Browse Articles

Analyzing energy transfer with density-functional theory in real time: Time scales for the energy transfer between B850 bacteriochlorophylls

We present techniques that allow for predicting energy transfer in multichromophoric systems based on density-functional-theory calculations in real-time. Our work respects that the time-dependent density is the basic quantity in density-functional theory. In the approach that we discuss here, the simulations are done for a complete multimolecular system, i.e., do not require an a priori decomposition into subsystems. Yet, our analysis tools allow one to reliably extract energy-transfer times between different regions or constituents of the multimolecular system, the structure of transition densities, and the relative degree of excitation of constituents. We demonstrate our approach by analyzing the excitation-energy transfer between six bacteriochlorophyll molecules from the B850 ring of the light-harvesting complex 2 of the purple bacterium Rhodoblastus acidophilus. Our analysis shows that energy is transferred through this system on a time scale of ∼45 fs. The spectral analysis reveals that mainly two supermolecular excitations drive the energy transfer in this system.

Terahertz spectroscopy study of the confining potential for methane in the endofullerene CH4@C60

We used terahertz spectroscopy to study the non-covalent interaction between CH4 and the confining fullerene cage in endofullerene CH4@C60. The temperature dependence of the THz absorption spectra of powdered CH4@C60 was measured between 5 and 300 K. At temperatures below 50 K, the THz spectrum of CH4 shows a single line centered at 214 cm−1, which broadens and shifts to a higher energy as the temperature increases. These effects are explained by the anharmonicity of the CH4–C60 interaction potential function. The model involves the center-of-mass motion of CH4 in a spherically symmetric potential well. Line intensities are modeled by invoking an electric dipole moment induced by the translational displacement of CH4 from the center of the cage C60. The potential function and the dipole moment parameters were derived from the temperature dependence of the THz absorption spectra and compared with the parameters of previously studied endofullerenes. The quantum chemistry calculations reproduce the CH4 translation motion frequency and the potential function remarkably well.

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Partitioning the electronic wave function using deep variational Monte Carlo

We propose a novel wave function partitioning method that integrates deep-learning variational Monte Carlo with ansätze based on generalized product functions. This approach effectively separates electronic wave functions (WFs) into multiple partial WFs representing, for example, the core and valence domains or different electronic shells. Although our ansätze do not explicitly include correlations between individual electron groups, we show that they accurately reproduce the underlying physics and chemical properties, such as dissociation curve, dipole moment, reaction energy, ionization energy, or atomic sizes. We identify the optimal number of core electrons and define physical core sizes for Li to Mg atoms. Our results demonstrate that core electrons can be effectively decoupled from valence electrons. We show that the core part of the WF remains nearly constant across different molecules and their geometries, enabling the transfer and reuse of the core part in WFs of more complex systems. This work provides a general framework for WF decomposition, offering potential advantages in computing and studying larger systems, and possibly paving the way for ab initio development of effective core potentials. Although currently limited to small molecules due to scaling, we highlight several directions for extending our method it to larger systems.

Ionization-driven collapse of xenon and argon foams in superfluid helium droplets

The electron diffraction studies of neutral and ionic rare gas clusters formed within superfluid helium droplets reveal several notable findings. Under various doping conditions and droplet sizes ranging from 104 to 106 helium atoms, both neutral argon and xenon clusters can exhibit foam-like structures, in which helium atoms reside between rare gas atoms and inhibit the formation of a fully bound structure. Under comparable doping conditions and droplet sizes, argon clusters exhibit even fewer bound structures, best described by more dispersed distributions compared to xenon clusters. Under low doping conditions—despite differences in droplet sizes (104 vs 106 helium atoms per droplet)—argon clusters yield broad, Gaussian distance distributions. Moreover, increasing the doping pressure while maintaining the same droplet conditions promotes the formation of more tightly bound clusters, with a greater contribution from the van der Waals distance in the overall pair-distance distributions. Finally, electron impact ionization of both rare gas clusters triggers a collapse of the foam-like structures, resulting in clusters that are most accurately described by a single dominant interatomic distance.

