham eigenvalues and, as a consequence, directly incorporates essential electron correlation into molecular orbitals. This approach paves the way toward a new class of approximations within Kohn–Sham Density Functional Theory (KS-DFT). However, since all existing quantum theory project functionals are derived from CAM-B3LYP, we pose the question: Can COT improve the hybrid versions of different exchange-correlation functionals as well? To that end, we explore two optimization strategies for adjusting the existing parameters within PBE0, TPSS0, and LC-PBE0: (i) the ionization potential condition and (ii) the HOMO–LUMO condition. In this sense, we critically assess how these functionals address the “Devil’s Triangle” of KS-DFT: self-interaction error, integer discontinuity, and one-particle spectra. We further examine how the COT influences the description of two challenging properties, charge transfer and reaction barrier heights. Overall, enforcing both COT conditions systematically enhances the performance of functionals within the PBE family, although the description of reaction barriers still leaves room for improvement.
Distinct pH-regulated conductance behaviors of PEI-coated PET and silica nanochannels
Polyethyleneimine (PEI)-functionalized nanochannels have been extensively exploited in ion gating and biosensing. It is of great significance to explore their conductance determining ion–surface interaction and ionic transport at the nanoscale. Here, pH-regulated conductance of PEI-coated nanochannels is theoretically studied. The results suggest that coating PEI may improve nanochannel conductance in different ways depending on the surface charge of nanochannels. The conductance of a PEI-coated PET nanochannel whose surface charge is assumed to be constant for simplicity is weakly changed and then decreased to that of a non-PEI-coated PET nanochannel with increasing pH, which is consistent with the reported experimental results. The conductance of a PEI-coated silica nanochannel, whose surface charge largely depends on pH, ion concentration, and nanochannel radius, is significantly increased and, subsequently, decreased to that of a non-PEI-coated silica nanochannel as pH rises, which is rarely reported in experiments. This work provides a fundamental framework to investigate the conductance of PEI-coated nanochannels, and the PEI-coated silica nanochannels with unique pH-dependent conductance may be explored in the construction of mesoporous silica thin films for biomedical analytical applications.
(Dis-)appearance of liquid–liquid phase transitions in a heterogeneous activated patchy particle model and experiment
The ion-activated patchy particle model is an important theoretical framework to investigate the phase behavior of globular proteins in the presence of multivalent ions. In this study, we examine and highlight the influence of patch heterogeneity on the extension, appearance, and disappearance of the liquid–liquid coexistence region of the phase diagram. We demonstrate that within this model the binding energy between salt ions and patches of different types is a key factor in determining the phase behavior. Specifically, we show under which conditions liquid–liquid phase separation (LLPS) in these systems can appear or disappear for varying binding energy and ion-mediated attraction energy between ion-occupied and unoccupied patches. In particular, we address the influence of the patch type dependence of these energies on the (dis)appearance of LLPS. These results rationalize our new results on ion-dependent liquid–liquid phase separation in solutions of bovine serum albumin with trivalent cations. In comparison with models with non-activated patches, where the gas–liquid transition disappears when the number of patches approaches two, we find the complementary mechanism that ions may shift the attractions from stronger to weaker patches (with an accompanying disappearance of the transition) if their binding energy to the patches changes. The results have implications for the understanding of charge-driven LLPS in biological systems and its suppression.
