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Fundamental bounds on many-body spin cluster intensities
Multiple-quantum coherence (MQC) spectroscopy is a powerful technique for probing spin clusters, offering insights into diverse materials and quantum many-body systems. However, prior experiments have revealed a rapid decay in MQC intensities as the coherence order increases, restricting observable cluster sizes to the square root of the total system size. In this work, we establish fundamental bounds on observable MQC intensities in the thermodynamic limit (N ≫ 1) outside the weak polarization region. We identify a sharp decay in the observable MQC intensities as the coherence order grows. This transition regime fragments the state space into two components consisting of observable and unobservable multiple-quantum coherences. Notably, we find that the center of the transition region is directly proportional to size N and polarization p of the system, suggesting that the aforementioned square root limitation can be overcome through hyperpolarization techniques. Our results provide important experimental guidelines for the selective observation of large spin cluster phenomena.
Contact mechanics for layered materials: Rubber film on hard substrate
I consider the contact mechanics for a layered material, consisting of an elastically soft film glued to a hard substrate. I calculate the area of real contact for surfaces with fractal-like roughness and for surfaces with roughness in narrow length scale regions. For the fractal-like surfaces, when the product q0d of the film thickness d and the low cut-off wavenumber q0 of the surface roughness power spectrum satisfy q0d < 0.1, the effective modulus becomes very large. This results in large contact stresses, which can induce plastic deformation or wear, in particular during sliding contact. I also calculate the probability distributions of the normal and tangential stresses at the film–substrate interface. If the tangential (shear) stress is too high, the adhesive bond between the film and the substrate will break. I compare the thin-film contact mechanics problem with the Gent solution for a thin elastic sheet confined between two flat solid surfaces, and discuss the origin of the difference in effective elastic modulus.
The bond capacity electronegativity equilibration charge model (EEQBC) for the elements <i>Z</i> = 1–103
The accurate and efficient assignment of atomic partial charges is crucial for many applications in theoretical and computational chemistry, including polarizable force fields, dispersion corrections, and charge-dependent basis sets. Classical charge models struggle to distinguish between neutral and zwitterionic fragments because, unlike quantum mechanical methods, there are no discrete electronic states. This limitation can lead to either reduced or additional artificial charge transfer (CT) at different interfragment distances. To address this issue, we propose a new version of a bond capacity electronegativity equilibration (EEQBC) model, which limits artificial CT between distant fragments in the simple EEQ framework. EEQBC offers excellent agreement with DFT-based reference charges for elements up to lawrencium (Z = 103) with mean absolute errors as low as 0.02 and 0.07 e− for random PubChem molecules and “mindless” molecules (MLMs), respectively. Thanks to its computational efficiency for both atomic charges and their analytical nuclear gradients, EEQBC is highly suitable as an initial charge guess for next-generation tight-binding methods. For seamless accessibility, EEQBC is implemented in the upcoming 0.5.0 release of the freely available multicharge program at github.com/grimme-lab/multicharge.
Solvent-induced transformations of assemblies of structured ionizable block co-polymers
Micelles formed by ionizable co-polymers are governed by both van der Waals and electrostatic forces. Slight tweaking of the solvent characteristics often drives large structural transformations. Here, the effects of modulating the electrostatic characteristics of solvents on polymeric assemblies formed by a co-polymer with ABCBA topology and an ionizable polystyrene sulfonate center (C) tethered to polyethylene propylene (B) end-capped by t-butyl styrene (A) are probed by small angle neutron scattering in cyclohexane/propanol solutions. With increasing propanol fraction, the spherical core–shell micelles, with the ionic block in their core, elongate and transition into large “swarms.” Surprisingly, as propanol becomes the major component of the solvent, reentrant spherical assemblies with a smaller polystyrene sulfonate core are formed. The propanol partitions across the interface between the core and the corona, affecting the polymer distribution in the corona and the core-corona boundary’s curvature, resulting in morphing of the shape of assemblies.
