Selected configuration interaction (SCI) methods are effective for treating strongly correlated electronic systems, yet their scalability has long been limited by implementations that replicate the configuration interaction (CI) vector across processes, leading to severe memory bottlenecks. Here, we present a fully distributed diagonalization framework tailored for extremely large selected determinant spaces, directly addressing this major scalability bottleneck of modern SCI methods. The method is grounded in a tensor-product bitstring (TPB) representation, in which determinants are organized through a TPB structure constructed from selected α- and β-bitstrings, and is referred to as tensor-product bitstring SCI (TBSCI). An efficient TBSCI eigensolver is developed based on a novel bitstring-based Hamiltonian evaluation algorithm together with a suite of MPI communication strategies designed to improve parallel efficiency. Large-scale full configuration interaction (FCI) benchmarks, employed as communication-intensive stress tests, demonstrate that the implemented TBSCI eigensolver continues to reduce the wall time for distributed diagonalization of 2.6 × 1012 determinants, reaching 54 000 nodes (more than 2.5 × 106 cores) on supercomputer Fugaku. Beyond scalability, we investigate the structural compactness of the TPB representation and show that selecting α- and β-bitstrings according to their collective weights in a reference SCI wavefunction yields TPB-based wavefunctions approaching the FCI limit while using only a small fraction of determinants. These results establish TBSCI as a scalable SCI methodology and provide evidence for the intrinsic compactness of the TPB representation.

Solvent-driven backbone chain distribution in bulk of P3HT film: Effect on the microstructure via molecular dynamics simulations.

Photoelectron-photoion (PEPICO) and photoion-photoion (PIPICO) coincidence measurements, coupled with a few-cycle intense laser field, were employed to investigate strong field ionization/dissociation dynamics of vinyl bromide (C2H3Br). The angular streaking technique was used to map the recoil-frame angle-dependent ionization rates to identify ionizing orbitals. Dissociative single ionization was mainly attributed to ionizing the lower lying molecular orbitals, suggesting that direct ionization dominates with few-cycle pulses. Three dissociative double ionization channels, both prompt and delayed fragmentation, were identified. Theoretical analysis using time-dependent configuration interaction with complex absorbing potential (TDCI-CAP) and high-level electronic structure methods was performed to elucidate the underlying ionization and dissociation mechanisms.

The lowest electronic states of the transition intermetallic TiV molecule have been studied by first principles employing the multireference configuration interaction technique and large correlation consistent basis sets. The ground state was found to be of X4Σ- symmetry with a binding energy of D00 = 41.1 kcal mol-1 and re = 1.910 Å. Full potential energy curves were constructed for a total of 45 low-lying Λ-S states of TiV, extracting spectroscopic constants, as well. In addition, an effort was made to rationalize the nature of the chemical bond in the different states of the system.

Quantum computing has emerged as a promising paradigm for tackling electronic structure problems, with most efforts to date focused on molecular energies and, more recently, first-order derivatives. However, the extension to second-order energy derivatives with respect to nuclear coordinates─essential for predicting vibrational spectra and identifying transition states─has remained relatively limited. Here, we present an analytic implementation for computing nuclear Hessians within the variational quantum eigensolver framework. Our approach produces harmonic vibrational frequencies and normal modes in excellent agreement with full configuration interaction (FCI) benchmarks, even for systems with challenging cases of orbital degeneracy or involving weak intermolecular interactions. We further assess the quantum measurement cost of these second-order derivative calculations and compare it with that of variational quantum eigensolver (VQE) energy calculations. Additionally, we demonstrate how point group symmetry can be incorporated to reduce measurement cost without loss of accuracy. This work extends quantum computing capabilities toward advanced quantum chemical simulations, such as the characterization of noncovalent interactions via low-frequency vibrational signatures and the identification of transition states.

The coordinate descent full configuration interaction (CDFCI) is an accurate and efficient method for calculating ground-state energies of molecular systems. In this work we introduce two perturbative extensions of original CDFCI. We first propose a direct CDFCI + PT to further correct variational energy by perturbation theory. We also propose an iterative perturbative CDFCI (IP-CDFCI) to avoid evaluating the overlarge perturbative space and catch the information encoded in variational stage. In IP-CDFCI, we store three sparse vectors in a Hash table: c, b and d, where c is the variational wave function, b is the compressed approximation of Hc and d is the perturbative wave function, respectively. By utilizing the redundant memory in the storage of c, we can calculate perturbation without any extra memory compared to original CDFCI. We also use a strategy to eliminate the expensive repetitive evaluation of diagonal elements of Hamiltonian by creating a vector h which stores the diagonal entries of Hamiltonian. This strategy trades memory usage for computational time and reduces the computing time significantly. Furthermore, we use quadruple-precision floating-point numbers to improve the numerical stability of perturbative energy. We demonstrate the effectiveness of the single-threaded IP-CDFCI and the multithreaded CDFCI + PT, respectively, by performing calculations on several small molecular benchmarks as well as challenging correlated system: the chromium dimer.