A variational formulation of the free energy of mixed quantum-classical systems: Coupling classical and electronic density functional theories
Combining classical density functional theory (cDFT) with quantum mechanics (QM) methods offers a computationally efficient alternative to traditional QM/molecular mechanics (MM) approaches for modeling mixed quantum-classical systems at finite temperatures. However, both QM/MM and QM/cDFT rely on somewhat ambiguous approximations, the two major ones being: (i) the definition of the QM and MM regions as well as the description of their coupling, and (ii) the choice of the methods and levels of approximation made to describe each region. This paper addresses the second point and develops an exact theoretical framework that allows us to clarify the approximations involved in the QM/cDFT formulation. We, therefore, establish a comprehensive density functional theory (DFT) framework for mixed quantum-classical systems within the canonical ensemble. We start by recalling the expression of the adiabatic equilibrium density matrix for a mixed system made of Nqm quantum and Nmm classical particles, which can be related to a partial Wigner transform. Then, we propose a variational formulation of the Helmholtz free energy in terms of the full, non-equilibrium, QM/MM density matrix. Taking advantage of permutational symmetry and thanks to constrained-search methods, we reformulate the computation of the Helmholtz free energy using only the quantum and classical one-body densities. Therefore, this paper generalizes both cDFT and electronic DFT (eDFT) to QM/MM systems. We then reformulate the functional to make the standard eDFT and cDFT Levy–Lieb functionals explicitly appear, together with a new universal correlation functional for QM/MM systems. A mean-field approximation is finally introduced in the context of solvation problems, and we discuss its connection with several existing mixed cDFT-eDFT schemes. An extension to the semi-grand canonical ensemble, where the number of classical particles is allowed to fluctuate, is provided in the supplementary material.
Efficient all-electron periodic Fourier-transformed Coulomb method
We present an efficient algorithm for constructing an all-electron periodic Coulomb matrix based on Ewald summation combined with the Fourier-transformed Coulomb method. The short-range contributions involving compact densities are evaluated in real space using standard Gaussian density fitting. For the long-range contributions, we introduce an integral-direct plane wave density fitting scheme that is applicable to both compact and diffuse densities. The resulting method achieves orders-of-magnitude speedups for prototypical solid-state systems compared to a closely related approach, the range-separated density fitting method. Using the dispersion-corrected PBE functional with all-electron Dunning and Karlsruhe basis sets, we apply our method to compute the cohesive energy of the benzene crystal and the adsorption energy of CO on the MgO(001) surface. These results are in good agreement with existing literature. Our approach enables efficient Gaussian-based semi-local density functional calculations using dense k-point meshes and traditional molecular Gaussian basis sets.
Polymer translocation through extended patterned pores in two dimensions: Scaling of the total translocation time
We study the translocation of a flexible polymer through extended patterned pores using molecular dynamics (MD) simulations. We consider cylindrical and conical pore geometries that can be controlled by the angle of the pore apex α. We obtained the average translocation time ⟨τ⟩ for various chain lengths N and the length of the pores Lp for various values α and found that ⟨τ⟩ scales as ⟨τ⟩∼NγFLpNϕ, with exponents γ = 3.00 ± 0.05 and ϕ = 1.50 ± 0.05 for both patterned and unpatterned pores, respectively.
pyRMG: A framework for high-throughput, large-cell DFT calculations on supercomputers
Exascale computing delivers the raw power to simulate ever larger and more chemically realistic systems, but realizing this potential requires codes that can efficiently use thousands of processors. Our real-space multigrid (RMG) density functional theory (DFT) code’s grid-decomposition approach scales nearly linearly with the number of graphics processing units (GPUs), even for simulations exceeding thousands of atoms. This scalability makes RMG a compelling tool for high-throughput DFT studies of materials that would otherwise be bottlenecked in other codes (for example, by global fast Fourier transforms in plane-wave DFT). However, the limited workflow infrastructure for RMG has thus far constrained its adoption to a small user community. In this work, we present pyRMG, a Python package designed to streamline the setup and execution of RMG DFT calculations. Built on the pymatgen and ASE (Atomic Simulation Environment) computational materials science Python packages, pyRMG automates input generation and convergence checking, and it integrates with modern job schedulers (e.g., Flux) on leadership-class platforms such as Frontier and Perlmutter. We demonstrate pyRMG for a high-throughput study of strain effects in 2D 2L-Bi2Se3/2L-NbSe2 heterostructures, which offers chemical insights into this system and shows that RMG-based workflows can converge with limited user intervention.