Characteristics of photoemission from radiative and sub-radiative localized surface plasmons in metal nanostructures
Revealing the mechanism of photoemission from plasmonic nanostructures that supports different localized surface plasmon modes is crucial for designing new ultrafast photoelectric cathodes, as well as for enhancing localized surface plasmon (LSP)-based photocatalysis, energy harvest, and photoluminescence, but the investigation on this topic is still lacking. In this paper, we directly investigated the photoemission yield and photoemission mechanism from a sub-radiative Fano mode in an asymmetric nanorod dimer, a radiative dipole mode in an isolated nanorod, and a radiative coupled dipole mode in a symmetric nanorod dimer, respectively, using time-of-flight photoemission electron microscopy. We found that the photoemission yield from the sub-radiative Fano mode is almost equal to that of the radiative dipole mode but more than four times higher than that of the radiative coupled dipole mode case. We reproduced the physical process using a two temperature model and Fowler–Dubridge theory and demonstrated that the thermal effects of the electron gas play an influential role in photoemission from LSP. Interestingly, it is found that the sub-radiative nanorod dimer, although exhibiting a much lower electron temperature, maintains a similar photoemission yield to that of the radiative dipole mode, and this feature of the sub-radiative nanorod dimer makes it potentially a high-brightness electron source with strong robustness. The demonstrated results in this work help to understand the LSP-assisted photoemission process in plasmonic nanostructures, which lays the foundation for designing new ultrafast photoelectric cathodes and many other applications.
Statistical mechanics of homologous pairing of long double-stranded DNA
The ability of homologous dsDNA to recognize and attract each other is a fundamental feature in DNA recombination and repair. A major unresolved question is how homologous genes initially locate and position themselves in front of each other—whether they do so at a distance without the use of proteins or unzipping their strands. One hypothesis suggests that such recognition is an innate property of DNA’s structure. DNA is not a perfect double-helix and distortions from helical structure are correlated with the sequence of base pairs. These distortions influence the patterns of charge distribution along the molecules. Those with identical sequences exhibit matching patterns of distortion, allowing them to align in a one-to-one register, which facilitates more favorable interactions. Conversely, uncorrelated sequences are unlikely to align perfectly, resulting in weaker attraction or greater repulsion. Consequently, the pairing of homologous sequences (i.e., positioning homologous genes in front of each other at a distance corresponding to free energy minimum) is more favorable than that of heterologous sequences. But how complete and stable would the pairing be? To address this, we present a model mapped on an Ising-like framework, which provides insight into the extent of pairing and its robustness. Our findings suggest that homologous dsDNAs, spanning multiple Kuhn lengths, can pair with some “bubbles”—regions of less tightly coupled sections. We compute the fraction of these unpaired sections and analyze the stability conditions to demonstrate that under physiological salt concentrations, heterologous double-stranded DNA cannot sustain long paired segments beyond the helical coherence length, further supporting the advantage of homologous pairing.
Pure spin current modulation via Fano resonance in a copper-coordinated single-molecule junction
The generation of pure spin current is a highly sought-after objective in spintronics as it holds significant promise for enabling high-performance spintronic devices with low power consumption and high circuit integration density. However, achieving pure spin current at the single-molecule level remains a formidable challenge. In this study, we demonstrate a novel approach to generate pure spin current through spin Fano resonance in a p-Ben-CuII molecule. Our results reveal that the spin-down Fano resonance in p-Ben-CuII is associated with the β-HOMO, while the spin-up Fano resonance originates from the α-HOMO. Furthermore, by precisely tuning the chemical potential to align with the resonance peak, a well-defined pure spin current can be achieved under specific chemical potential or temperature conditions. This strategy offers a promising pathway for the precise manipulation of pure spin current in single-molecule junctions, paving the way for advanced spintronic applications.