Nanohybrid systems in which semiconductor quantum dots (QDs) functionalize molecular photoswitches (PhSs) offer a promising platform for light-responsive materials. These systems leverage the reversible photoisomerization of PhSs and the size-tunable optical properties of QDs to enable functionalities in biomedicine, catalysis, and sensing. While strong light-matter coupling has been used to modulate photoisomerization in PhSs, such approaches are limited by ultrafast dynamics and the requirement for resonant cavity architectures. Here, we propose intrinsic excitonic coupling to shape photoisomerization pathways, taking advantage of the nanosecond-scale lifetimes of such hybrid states and the lower structural complexity of QD-based systems. Specifically, by applying the recently developed hybrid configuration interaction - a non-perturbative multiscale approach - to azobenzene and cadmium selenide quantum dots, we show avoided crossings near resonance between the photoswitch M0 → M1 transition and the lowest QD exciton, accompanied by excitonic splittings in the few meV range. Analysis of hybrid dipoles shows a redistribution of oscillator strength between the molecular and QD components, confirming the delocalized nature of the excitations. These results demonstrate that cavity-free PhS-QD nanohybrids can exhibit coherent excitonic reshaping of molecular photoisomerization, highlighting their potential as tunable, light-driven nanodevices.

Composite recipes based on ground-state coupled cluster methods can be used to calculate thermochemically relevant quantities such as heats of formation and ionization potentials to within 1 kJ/mol of experimental values. Achieving this level of accuracy with composite recipes typically requires inclusion of electron correlation of up to quadruple excitations. In this study, composite recipes based on the equation of motion coupled cluster theory (EOM-CC) have been designed to calculate electronic excitation energies of cations and ionization potentials (IP) of closed-shell molecules. The adiabatic ionization energies were calculated using two different recipes, and the results were compared to experimental values. With the more rigorous recipe, predictions of excitation and ionization energies were consistently within subchemical accuracy, with the ionization energies predicted with a mean absolute error (MAE) of 25 cm-1. An alternative recipe, designed to be more affordable, was applied to calculate IPs for a set of 16 molecules; it predicted ionization energies with an MAE of 132 cm-1. The level of electron correlation required in these recipes to reach subchemical accuracy is indicative of the slower convergence of EOM-CC to the full configuration interaction limit compared to ground-state CC methods.

Polyenes serve as a rigorous test for theoretical models and electronic structure methods, playing a key role in advancing computational and theoretical chemistry. Here, we present a high-level theoretical investigation of linear, all-trans polyenes using energy gradients and nonadiabatic coupling vectors based on an MR-CISD wave function to describe electronic transitions involving the ground state (11Ag-) and three low-lying excited states (21Ag-, 11Bu+, and 21Bu-) of hexatriene, octatetraene, and decapentaene. This approach enables accurate evaluation of both adiabatic and vertical excitation and emission energies, yielding results in excellent agreement with experiment, as well as locating minima on the crossing seam between adiabatic states. Our results show that vertical excitation energies to the 11Bu+ state are blue-shifted by 0.2-0.3 eV relative to the experimental absorption maximum, whereas the vertical emission energy from the 21Ag- state is red-shifted by ∼0.2 eV relative to the experimental emission maximum. Upon relaxation from the Franck-Condon geometry, the 21Ag- state stabilizes by around 1 eV, compared to 0.2-0.3 eV for the 11Bu+ state. An analysis of the S1/S0 crossing seam in hexatriene shows that its minimum involves asymmetric backbone deformations and provides an efficient channel for ultrafast internal conversion to the ground state, consistent with the absence of detectable fluorescence in this molecule. These results demonstrate the power of analytic gradients and nonadiabatic coupling vectors based on an MR-CISD wave function for accurately characterizing the electronic structure and photophysics of polyenes.