Modification of Rys quadratures for two- and three-center electron repulsion integrals calculation
Integrals for Gaussian-type orbitals are the basis of every quantum chemical calculation. In this work, we present the efficient implementation of nuclear attraction and of two- and three-center electron repulsion integral calculations based on the Rys quadratures. It was found that some types of electron repulsion integrals can be calculated using modified Rys quadratures with fewer roots. Despite the overall effect of this modification being shown to be small, a 1.6-fold speed-up of some types of integral calculations was achieved. Overall, nuclear attraction integral calculations were shown to be 8–10 times faster compared to LIBINT in electrostatic potential calculation tasks. The speed of three-center electron repulsion integrals calculation was enhanced by a factor of 2–4 compared to LIBINT.
A practical framework for rapid calculation of protein polarization energies with anisotropic atomic polarizabilities
An accurate description of electronic polarization is fundamental to modeling protein interactions and dynamics. Despite its importance, polarization is frequently omitted in classical force fields, while existing polarizable models are hindered by parameterization complexity, high computational cost, or limited resolution. We present a novel, efficient method for calculating protein polarization energies. Our method computes local electric fields at atomic sites, decomposes them into bond-parallel and perpendicular components, and uses environment-specific, anisotropic atomic polarizabilities fitted to accurate quantum-chemical calculations. The model accurately reproduces quantum-mechanical polarization energies for amino-acid monomers, dimers, trimers, and small proteins. Its computational efficiency enables application to large, explicitly solvated proteins, achieving excellent agreement with benchmark data at a minimal computational cost. We demonstrate its scalability by applying it to large, solvated proteins, providing a practical path for incorporating polarization into biomolecular simulations.
Light absorption and emission by weakly bound heteronuclear molecular ions in the superlinear crossing transition regime. The example of NeXe+
We develop a semi-analytic theory for describing nonadiabatic bound–bound, free–bound, bound–free, and free–free photoprocesses in heteronuclear ions in the regime of the superlinear potential energy curve crossing. It extends the previous semiclassical method for calculating the absorption and emission spectra of strongly and moderately bound diatomic species based on the linear curve crossing model and can be used for molecules and ions with small dissociation energies, D0 ≲ kBT. The use of quasicontinuum approximation for rovibrational levels allows us to give a unified description of the integral contributions of the discrete and continuous spectra of the molecular species with linear and superlinear crossings to the effective cross sections and rate coefficients of the radiative processes in the systems studied. Specific calculations were performed for the excimer-like NeXe+ ion (DeNeXe+=37.3 meV). Potential energy curves and dipole transition matrix elements are evaluated using ab initio multi-reference calculations with a perturbative description of relativistic effects. In contrast to ArXe+ and KrXe+ ions studied previously, the main contributions to the absorption spectra of NeXe+ are due to bound–bound transitions and photoassociation. The emission spectra at room temperatures are determined predominantly by the bound–bound transitions, while at temperatures above 450 K, the most significant contribution to the radiation is made by bound–free phototransitions. Our calculations are in good agreement with the available experimental data. The results obtained are of interest for chemical physics, spectroscopy of weakly bound molecular systems, and physics of radiative processes in gases and plasmas, as well as for the kinetics of active media of excilamps and gas lasers based on noble gas mixtures.
Vibrational dynamics of liquid nitromethane at fundamental and overtone band studied by femtosecond time-resolved coherent anti-Stokes Raman spectroscopy
Time-resolved third-order and fifth-order CARS experiments were employed to systematically investigate the vibrational dynamics of liquid nitromethane (NM) molecules in both fundamental and overtone bands, and the overtone vibrational dephasing parameters of NM were reported for the first time. The experimental findings reveal that overtone vibrations exhibit a notably faster dephasing process than corresponding fundamental vibrations. This phenomenon arises from the higher energy levels of overtone vibrations, which render them more susceptible to strong intermolecular interactions within the condensed-phase environment. Among the main vibrational modes of NM, the dephasing lifetime of the C–N stretching vibration is remarkably longer than that of other modes, and even the overtone lifetime is longer than the fundamental lifetime of other modes. It is indicated that when NM is excited by external stimuli, energy has a high probability of depositing on the C–N bond, which in turn brings this bond to a high vibrational level. This inference helps account for the observation that C–N bond cleavage acts as the primary initial reaction channel in the pyrolysis and photolysis of NM. Molecular vibrations, especially the vibrations in high vibrational excited states, are closely associated with the chemical reactions of molecules. This study contributes to a better understanding of the vibrational energy localization mechanism and the subsequent molecular dissociation in condensed-phase energetic materials.
