Spin-Adapted Externally Contracted Multireference Configuration Interaction Method Based on Selected Reference Configurations.

As one kind of approximation of the full configuration interaction solution, the selected configuration interaction (sCI) methods have been shown to be valuable for large active spaces. However, the inclusion of dynamic correlation beyond large active spaces is necessary for more quantitative results. Since the sCI wave function can provide a compact reference for multireference methods, previously, we proposed an externally contracted multireference configuration interaction method using the sCI reference reconstructed from the density matrix renormalization group wave function [J. Chem. Theory Comput. 2018, 14, 4747-4755]. The DMRG2sCI-EC-MRCI method is promising for dealing with more than 30 active orbitals and large basis sets. However, it suffers from two drawbacks: spin contamination and low efficiency when using Slater determinant bases. To solve these problems, in this work, we adopt configuration state function bases and introduce a new algorithm based on the hybrid of tree structure for convenient configuration space management and the graphical unitary group approach for efficient matrix element calculation. The test calculation of naphthalene shows that the spin-adapted version could achieve a speed-up of 6.0 compared with the previous version based on the Slater determinant. Examples of dinuclear copper(II) compound as well as Ln(III) and An(III) complexes show that the sCI-EC-MRCI can give quantitatively accurate results by including dynamic correlation over sCI for systems with large active spaces and basis sets.

Kylin-V: An open-source package calculating the dynamic and spectroscopic properties of large systems.

Quantum dynamics simulation and computational spectroscopy serve as indispensable tools for the theoretical understanding of various fundamental physical and chemical processes, ranging from charge transfer to photochemical reactions. When simulating realistic systems, the primary challenge stems from the overwhelming number of degrees of freedom and the pronounced many-body correlations. Here, we present Kylin-V, an innovative quantum dynamics package designed for accurate and efficient simulations of dynamics and spectroscopic properties of vibronic Hamiltonians for molecular systems and their aggregates. Kylin-V supports various quantum dynamics and computational spectroscopy methods, such as time-dependent density matrix renormalization group and our recently proposed single-site and hierarchical mapping approaches, as well as vibrational heat-bath configuration interaction. In this paper, we introduce the methodologies implemented in Kylin-V and illustrate their performances through a diverse collection of numerical examples.

Synergistic Al-Al Dual-Atomic Site for Efficient Artificial Nitrogen Fixation.

Synthesis of ammonia by electrochemical nitrogen reduction reaction (NRR) is a promising alternative to the Haber-Bosch process. However, it is commonly obstructed by the high activation energy. Here, we report the design and synthesis of an Al-Al bonded dual atomic catalyst stabilized within an amorphous nitrogen-doped porous carbon matrix (Al2NC) with high NRR performance. The dual atomic Al2-sites act synergistically to catalyze the complex multiple steps of NRR through adsorption and activation, enhancing the proton-coupled electron transfer. This Al2NC catalyst exhibits a high Faradaic efficiency of 16.56±0.3 % with a yield rate of 29.22±1.2 µg h-1 mgcat -1. The dual atomic Al2NC catalyst shows long-term repeatable, and stable NRR performance. This work presents an insight into the identification of synergistic dual atomic catalytic site and mechanistic pathway for the electrochemical conversion of N2 to NH3.

Mechanistic Insights into Water Autoionization through Metadynamics Simulation Enhanced by Machine Learning.

Characterizing the free energy landscape of water ionization has been a great challenge due to the limitations from expensive ab initio calculations and strong rare-event features. Lacking equilibrium sampling of the ionization pathway will cause ambiguities in the mechanistic study. Here, we obtain convergent free energy surfaces through nanosecond timescale metadynamics simulations with classical nuclei enhanced by atomic neural network potentials, which yields good reproduction of the equilibrium constant (pK_{w}=14.14) and ionization rate constant (1.369×10^{-3} s^{-1}). The character of transition state unveils the triple-proton transfer occurs through a concerted but asynchronous mechanism. Conditional ensemble average analyses establish the dual-presolvation mechanism, where a pair of hypercoordinated and undercoordinated waters bridged by one H_{2}O cooperatively constitutes the initiation environment for autoionization, and contributes extremely to the local electric field fluctuation to promote water dissociation.

Benzo[g]quinoxaline-Based Complexes [Ir(pbt)2(dppn)]Cl and [Ir(pt)2(dppn)]Cl: Modulation of Photo-Oxidation Activity and Light-Controlled Luminescence.