Experimental and theoretical investigations on the vibrational and electronic spectra of 1,3-dibromobenzene
We report here a comprehensive spectroscopic study of the vibrational and electronic spectra of 1,3 dibromobenzene. The vibrational spectrum is studied using Fourier transform infrared and Raman techniques and analyzed using density functional theory (DFT) calculations incorporating anharmonicity effects via the second-order vibrational perturbation theory method, resulting in a set of consistent assignments for all the fundamentals and several overtone and combination bands. The electronic spectrum is studied using synchrotron radiation based photoabsorption spectroscopy spanning the spectral region 1150–3000 Å (86 956–33 333 cm−1), for which the spectrum in the 1150–1700 Å (86 956–58 823 cm−1) region is reported here for the first time. The electronic absorption spectrum is richly structured and comprises valence, Rydberg, and charge transfer excitations, along with distinct vibronic features. Quantum defect analysis is used to assign Rydberg series converging to the first four IPs of 1,3 dibromobenzene, while charge transfer and valence transitions are assigned using theoretical calculations at the TDDFT/CAMB3LYP/aug-cc-pVTZ level. The extensive vibrational bands accompanying the first valence transition are analyzed and assigned using Franck–Condon factor calculations incorporating the Herzberg–Teller effect. In addition, time dependent DFT studies of excited state potential energy curves yield some new insights into the role of internal conversions and intersystem crossings in the UV photodissociation dynamics of 1,3 dibromobenzene.
Size-dependent wettability of carboxyl alkyl chain-modified gold nanoparticles
The wettability of organic ligand-capped metal nanoparticles plays a crucial role in determining their behavior in diverse applications, including protein adsorption, protein corona formation, cellular uptake, toxicity, immune system recognition, drug release kinetics, and bioavailability. However, a comprehensive understanding of the size-dependent effects in metal nanoparticles with specific ligand modifications remains elusive. Here, we leverage molecular dynamics simulations to delineate the size-dependent wettability of Au nanocrystals modified with carboxyl-terminated alkyl chains at the same grafting density. Our results reveal a negative correlation between the water contact angle of gold nanoparticles and the radius of the Au-core for a given ligand length. This trend is determined by the spatial arrangement of carboxyl groups, where an increase in the size of Au-core leads to a reduction in the intermolecular spacing between carboxyl groups, promoting the formation of hydrogen bonds between the carboxyl groups and water molecules, thereby conferring a more hydrophilic character to the nanoparticles. Conversely, for a fixed Au-core radius, the water contact angle of nanoparticles increases as the length of alkyl chains increases. This distinct relationship arises from the extended separation between carboxyl groups in longer alkyl ligands, which reduces the hydrogen bond formation with water molecules and renders the nanoparticle more hydrophobic. These findings lay the groundwork for how nanoparticle wettability can be precisely modulated, a critical factor for optimizing their performance in various biomedical and industrial applications.
Minimal pole representation for spectral functions
Representing spectral densities, real-frequency, and real-time Green’s functions of continuous systems by a small discrete set of complex poles is a ubiquitous problem in condensed matter physics, with applications ranging from quantum transport simulations to the simulation of strongly correlated electron systems. This paper introduces a method for obtaining a compact, approximate representation of these functions, based on their parameterization on the real axis and a given approximate precision. We show applications to typical spectral functions and results for structured and unstructured correlation functions of model systems.
Structural comparison of homomolecular systems on surfaces using a fingerprint-based method
This work presents an adaptation of the Smooth Overlap of Atomic Positions (SOAP) method to improve the efficiency of (dis)similarity quantification in homogeneous molecular (homomolecular) systems. SOAP, a fingerprint-based approach, is widely used to measure molecular similarity. We propose variants of SOAP kernels that leverage the structural architecture of homomolecular systems to minimize irrelevant comparisons of atomic environments. To evaluate its performance, we apply this adapted SOAP-based method to a synthetic dataset consisting of two identical tripeptides deposited on a copper surface, simulating different molecular states. The results demonstrate that the adapted method not only improves computational efficiency but also yields more meaningful clustering outcomes by better capturing the key structural differences between states. These findings suggest that the proposed method is well-suited for the study of homomolecular systems, particularly those involving surface interactions, and has the potential to enhance the use of diverse types of molecular modeling and analysis methods that rely on (dis)similarity measures.