Intersystem crossing in the molecular channel of ammonia photodissociation manipulated by an external laser field

Conical intersections (CIs) play an important role in photochemistry, allowing for ultrafast radiationless decay in processes such as photodissociation. In addition to these natural CIs, an external electric field can create light-induced conical intersections (LICIs), as the dipole–field interaction shifts the coupled potential energy surfaces. This work explores the effect of LICIs on the minor molecular (NH + H2) channel of ammonia photodissociation, building on prior work that studied the major radical (NH2 + H) channel. The molecular channel can involve intersystem crossing to the low-lying triplet state. A Floquet Hamiltonian was used to simulate the dynamics of ammonia photodissociation in the presence of an external laser field. A total of 230 000 quasi-classical trajectory surface-hopping calculations were performed with SHARC using recently reported diabatic potential energy matrices, dipole matrices, and spin–orbit coupling matrices, which were fitted using neural networks. With the field off, 0.04% of trajectories with a total energy of 8.4 eV yielded triplet NH. With an electric field strength of 0.02 Ha/ea0, over 0.3% of trajectories yielded triplet NH (an eightfold increase), and this value was even larger for higher field strengths.

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The coadsorption of chlorine and oxygen on Ag(111): Detection of electrophilic oxygen in the ClO and ClO3 quasimolecules

The coadsorption of chlorine and oxygen on the Ag(111) surface has been studied with low-temperature scanning tunneling microscopy in combination with density functional theory (DFT) calculations. The coadsorption systems were created in two ways: either by the oxidation of the chlorinated Ag(111) surface or by chlorination of the oxidized sample. Depending on the stoichiometry, a series of new well-ordered coadsorbed structures containing both oxygen and chlorine atoms has been discovered. The DFT analysis has shown that all the structures correspond to the reconstruction of the silver (111) surface. Moreover, the reconstruction is formed by silver triangles with atoms occupying either fcc or hcp positions. Most chlorine and oxygen atoms occupy nearly fourfold positions similar to those in the p(4 × 4)-O and (3 × 3)-Cl reconstructions. A characteristic feature of all coadsorbed structures is the presence of bright objects, which we associate with ClO3 and ClO quasimolecules. Using DFT, we calculated the O 1s binding energies for all oxygen atoms in the structures. The binding energy of oxygen atoms in the nearly fourfold positions is 528 eV [as in the Ag(111)-p(4 × 4)-O phase], while the binding energies of oxygen atoms in ClO3 and ClO quasimolecules are 531 and 530 eV, respectively, indicating their electrophilic nature.

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Single-atom catalysts based on one-dimensional metal porphyrin chains toward oxygen reduction reactions

Single-atom catalysts has emerged as a groundbreaking concept in catalysis, where individual metal atoms are anchored on supports such as carbon-based materials, oxides, or nitrides, serving as isolated active sites for catalytic reactions. In the present study, we theoretically explore the geometries, bonding properties, electronic structures, and potential catalytic performances of the recently synthesized one-dimensional (1D) M-porphyrin chains, in which the M–N4–C motif acts as the active site for oxygen reduction reactions (ORRs). Three configurations of 1D M-porphyrin chains (M = Ni, Zn) were investigated, including (a) M-porphyrin ribbon, (b) butadiyne-linked M-porphyrin, and (c) M-porphyrin-fused graphene nanoribbons. The calculation results reveal that all those 1D M-porphyrin chains are semiconductors. Energy decomposition analysis combined with natural orbital for chemical valence (EDA-NOCV) shows that the metal–ligand interaction in Ni-porphyrin is stronger than that in Zn-porphyrin. Compared to Zn-porphyrin chains, Ni-porphyrin chains exhibit stronger adsorption and superior electron transfer capabilities, which is attributed to enhanced orbital hybridization between the Ni 3d atomic orbitals and adsorbed oxygen 2p orbitals. The catalytic reaction pathways of these chains are similar for all those SACs and depend slightly on linker types, highlighting the importance of the local environment of the M–N4–C coordination framework. These findings provide valuable insights into the design of SACs with tailored properties, offering significant potential for applications in energy conversion and environmental catalysis.