Based on benzo[i]dipyrido[3,2-a:2',3'-c]phenazine (dppn) with photo-oxidation activity, complexes [Ir(pbt)2(dppn)]Cl (1) and [Ir(pt)2(dppn)]Cl (2) have been synthesized (pbtH = 2-phenylbenzothiazole, and ptH = 2-phenylthiazole), with two aims, including studying the influence of the cyclometalating ligands (pbt- in 1, pt- in 2) on the photo-oxidation activity of these complexes and exploring their photo-oxidation-induced luminescence. Both 1H nuclear magnetic resonance (NMR) and electrospray (ES) mass spectrometry indicate that the benzo[g]quinoxaline moiety in complex 1 can be oxidized at room temperature upon irradiation with 415 nm light. Thus, this complex in CH2Cl2 shows photo-oxidation-induced turn-on yellow luminescence. In contrast, complex 2 reveals significant structural decomposition during the process of photo-oxidation due to incorporating a cyclometalating ligand pt- instead of pbt- in complex 1. In this paper, we report the photo-oxidation behaviors and the related luminescence modulation in 1 and 2 and discuss the relationship between structure and photo-oxidation activity in these complexes.

Enhancing the Understanding of Hydrogen Evolution and Oxidation Reactions on Pt(111) through Ab Initio Simulation of Electrode/Electrolyte Kinetics.

The hydrogen oxidation reaction (HOR) and hydrogen evolution reaction (HER) play an important role in hydrogen-based energy conversion. However, the sluggish kinetics in alkaline media has raised debates on the relevant mechanism, especially on the role of surface hydroxyl (OH*). With the potential-related free energy profiles obtained with density functional theory calculations, the full pH range transient kinetics simulation of HER/HOR polarization curves on Pt(111) agrees well with experimental observations. Studying model systems with varying metal-OH* binding energies confirms that the current near the HOR onset potential is contributed from the pathway through OH- rather than OH*, suggesting that OH* is unlikely an effective activity descriptor for HOR. The degree of rate control analyses reveal that, while acidic current is controlled solely by the Tafel step, alkaline current is controlled jointly by Tafel and Volmer steps, as the Volmer barrier is considerably increased in alkaline conditions. Finally, based on a model study, we draw up a scheme of reducing the overpotential of alkaline HER/HOR by accelerating the Tafel step.

High-Performance Inverted Planar Perovskite Solar Cells Enhanced by Thickness Tuning of New Dopant-Free Hole Transporting Layer.

A new hole transporting material (HTM) named DMZ is synthesized and employed as a dopant-free HTM in inverted planar perovskite solar cells (PSCs). Systematic studies demonstrate that the thickness of the hole transporting layer can effectively enhance the morphology and crystallinity of the perovskite layer, leading to low series resistance and less defects in the crystal. As a result, the champion power conversion efficiency (PCE) of 18.61% with JSC = 22.62 mA cm-2 , VOC = 1.02 V, and FF = 81.05% (an average one is 17.62%) is achieved with a thickness of ≈13 nm of DMZ (2 mg mL-1 ) under standard global AM 1.5 illumination, which is ≈1.5 times higher than that of devices based on poly(3,4-ethylenedioxythiophene)/poly(styrene sulfonic acid) (PEDOT:PSS). More importantly, the devices based on DMZ exhibit a much better stability (90% of maximum PCE retained after more than 556 h in air (relative humidity ≈ 45%-50%) without any encapsulation) than that of devices based on PEDOT:PSS (only 36% of initial PCE retained after 77 h in same conditions). Therefore, the cost-effective and facile material named DMZ offers an appealing alternative to PEDOT:PSS or polytriarylamine for highly efficient and stable inverted planar PSCs.

How intermolecular interactions influence electronic absorption spectra: insights from the molecular packing of uracil in condensed phases.

The photoexcitation mechanism in photochemistry and photophysics is a key to understanding the photostability and photoreaction of nucleobases. Using a combination of the generalized energy-based fragmentation (GEBF) and quantum mechanical and molecular mechanical (GEBF-QM/MM) approach and the QM/MM approach, we have investigated the electronic absorption spectra for the π-π* transition of uracil in aqueous solution, amorphous solid, and crystal. Our results indicate that the intermolecular interactions in terms of molecular packing are crucial for the investigation of the absorption spectra of uracil in different environments. There is a large red-shift (relative to uracil in the gas-phase) for uracil in the amorphous phase, which arises from hydrogen-bonding (HB) and close π-π stacking interactions. In contrast, the relatively smaller red-shift of uracil in aqueous solution can be attributed to the cooperative HB and long-range electrostatic and polarization interactions. Due to the HB and weak π-π interactions, the red-shift of the crystal is smaller than that of amorphous uracil. Furthermore, the results suggest that a large system is required to obtain the accurate absorption spectra of solutions, whereas small electrostatically embedded cluster models could be used to obtain the corresponding results for amorphous solids and molecular crystals.