Modeling the impact of drug-nanocarriers in lipid membranes
A coarse-grained framework for molecular dynamics (CG-MD) simulations based on the MARTINI force field was developed to tackle interactions between ionic G5 dendrimers and nonionic Pluronic micelles with diverse amphiphilic characters as drug-loaded nanocarriers in contact with two biological membranes, the anionic 1-palmitoyl-2-oleoyl-glycero-3-phosphocholine (POPC) + 1-palmitoyl-2-oleoyl-glycero-3-phosphatidylglycerol (POPG) and the neutral dipalmitoyl phosphatidylcholine. Simulations showed that drug-nanocarrier stability relies on a delicate balance of their amphiphilic character and Coulombic interactions. Pluronic micelles yielded lower undesired drug leaks into the water phase compared with the cationic G5 dendrimers, which either remained attached to Pluronic moieties or remained between them and the membrane surface. An interesting feature of Pluronic micelles was their tendency to be disrupted and absorbed into the membranes. Hydrophilic micelles showed improved drug stability, avoiding early release of doxorubicin and gemcitabine drugs. When the Pluronic micelles are disrupted and absorbed into the charged membrane, the hydrophilic pluronic segments were depleted toward the membrane surface, retaining the drugs within. Overall, the CG-MD framework yields a detailed molecule-scale picture of the cooperative Coulombic and amphiphilic effects between charged moieties in contact with membrane surfaces.
Additive strategy for nucleation pathway control based on the understanding of molecular size effect on crystallization
Understanding mechanisms involved in particle formation processes is crucial to effectively control crystalline particle characteristics. This study highlights the significant effect of slight changes in molecular size on the crystallization pathway. Molecular dynamics simulations are performed in a binary Lennard-Jones system as a model for systems that undergo two-step nucleation via an intermediate droplet structure. This study analyzed two cases with different solute–solute interaction strengths and found that a larger solute-to-solvent size ratio delayed droplet crystallization in both cases. In systems with strong solute–solute interactions, this delay shifted the pathway from one-step-like to two-step-like nucleation, as droplets with larger solute molecules incorporated more solvent, thereby hindering crystallization. We explained this change in droplet composition by considering the mixing free energy between the solute and solvent. Larger solute molecules form entropically and enthalpically favorable structures by accommodating solvent molecules, which increase the solvent fraction of the droplet. We used a thermodynamic model based on the classical nucleation theory with a core–shell nucleus and revealed that this increased solvent fraction in the droplet lowered the freezing point of the droplet and raised the solid–liquid interfacial tension, ultimately delaying and suppressing crystallization. Based on these findings, we proposed a strategy to control the nucleation pathway using additives. Introducing appropriate additives to modify the stability of intermediates is a promising strategy to control nucleation pathways in various systems.
Noncollinear generalization of nonlocal pure exchange–correlation functionals
We have recently proposed a method for extending collinear functionals to noncollinear functionals, referred to as the multicollinear approach. While previous studies have applied this method to local and semilocal functionals, demonstrating its effectiveness, the present work extends the approach to nonlocal pure functionals via a novel implementation scheme. As an example, we generalize the weighted spin density approximation functional from its original collinear form to a noncollinear one, introducing the first pure functional that is both nonlocal and noncollinear. Our numerical tests demonstrate that the generalized noncollinear functional satisfies the correct collinear limit, preserves spin rotational invariance, provides nonzero local torque, and maintains numerical stability. This work highlights the broad applicability of the multicollinear approach and provides a reference for developing more sophisticated noncollinear functionals beyond local and semilocal forms.