Rotational excitation of AlCl induced by H2 collisions
We present the first study of the rotational excitation of AlCl(X1Σ+) induced by collisions with H2. We have calculated a four dimensional potential energy surface (PES) to describe the AlCl–H2 interaction, with AlCl and H2 being treated as rigid rotors. This PES has been computed using the explicitly correlated coupled-cluster method with single, double, and perturbative triple excitation in conjunction with the augmented-correlation consistent-polarized valence triple zeta basis set. Then, we have calculated the rotational excitation cross sections between the first 41 rotational levels of AlCl induced by H2 collisions using the time-independent quantum mechanical close-coupling and coupled-states formalisms. Convergence tests revealed that the inclusion of excited para-H2 energy levels in the rotational basis has a minor effect on the magnitude of the excitation cross sections. We also found that AlCl excitation cross sections induced by ortho-H2 collisions are in good agreement with those induced by para-H2 collisions. Hence, we limited the calculations to the excitation cross sections induced by H2 in its ground rotational state. To derive excitation rate coefficients for temperatures up to 250 K, the cross sections computed for energies up to 1500 cm−1 were averaged over a Maxwell–Boltzmann velocity distribution. The new rate coefficients were compared to those available for the AlCl–He collisional system, and major differences were found at high temperature, showing that actual AlCl–H2 rate coefficients should be used for accurate astrophysical models. The new data are expected to play a crucial role in the modeling of AlCl observational spectra and will help in better constraining its abundance in space.
Structural transitions in liquid water at high temperatures and pressures: Evidence from molecular simulations
Classical molecular dynamics simulations were carried out for liquid water using the TIP4P-2005 potential model along the 50 MPa isobar, covering a wide temperature range from ambient to near-critical conditions. Particular attention was given to the behavior of various local structural descriptors of liquid water, as well as to the corresponding dynamics and entropic quantities. The results obtained reveal the existence of two distinct structural transitions, located in the temperature range around 423.15 and 498.15 K, respectively. The observed transitions have been characterized by local extrema and crossovers in many of the above-mentioned quantities. Significant changes in the hydrogen bond network were also observed across these transitions. These structural rearrangements are reflected in the calculated intermolecular vibrational and librational dynamics, as evidenced by clear modifications in the spectral densities of atomic velocity correlation functions and in the translational and rotational densities of states.
Observation of Penning electron detachment by electronically excited potassium atoms in high Rydberg states
Penning electron detachment is a process in which an electronically excited neutral species collides with an anion and detaches an electron from it by transferring its excitation energy, viz., A− + N* → A + N + e−. While there have been theoretical studies relating to the mechanism of Penning detachment, there have been few experimental investigations of this fundamental process. In this work, Penning electron detachment of sulfur pentafluoride anions by potassium atoms, which had been selectively excited to high Rydberg electronic states, was investigated by directly observing its occurrence via the depletion of the SF5− anion signal in mass spectra, viz., SF5− + K** → SF5 + K + e−. Scanning the excited states of potassium in the range of 8d–32d and 10s–33s (excitation energy 4.11–4.33 eV) showed a significant dependence of the Penning detachment cross section on the excitation energy of potassium. Combining quantitative Penning detachment results with the electron affinity of SF5 (4.4 eV), which we had determined using anion photoelectron spectroscopy, demonstrated that Penning detachment occurred when the total available energy, that is, the SF5− + K** collision energy + the K** excitation energy, exceeded the electron affinity of SF5.