Time-dependent density matrix renormalization group quantum dynamics for realistic chemical systems.

Electronic and/or vibronic coherence has been found by recent ultrafast spectroscopy experiments in many chemical, biological, and material systems. This indicates that there are strong and complicated interactions between electronic states and vibration modes in realistic chemical systems. Therefore, simulations of quantum dynamics with a large number of electronic and vibrational degrees of freedom are highly desirable. Due to the efficient compression and localized representation of quantum states in the matrix-product state (MPS) formulation, time-evolution methods based on the MPS framework, which we summarily refer to as tDMRG (time-dependent density-matrix renormalization group) methods, are considered to be promising candidates to study the quantum dynamics of realistic chemical systems. In this work, we benchmark the performances of four different tDMRG methods, including global Taylor, global Krylov, and local one-site and two-site time-dependent variational principles (1TDVP and 2TDVP), with a comparison to multiconfiguration time-dependent Hartree and experimental results. Two typical chemical systems of internal conversion and singlet fission are investigated: one containing strong and high-order local and nonlocal electron-vibration couplings and the other exhibiting a continuous phonon bath. The comparison shows that the tDMRG methods (particularly, the 2TDVP method) can describe the full quantum dynamics in large chemical systems accurately and efficiently. Several key parameters in the tDMRG calculation including the truncation error threshold, time interval, and ordering of local sites were also investigated to strike the balance between efficiency and accuracy of results.

Origin of the overpotentials for HCOO- and CO formation in the electroreduction of CO2 on Cu(211): the reductive desorption processes decide.

Electroreduction of CO2 on Cu surface provides the potential in producing hydrocarbons and other multi-carbon products. However, a comprehensive understanding of the potential-related mechanism is required to improve the product selectivity as well as to reduce the overpotentials. Herein, we systematically characterize the potential effect on the complete reaction pathways to CO and HCOO- on the Cu(211) surface. Reaction free energy and activation barrier are computed as functions of electric potential. It is found that chemical adsorption state of CO2 is effectively stabilized by the substrate, which is expected to be dominant at potentials below -0.27 V vs. SHE, much earlier than that previously reported on Cu(100). Considering that the activation barriers of the other surface processes are small enough to be overcome at room temperature, the large reductive desorption free energies of OH- and HCOO- are suggested as the origin of high overpotentials.

Photoisomerization of silyl-substituted cyclobutadiene induced by σ → π* excitation: a computational study.

Photoinduced chemical processes upon Franck-Condon (FC) excitation in tetrakis(trimethylsilyl)-cyclobutadiene (TMS-CBD) have been investigated through the exploration of potential energy surface crossings among several low-lying excited states using the complete active space self-consistent field (CASSCF) method. Vertical excitation energies are also computed with the equation-of-motion coupled-cluster model with single and double excitations (EOM-CCSD) as well as the multireference Møller-Plesset (MRMP) methods. Upon finding an excellent coincidence between the computational results and experimental observations, it is suggested that the Franck-Condon excited state does not correspond to the first π-π* single excitation state (S1, 1(1)B1 state in terms of D2 symmetry), but to the second (1)B1 state (S3), which is characterized as a σ-π* single excitation state. Starting from the Franck-Condon region, a series of conical intersections (CIs) are located along one isomerization channel and one dissociation channel. Through the isomerization channel, TMS-CBD is transformed to tetrakis(trimethylsilyl)-tetrahedrane (TMS-THD), and this isomerization process could take place by passing through a "tetra form" conical intersection. On the other hand, the dissociation channel yielding two bis(trimethylsilyl)-acetylene (TMS-Ac) molecules through further stretching of the longer C-C bonds might be more competitive than the isomerization channel after excitation into S3 state. This mechanistic picture is in good agreement with recently reported experimental observations.

A non-adiabatic dynamics study of octatetraene: the radiationless conversion from S2 to S1.