Theoretical investigation into the interaction mechanism of hexavalent americium with covalent organic framework (PyN-DAB)
The separation of radionuclide americium (Am) is a crucial challenge in the reprocessing of spent nuclear fuel, due to the complex speciation. Selective coordination of hexavalent americium [Am (VI)] with covalent organic frameworks (COFs) has emerged as a promising strategy to address this issue. In this work, we employed first-principles simulations combined with density functional theory (DFT) to investigate the adsorption stability of COF (PyN-DAB) for the linear americyl ion AmO22+. Our results demonstrate that COFs can effectively and stably coordinate with Am (VI), highlighting their potential for Am separation. Furthermore, we explored the high-oxidation-state model of AmO22+ complexes with the PyN-DAB ligand to elucidate the underlying microscopic interaction mechanisms between AmO22+ and the PyN-DAB monomer. Comprehensive analyses revealed a strong attraction between the PyN-DAB and AmO22+, which is attributed to the synergistic effects of electrostatic interactions, orbital interactions, and π-electron-rich aromatic rings within the PyN-DAB framework. These findings not only provide fundamental insights into the Am separation process but also offer a novel perspective on the potential applications of COFs in the efficient extraction of actinides. Thus, this study contributes to the ongoing efforts to develop advanced materials for nuclear fuel reprocessing and waste management.
Fast calculation of the permittivities of gold thin films in the frequency range of 0–6 eV
The permittivity tensor of gold nanofilms of different orientations and thicknesses in the frequency range of 0–6 eV is theoretically studied, revealing significant differences from the bulk gold permittivity. Two models are proposed to calculate the longitudinal ɛ‖(h, ω) and transverse ɛ⊥(h, ω) parts of the permittivity tensor in the specified frequency range for gold nanofilms of different thicknesses and surface orientations (001), (110), and (111). These models explain intense peaks in the real and imaginary parts of permittivity at 0–2 eV. The model for calculating the transverse permittivity does not use the Drude model but uses the interband contribution of the bulk material determined through DFT calculations and the contribution of electron motion perpendicular to the nanoslab surface. This contribution takes into account the electron motion inside an infinitely deep one-dimensional potential well with a set of discrete electron levels and makes it possible to calculate the imaginary part of the permittivity using Fermi’s golden rule. The model for calculating the longitudinal permittivity employs an interpolation scheme using the tabulated permittivity of bulk gold and that of several plates with different thicknesses. The difference between experimental permittivity values and those calculated using DFT and the proposed models is discussed. The proposed algorithms enabled a Python program for fast calculation of ɛ⊥(h, ω) and ɛ‖(h, ω) of gold nanofilms of any thickness and above-mentioned orientations in the 0–6 eV range without computationally expensive DFT calculations. This program is included in the supplementary material. The proposed approaches can be easily applied to nanofilms made of other metals.
Gold(I)–N–heterocyclic carbene hydration process; <i>ab initio</i>, DFT, and QM/MM molecular dynamics study
This study investigates the hydration reaction of a gold(I)–N-heterocyclic carbene [Au(I)–NHC] complex at both the quantum mechanical (QM) level and combined Quantum Mechanics/Molecular Mechanics (QM/MM) MD simulations. The main goals are to analyze the differences between implicit (PCM) and explicit solvation models and to compare the advantages and disadvantages of both approaches. Regarding the QM part, the B97D3 and B3PW91 functionals are combined with double-zeta basis sets and the C-PCM/UFF implicit solvation model and compared with the CCSD(T)/TZP computational level supplemented with the C-PCM (COSMO/Klamt radii) or D-PCM/scaled-UAKS solvation model. In addition, reaction force and reaction electronic flux (REF) analyses are performed along the intrinsic reaction coordinate (IRC) determined at the B3PW91/6-31+G(d)/SDD/C-PCM/UFF computational model for deeper insights into the reaction mechanism. Despite relatively high endergonicity, the TS structure is quite close to the center of the reaction coordinate, contrary to the Hammond principle. In the QM/MM MD part of the study, the B97D3 computational setting from the previous part is used as a QM core, and several different explicit water solvation models are explored in the MM environment. The TIP3P water model is compared with the OPC, POL3, TIP4P, and SPCE ones. Nevertheless, they all lead to very low activation barriers and mild endergonicity. Both ΔGr and ΔGa energies are visibly reduced compared to QM values when PCM models are applied. Since partial charges of water atoms within the QM calculations are visibly smaller than point charges in all the explored force-field water models, a modified TIP3P (with partial charges close to DFT RESP values and a LJ parameter conserving the correct water density) is used. In this manner, the energy profile is closer to QM results (with ΔGa = 8.2 and ΔGr = 6.4 kcal mol−1)—especially to the CCSD(T)/TZP/D-PCM/scaled-UAKS model (ΔGa = 14.6 and ΔGr = 9.1). Nevertheless, the hydration process is predicted to be endoergic in all explored models.