The microcanonical Lindemann mechanism for unimolecular reactions

The microcanonical analog of the Lindemann mechanism for unimolecular reactions, or microcanonical Lindemann mechanism (MLM), is discussed. The mechanism makes explicit a central role of intramolecular vibrational redistribution (IVR) in mediating rates of unimolecular reactions. Under conditions of ergodic dynamics, the MLM predicts single-exponential kinetics with a rate constant that is generally smaller than the Rice–Ramsperger–Kassel–Marcus (RRKM) theory estimate. The RRKM theory rate is reached in the limit where rates of IVR from states of the activated complex to non-reactive states of the reactant are much greater than rates of transition from states of the activated complex to states of the product. We discuss the MLM in the context of experimental results for conformational isomerization, one irreversible, the photoisomerization of trans-stilbene, and another reversible, cyclohexane ringinversion. This Perspective aims to address and clarify misunderstandings that persist as to the role of IVR in unimolecular reaction kinetics. We also discuss, in the context of the MLM, a means to estimate rates of vibrational relaxation from states of the activated complex to non-reactive states from available energy-dependent unimolecular reaction rate data, which we believe is new.

Levels of symmetry-adapted perturbation theory (SAPT). II. Convergence of interaction energy components

Symmetry-adapted perturbation theory (SAPT) is a valuable theoretical technique useful in quantifying intermolecular interaction energies in terms of four physically meaningful components: electrostatics, exchange-repulsion, induction/polarization, and London dispersion. We present a systematic analysis of the convergence of SAPT total and component energies with respect to the level of theory and basis set using an extended database of 4569 van der Waals dimer geometries. Our analysis supports the use of SAPT0/aug-cc-pVDZ over previously recommended sSAPT0/jun-cc-pVDZ as an economical level of SAPT. Our previous recommendations of SAPT2+/aug-cc-pVDZ and SAPT2+(3)δMP2/aug-cc-pVTZ as medium and high cost variants, respectively, remain unchanged. However, SAPT0/aug-cc-pVDZ and SAPT2+/aug-cc-pVDZ total interaction energies on average rely on error cancellations, so they should be used with caution when parameterizing SAPT-based force fields and intermolecular potentials. SAPT2+(3)/aug-cc-pVTZ shows quantitatively accurate component energies, making it the preferred choice for applications when feasible. Finally, we examine a focal point approximation that approaches the accuracy of SAPT2+(3)δMP2/aug-cc-pVTZ with a significantly reduced cost.

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Tunable interfacial thermal conductance in graphene/germanene van der Waals heterostructure using an optimized interlayer potential

Accurately modeling interfacial thermal transport in van der Waals heterostructures is challenging due to the limited availability of interlayer interaction potentials. We develop a pairwise interlayer potential for graphene/germanene van der Waals heterostructures using the binding energy obtained from ab initio density functional theory calculations and use it to calculate the interfacial thermal conductivity. Our calculations reveal that the interfacial thermal conductivity shows superior tunability with external strain. The phonon density of states calculations show a blueshift in the phonon spectra with an applied compressive strain in the direction of heat flow, increasing the interfacial thermal conductance (ITC) to ∼136% of the unstrained value. In contrast, a tensile strain is found to cause an opposite effect, reducing the conductance to ∼70% of the unstrained value. Moreover, due to increased availability of phonons for heat transfer, both temperature and interaction strength are found to correlate positively with the ITC for both directions of heat flow.

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On the anomalous behavior of aqueous solutions: Rigorous formal results and the unraveling of their microstructural origin

We unravel the formal, unambiguous relationships between the evolution of the solute-induced perturbation of the solvent environment and the consequent macroscopic manifestation of the anomalous (aka non-monotonous composition) behavior of the second and third derivatives of the excess Gibbs free energy of a binary mixture. We address some relevant issues regarding the anomalous behavior of aqueous solutions, including the identification of their molecular-based manifestation and the rigorous interpretation according to explicit cause–effect relations between the thermodynamic evidence and the molecular-based behavior in terms of the relevant fundamental structure-making/structure-breaking functions and their temperature/pressure derivatives. Because this novel approach involves no restrictions on the type of interactions and the nature of the species in solution, it provides a powerful tool to gain understanding of the role played by solute–solvent intermolecular interaction asymmetries either in the presence or absence of hydrogen bonding interactions. Finally, we illustrate the formalism through a detailed analysis of the anomalies of aqueous ethanol solutions and test some conjectured hypotheses underlying the anomalies that point to their plausible connection to changes in the hydrogen bonding between species.