Simulation of the excited state dynamics of all-trans-1,3,5,7-octatetraene has been performed to investigate the ultrafast radiationless S2 → S1 internal conversion process. Multireference configuration interaction with single excitation method has been employed to optimize the equilibrium structure of the excited states, as well as the S2/S1 conical intersection, and to investigate the non-adiabatic molecular dynamics of the S2/S1 state transition. At the conical intersection, the molecule is found to be distorted from the original planar trans structure to a nearly perpendicular conformation around C3-C4 bond, with the torsion angle being about 107°. Such structural change can result in mutual approaching of states S2 and S1 in energy, and drastically increase the nonadiabatic coupling between the two states by destroying the inter-state symmetry prohibition in the electronic wavefunctions. Surface-hopping molecular dynamics simulations are performed to describe the non-adiabatic process. Upon the Franck-Condon excitation to the S2 state, the molecule quickly twists its C3-C4 bond and approaches the conical intersection region, where it can undergo efficient internal conversion to S1. The decay time constant (τ) of S2 state is estimated to be around 251 fs by fitting the occupation number of average fraction of trajectories using an exponential damping function. This value is reasonably consistent with previous experimental measurements of around 300-400 fs.

Hierarchical Mapping for Efficient Simulation of Strong System-Environment Interactions.

Quantum dynamics (QD) simulation is a powerful tool for interpreting ultrafast spectroscopy experiments and unraveling their microscopic mechanism in out-of-equilibrium excited state behaviors in various chemical, biological, and material systems. Although state-of-the-art numerical QD approaches such as the time-dependent density matrix renormalization group (TD-DMRG) already greatly extended the solvable system size of general linearly coupled exciton-phonon models with up to a few hundred phonon modes, the accurate simulation of larger system sizes or strong system-environment interactions is still computationally highly challenging. Based on quantum information theory (QIT), in this work, we realize that only a small number of effective phonon modes couple to the excitonic system directly regardless of a large or even infinite number of modes in the condensed phase environment. On top of the identified small number of direct effective modes, we propose a hierarchical mapping (HM) approach through performing block Lanczos transformations on the remaining indirect modes, which transforms the Hamiltonian matrix to a nearly block-tridiagonal form and eliminates the long-range interactions. Numerical tests on model spin-boson systems and realistic singlet fission models in a rubrene crystal environment with up to 7000 modes and strong system-environment interactions indicate HM can reduce the system size by 1-2 orders of magnitude and accelerate the calculation by ∼80% without losing accuracy.

Visualization of Electron Density Changes Along Chemical Reaction Pathways.

We propose a simple procedure for visualizing the electron density changes (EDC) during a chemical reaction, which is based on a mapping of rectangular grid points for a stationary structure into (distorted) positions around atoms of another stationary structure. Specifically, during a small step along the minimum energy pathway (MEP), the displacement of each grid point is obtained as a linear combination of the motion of all atoms, with the contribution from each atom scaled by the corresponding Hirshfeld weight. For several reactions (identity SN2, Claisen rearrangement, Diels-Alder reaction, [3+2] cycloaddition, and phenylethyl mercaptan attack on pericosine A), our EDC plots showed an expected reduction of electron densities around severed bonds (or those with the bond-order lowered), with the opposite observed for newly-formed or enhanced chemical bonds. The EDC plots were also shown for copper triflate catalyzed N2O fragmentation, where the N-O bond weakening initially occurred on a singlet surface, but continued on a triplet surface after reaching the minimum-energy crossing point (MECP) between the two potential energy surfaces.

The Existence of Periodic Solutions for Second-Order Delay Differential Systems.

In this paper, we consider a kind of second-order delay differential system. By taking some transforms, the property of delay is reflected in the boundary condition. The wonder is that the corrseponding first-order system is exactly the so-called P-boundary value problem of Hamiltonian system which has been studied deeply by many mathematicians, including the authors of this paper. Firstly, we define the relative Morse index µ Q ( A , B ) for the delay system and give the relationship with the P-index i P ( γ R ) of Hamiltonian system. Secondly, by this index, topology degree and saddle point reduction, the existence of periodic solutions is established for this kind of delay differential system.

Electronic structures of low-lying Bu excited states in trans-oligoenes: Pariser-Parr-Pople and ab initio calculations.