People rarely associate a geometric phase with nuclear configurations that lie far away from a conical intersection, where the ground state is energetically well-separated from excited states. However, the geometric phase is a nonlocal impact of a conical intersection that should emerge whenever a nuclear configuration travels on a closed loop trajectory that encloses the intersection in the branching plane, regardless of the distance from the intersection. In particular, if a trajectory starts at a structure with a stable closed-shell electronic configuration, how could the continuous closed-shell configuration possibly gain a sign change on finishing the loop? In this work, inspired by a recent paper by Kjønstad; ; Koch ( J. Chem. Phys. 2025, 163, 194104)., we performed full configuration interaction calculations and closed-shell single-reference calculations for the toy models of H4 and LiH3 to elucidate this problem. The investigation leads to the conjecture that restricted Hartree-Fock solutions necessarily encounter a point of degeneracy along closed loops that encircle a (or an odd number of) conical intersection(s). Single-reference wave functions need to change character close to such points and cusps or discontinuities in a potential energy surface and multireference characters are a likely result, even when the energy gap between ground and excited states is large.

Accurate many-body treatments of condensed-phase systems are challenging because correlated solvers such as the full configuration interaction (FCI) and the density matrix renormalization group (DMRG) scale exponentially with system size. Downfolding and embedding approaches mitigate this cost but typically require prior selection of a correlated subspace, which can be difficult to determine in heterogeneous or extended systems. Here, we introduce a stochastic cluster expansion framework for efficiently recovering the total correlation energy of large systems with near-DMRG accuracy and without the need to select an active space a priori. By combining correlation contributions from randomly sampled environment orbitals with an exactly treated subspace of interest, the method reproduces total energies for nonreacting and reactive systems while drastically reducing computational cost. The approach also provides a quantitative diagnostic for molecule-solvent correlation, guiding principled embedding decisions. This framework enables systematically improvable many-body calculations in extended systems, opening the door to high-accuracy studies of chemical processes in condensed phase environments.

We employ correlation-consistent effective core potentials (ccECPs) to perform exact or nearly exact correlation and total energy calculations for the fifth-row elements (Rb-Xe). Total energies are calculated using various correlated methods: configuration interaction (CI), coupled-cluster (CC) up to perturbative quadruple excitations whenever feasible, and stochastic quantum Monte Carlo (QMC) approaches. In order to estimate the energy at the complete basis set (CBS) limit, the basis sets are employed systematically through aug-cc-p(C)VnZ for each ccECP and further extrapolated to the CBS limit within the corresponding methods. Kinetic energies are evaluated at the FCI/CISD level to provide insights into the electron density and localization of the ccECPs. We also provide data sets for the widely used diffusion Monte Carlo (DMC) to quantify fixed-node biases with single-reference trial wave functions. These comprehensive benchmarks validate the accuracy of ccECPs within the CC, CI, and QMC methodologies, thereby providing accurate and tested valence-only Hamiltonians for many-body electronic structure calculations.

Quantum SENiority-based Subspace Expansion (Q-SENSE) is a hybrid quantum-classical algorithm that interpolates between the Variational Quantum Eigensolver (VQE) and Configuration Interaction (CI) methods. It constructs Hamiltonian matrix elements on a quantum device and solves the resulting eigenvalue problem classically. Unlike other expansion-based methods, such as Quantum Subspace Expansion (QSE), Quantum Krylov Algorithms, and the Non-Orthogonal Quantum Eigensolver, Q-SENSE introduces seniority operators as artificial symmetries to construct orthogonal basis states. This seniority-symmetry-based approach reduces one of the primary limitations of VQE on near-term quantum hardware─circuit depth─at the cost of measuring additional matrix elements. The artificial symmetries also reduce the number of Hamiltonian terms that must be measured, as only a small fraction of the terms couple basis states in different seniority subspaces. With all these merits, Q-SENSE offers a scalable and resource-efficient route to quantum advantage on near-term quantum devices and in the early fault-tolerant regime.

We report on a joint experimental and theoretical investigation of the metal 2p X-ray photoelectron spectra of transition-metal phthalocyanines MnPc, FePc, and CoPc. Using a multiconfigurational approach including spin-orbit coupling, we obtain nearly quantitative agreement with experiment. In all investigated systems, we found an unexpected and robust physical effect induced by core ionization, namely a drastic reorganization of the valence electronic structure. This reorganization is characterized by a pronounced stabilization of doubly excited ligand-to-metal charge-transfer configurations, which become energetically favored in the final states. This ionization-induced double charge transfer is found to be common to all metallophthalocyanines studied. Furthermore, we identify a nearly linear correlation between the stabilization energies obtained from restricted active space configuration interaction (RAS-CI) calculations and a simple hydrogenic/Slater screening estimate, providing an intuitive and practical descriptor to assess when such charge-transfer states are expected to dominate after core ionization.