The effect of acceptor–donor–acceptor molecule bending on the symmetry breaking charge transfer and transition dipole moment

This study investigates the influence of molecular bending on excited-state symmetry-breaking charge transfer (SBCT) and transition dipole moments (TDMs) in donor–acceptor (A–D–A/D–A–D) systems with identical acceptors (or donors). The main conclusion is that SBCT, or exciton localization, is hindered in bent molecules. The proposed model introduces a key novelty: two reaction coordinates are required to describe SBCT in bent molecules, whereas linear systems can be adequately described by using a single coordinate. The model predicts that (i) bending reduces the degree of dissymmetry and induces nonmonotonic changes in the dipole moment, and (ii) the TDM of fluorescence exhibits monotonic increase or decrease with bending, contingent on the parity of the lower excited state. Crucially, the two-dimensional framework accounts for both the magnitude changes and rotational effects of the dipole moment and the TDM. The predictions of the model are aligned with available experimental data, validating its conclusions. Additionally, the article provides simple formulas for quantifying symmetry-breaking extent and TDM magnitudes (for absorption and fluorescence), offering practical utility for experimentalists in designing studies and interpreting results. These insights advance the design of optoelectronic materials, where controlled symmetry breaking is critical.

Dynamic signature of the thermodynamic transition in a novel mean field system

Understanding the connection between thermodynamics and dynamics in glass-forming liquids remains a central challenge in condensed matter physics. In this study, we investigate a novel model system that enables a continuous crossover from a standard three dimensional liquid to a fully connected mean field like system by introducing pseudo neighbors. These pseudo neighbors enhance the effective connectivity of the system without altering its local structure. While their presence slows down the dynamics, they influence thermodynamic properties even more significantly. In particular, the configurational entropy obtained via thermodynamic integration vanishes at a temperature much higher than the temperature where the dynamics begin to slow down, leading to a clear breakdown of the Adam–Gibbs relation. To uncover a possible dynamical signature of this thermodynamic transition, we analyze bond breakage dynamics. Unlike real–real bonds, which decay similarly in both the parent Kob–Andersen model and its mean field variant, real–pseudo bonds exhibit long lived, persistent behavior with strong temperature dependence. These bonds do not fully decay over time, leading to a finite saturation value of the bond breakage correlation function. Remarkably, we show that the number of surviving pseudo bonds can be analytically estimated and correlates directly with the thermodynamic transition temperature TK. We propose a phenomenological relation between TK and the number of surviving pseudo-bonds, establishing a novel link between thermodynamic and dynamic observables. Our results suggest that these persistent pseudo bonds serve as a robust dynamical signature of the thermodynamic transition, and the system might have properties analogous to those of randomly bonded ultrastable glasses.

The construction of spin eigenfunctions for fermion systems using modular tensor diagram

The construction of spin eigenfunctions for multi-spin systems can become highly nontrivial as the system size increases. Recently, a modular tensor diagram approach was proposed to hierarchically decompose the state space in terms of tensorial modules based on spin pairs, resulting in an effectively organized state space and efficiently constructed symmetry-adapted basis states. Here, it is generalized to treat fermion systems with odd numbers of spins. Elementary modules made of primitive spin-pair modules augmented by an odd-spin tag module are classified into various module classes and further mapped to geometric building blocks of various sizes and shapes to illustrate the hierarchical structure and symmetry of the state space. Spin eigenfunctions are generated from linear combinations of single-spin-tagged or triple-spin-tagged elementary modules using only symmetry and orthogonality conditions, and universal recursive relations for systems with arbitrary odd numbers of spins can be obtained. This work explores the structure and symmetry of the state space of fermion systems that complement previous studies, which may provide new insights into general quantum many-body systems and spin dynamics.