Two lowest-lying excited singlets with B(u) symmetry of all-trans-oligoenes, the well-known ionic 1(1)B(u)(+) state as well as the "hidden" ionic-covalent-mixed 1(1)B(u)(-) state, are calculated within both the Pariser-Parr-Pople (PPP) model at full configuration interaction (FCI) level and ab initio methods. The vertical excitation energies as well as wavefunctions from PPP-FCI calculations are found to be in good agreement with those from high-level multi-reference methods, such as multi-reference complete active space self-consistent field (CASSCF) with second order perturbative corrections (CASPT2), multi-reference Møller-Plesset perturbation theory (MRMP), and complete active space valence bond theory (CASVB). The oscillator strengths from PPP calculation are in good agreement with spectroscopy experiments. The relatively small oscillator strength of 1(1)B(u)(-) is due to the approximate electron-hole symmetry of this state. In addition, the bond lengths in both states are found to show remarkable relativity with the bond orders calculated with ground state geometries, which suggests a possible strategy for initial guess in geometry optimization of excited states.

Excitation energy calculation of conjugated hydrocarbons: a new Pariser-Parr-Pople model parameterization approaching CASPT2 accuracy.

A new parameterization for the Pariser-Parr-Pople (PPP) model for conjugated hydrocarbons is proposed in this work. The distance-dependence of PPP parameters are obtained from CASPT2 ground state and low-lying excited state energies of ethylene and its cation at various C-C single bond lengths and are fitted to a set of carefully chosen mathematical functions. Our new PPP model is applied to the calculation of vertical singlet-triplet energy gaps and the excitation energies for low-lying π→π(*) valence excitations in various π-conjugated molecules. Results with the new PPP model are consistently better than the standard PPP model in use. It often surpasses density functional theory and single-reference excited state methods such as configuration interaction singles or time-dependent density functional theory in terms of its accuracy and agrees reasonably well with high-level theories or experiments.

Communication: A dramatic transition from nonferromagnet to ferromagnet in finite fused-azulene chain.

One-dimensional fused-azulene oligomers (n = 2-6) are studied with the effective valence bond as well as density functional theory methods. A nonferromagnetic (closed-shell singlet) to ferromagnetic (triplet) ground state transformation is witnessed with increasing length of oligomers. The computational results are interpreted in terms of spin coupling between the unpaired electrons of two nonbonding molecular orbitals localized, respectively, on the top and bottom chains of the oligomers. The present study provides a theoretical suggestion for understanding the ferromagnetic spin polarizations that has been observed very recently in graphene nanoribbons.

Controlled Fluorescence Enhancement of DNA-Binding Dye Through Chain Length Match between Oligoguanine and TOTO.

Fluorescent DNA-binding dyes are extensively employed as probe and biosensing in biological detection and imaging. Experiments and theoretical calculations of thiazole orange homodimeric (TOTO) dye binding to a single-strand DNA (ssDNA), poly(dG)n (n = 2, 4, 6, 8), reveal that the n = 6 complex shows about 300-fold stronger fluorescence than n = 2, 4 and a slightly stronger one than n = 8 complexes, which is benefited from the length match between TOTO and poly(dG)6. The machine learning, based on molecular dynamics trajectories, indicates that TOTO is featured by the dihedral angle along its backbone and its end-to-end distance, in which the latter one defines the stretch and hairpin structures of TOTO, respectively. The time-dependent density functional theory calculations on the low-lying excited states show that the stretched TOTO with π-π end-stacking binding mode can bring about strong fluorescence with localized π-π* transitions. For the n = 2, 4, and 8 complexes, the linear scaling quantum mechanics calculations indicate that the dominant hairpin TOTO with intercalative binding modes have relatively larger binding energies, leading to fluorescence quenching by intramolecular charge transfer. Our results may provide an insight for modulating the DNA-dye binding modes to tune the degree of charge transfer and designing fluorescent probes for the recognition of specific DNA sequences.

Free-Standing Single Ag Nanowires for Multifunctional Optical Probes.

Miniaturized and manipulable optical probes are the foundation for developing in situ characterization devices in confined space. We developed two methods for fabricating free-standing single Ag nanowires (AgNWs) directly at the tip of a glass capillary either by chemical or electrochemical reduction. The electrochemical nature of both methods resulted in a rapid growth rate of AgNWs up to 1.38 µm/s and a controllable length from 5 to 450 µm. The AgNWs with a unique anisotropic structure allow localized surface plasmon resonance and surface plasmon waveguides in the radial direction and axial direction, respectively. We verified the possibility of using single AgNWs as an optical dispersion device and waveguide probe. By controlling the experimental conditions, rough-surface AgNWs with high surface-enhanced Raman scattering (SERS) activity were also fabricated. These SERS-active probes also exhibited advantages in acquiring molecular information from a single living cell.