Charge screening as the key effect governing physical properties of deep eutectic solvents
In this work, we performed all-atom molecular dynamics simulations of the ionic liquids choline chloride (ChCl) and 1-ethyl-3-methylimidazolium chloride, as well as their mixtures with ethylene glycol (ETG) and urea (UR), to obtain deep eutectic solvents (DESs). We calculated the structural and transport properties of all systems studied to explore the differences between pure ILs and DESs. From these results, we discuss the ability of ETG and UR to orient themselves along the ionic electric field, similarly to water in classical electrolytes. This orientational behavior promotes an effective reduction of the electrostatic interactions (usually called “screening” within the context of classical electrolytes). We found that, similar to inorganic salts dissolved in water, the hydrogen-bond donors reduce electrostatic interactions between ions, leading to significant changes in their structural and transport properties.
Association and phase transitions in simple models for biological and soft matter condensates
We investigate a set of design principles that link specific features of interparticle interactions to predictable structural and dynamic outcomes in two-dimensional self-assembly, a framework relevant to soft matter and biological condensates. Using extensive molecular dynamics simulations of single- and two-component systems, we systematically dissect how modifications to competing short-range attraction and long-range repulsion (SALR) potentials (both isotropic and anisotropic) serve as independent control parameters. In particular, we have focused on tuning the repulsive barrier height, decorating the attractive well with oscillatory components, and changing particle geometry. We demonstrate that these modifications dictate cluster size distributions, the degree of intra-cluster ordering, the geometry of the clusters, and the propensity for inter-cluster crystallization. A key finding is the decoupling of internal and global dynamics: oscillatory wells promote solid-like order within clusters while maintaining liquid-like cluster mobility. Furthermore, we show how asymmetric interactions in a binary SALR mixture can be designed to induce internal phase segregation within condensates. Complementing this, we observe that in anisotropic models in which the short-range component of the interaction stems from the presence of attractive patchy sites, stoichiometry and the geometric distribution of the patches are essential to control self-assembly and cluster morphology, whereas long-range repulsion can be used to tune cluster size and polydispersity. The extracted principles provide a causal road-map for engineering self-assembled materials and a set of basic physical concepts for interpreting the complex phase behavior of biomolecular condensates.
Machine learning potential as a guide for eutectic in ultra-refractory multicomponent ceramics
The experimental determination of eutectic points is a long-established and widely used technique, but it is generally only practical for systems with relatively low melting points. Many modern, promising materials, however, are ultra-refractory, with melting points exceeding 3000 K. For these systems, conventional melting experiments become prohibitively expensive and technically challenging. Advanced AI modeling can serve as a powerful precursor to guide successful experimentation in such cases. This work proposes a novel criterion for determining the eutectic point concentration in ultra-refractory alloys. The approach is verified using the Ti–B–C system—the most thoroughly studied three-component refractory system to date. The core of the algorithm is a machine-learning interatomic potential, based on a neural network, which achieves accuracy comparable to ab initio methods. Crucially, the algorithm operates effectively in the liquid phase, eliminating the need for information about the solid alloy’s crystalline structure to estimate eutectic points.
Extended perturbative approach for quantitative calculation of the coherence excitation energy transfer in photosynthetic light harvesting complex
There are non-negligible exciton couplings and coherent oscillations in the excitation energy transfer within the light-harvesting complex and the exciton dynamics. In addition, the exciton dynamics depends on a variety of system parameters, such as electron coupling, electron–phonon coupling, and the site energy of each chromophore. In order to investigate the detailed interaction process, we propose an extended perturbation theory approach and derive the coherent dynamics of reduced density matrices under arbitrary orthogonal transformations. Our method quantitatively presents the dependence of system parameters on exciton state dynamics and the mathematical analysis of coherent dynamics, expands the application boundary of standard Förster and modified Redfield theories, and provides a mathematical description in the general case with an arbitrary basis. In the two-level dimer system, our model provides an excellent correspondence with the calculation of numerically exact hierarchical equations of motion. As an application, we quantitatively describe the long-lived vibronic coherence within recombinant allophycocyanin and the electron coherence process between chromophores within cryptophyte phycocyanin 645.