Investigating ferromagnetic response in monolayer CVD grown MoS2 flakes using quantum weak measurement
We synthesize MoS2 atomic layer flakes at different growth conditions to tailor S-terminated and Mo-terminated edge defect states that are investigated for their ferromagnetic response. We leverage quantum weak measurement principles to construct a spin Hall effect of light-based magneto-optic Kerr effect (SHEL-MOKE) setup to sense the ultra-small magnetic response from the synthesized atomic layers. We establish that Mo-terminated edge states are the primary source of ferromagnetic response from MoS2 flakes, which is consistent with x-ray photoelectron, Raman, and photoluminescence spectroscopic results. In the process, we demonstrate SHEL-MOKE to be a robust technique to investigate ultra-weak magnetic properties in novel atomic-scale materials. Our findings highlight the importance of controlling edge terminations in engineering magnetism at the nanoscale and underscore the potential of weak value amplification based optical measurements.
Uniaxial ordering by self-assembly of isotropic octahedral junctions
We demonstrate that isotropic octahedral (sixfold branched) junctions with three diagonal end point pairs of different colors almost inevitably form a macroscopic assembly of uniaxial order, exhibiting the perfect order of a single color. Monte Carlo simulations of the antiferromagnetic three-state Potts model on the tripartite reo net, consisting of corner-sharing regular octahedrons, confirm this counterintuitive prediction while showcasing switching self-assembly upon an ordering phase transition. The possible inequivalence of three directions, i.e., more symmetry breaking than uniaxiality, is found and discussed for the ordered phase of this model at finite temperatures. Some additional analyses of the model are provided, including the possibility of a metastable isotropic order, which aligns better with intuition.
Experimental evidences for the correlation between chair-to-chair conversion and dynamic fragility in poly(cyclohexyl methacrylate)/hindered phenol blends
Polymers with cyclic or ring topologies are typically endowed with unusual molecular dynamics considerably different from their linear or branched counterparts. Here, the primary and secondary dynamics of neat poly(cyclohexyl methacrylate) (PCHMA) and its blends with hindered phenols were systematically investigated by a combination of dielectric and enthalpy relaxation. Notably, neat PCHMA owns a glass transition temperature Tg ∼ 299 K, while the segmental fragility m can be as low as about 42. After introducing hindered phenols capable of forming intermolecular hydrogen bonds, m increases in the same direction as Tg. Surprisingly, the observed increase in segmental fragility exhibits a strong correlation with the suppression of the dielectric γ-relaxation process, which has been previously attributed to chair-to-chair transitions of the cyclohexyl side groups. The concomitant decrease in accessible configurational states is further evidenced by enthalpy relaxation. We suppose that the γ-relaxation may involve less degree of intermolecular freedom, which can be speculated from its increased activation energy that is comparable to that of the Johari–Goldstein secondary process, as well as the broadened γ-peak breadth. These results provide solid experimental evidence that a correlation does exist between the conformational change and fragility and therefore pave a new routine to fabricate strong polymeric glass formers by introducing the ring-containing topology structures.