Modeling propagation of ultrahigh-energy cosmic rays using the input from the configuration interaction shell model

We present a new paradigm for modeling electronically nonadiabatic molecular dynamics by extending the specific reaction parameter (SRP) approach to active space configuration interaction with single, double, and triple excitations in order to model coupled potential energy surfaces. Using the photoisomerization of 1,3-cyclohexadiene (CHD) as an example application, we develop ODM3.25(CHD), a system-specific reparameterization of the semiempirical ODM3.25 method trained against XMS-CASPT2 energies for the S0, S1, and S2 states at 1001 geometries. The resulting model achieves a mean unsigned error of 0.195 eV─comparable to the intrinsic accuracy limits of multireference perturbation theory─while automatically preserving (without diabatization) the correct topology of conical intersections through its configuration-interaction framework. Coupled with curvature-driven coherent switching with decay of mixing (κCSDM), ODM3.25(CHD) enables large-ensemble, 800 fs nonadiabatic trajectory simulations that reproduce key experimental and theoretical features of CHD photochemistry, including excited-state lifetimes of 60-70 fs, early passage through S1/S2 intersections, and ring-opening quantum yields of 43-53%. ODM3.25-SRP enables nonadiabatic dynamics of cyclohexadiene with XMS-CASPT2 quality and an expanded active space while reducing the computation time by a factor of 65. This work demonstrates that treating ODM3.25 as a domain-specific foundation model and fine-tuning it via SRP yields a computationally efficient, chemically accurate multistate potential capable of describing complex photodynamics at a fraction of the cost of high-level ab initio methods. This establishes SRP-based custom excited-state models as a powerful new tool for large-scale nonadiabatic dynamics.

Selected Configuration Interaction (SCI) is a method in molecular electronic structure theory that allows for the construction of configuration subspaces adapted to the particular system under study. This adaptability is achieved by guiding subspace construction with respect to a selection criterion designed to identify and retain the most important configurations for an accurate description of the system. Quantum-SCI (QSCI) introduces quantum resources to inform the construction of these subspaces, motivated by the classical hardness of state sampling and system dynamics. As with conventional SCI, the CI routine is still performed classically and is therefore not corrupted by hardware noise; only the subspace quality is affected by deficiencies in the quantum hardware. Previous QSCI approaches take the measurements produced by a physically motivated circuit construction, often with a recovery step to mitigate against errors, and form a subspace from the resulting configurations. We propose an alternative approach that is more aligned with conventional selection criteria but attempts to inject classically inaccessible information into the selection step with the aim of enabling new subspace expansion pathways. The approach presented in this work uses the population statistics of a time-evolved quantum state to predict likely single and double excitations away from existing configurations to bias the subspace expansion procedure. Importantly, this occupancy-guided expansion complements rather than replaces the direct inclusion of valid configurations sampled from the time-evolved quantum state, so determinants containing higher-order excitations can still enter the variational space in a single iteration. We also include multireference perturbation theory to capture missed correlations outside the configuration subspace. This is demonstrated on hardware by using 42 qubits of an IQM superconducting device to compute the potential energy curve of SiH4 in a 6-31G basis set as the Si-H bonds are stretched. We benchmark against the best-in-class Heatbath CI algorithm to assess the compactness of the resulting wave function.

We derive analytical nuclear gradients for state-averaged orbital-optimized configuration interaction singles (SACIS) and its spin-projected extension (SAECIS), enabling efficient geometry optimization and minimum-energy conical intersection (MECX) searches within a low-cost CIS-based framework. The formulation employs a Lagrangian approach and explicitly removes null-space contributions in the coupled-perturbed equations to ensure numerically stable gradients. For twisted-pyramidalized ethylene, both SACIS and SAECIS qualitatively reproduce the correct conical intersection topology, in sharp contrast to conventional CIS and ECIS. Benchmark calculations on 12 MECXs demonstrate that both methods reproduce geometries with mean RMSDs below 0.1 Å relative to high-level reference methods. SACIS captures the essential degeneracy through variational orbital relaxation, which alleviates ground-state Hartree-Fock (HF) orbital bias and effectively incorporates static correlation through localization effects; notably, spin projection is found to be nonessential for the qualitative description of these intersections. Overall, SACIS and SAECIS provide qualitatively reliable CX descriptions at mean-field computational cost in a black-box manner. Given their comparable accuracy and the additional overhead associated with spin projection, SACIS offers a more favorable cost-performance balance for general applications, whereas SAECIS may become advantageous when higher excited states with significant double-excitation character are involved.

The study presents an exploratory design-space mapping approach for analysing knitted fabrics through the combined consideration of structural, comfort-related, and optical parameters. The methodology addresses the multi-parameter nature of knitted macrostructures, where functional behaviour emerges from the interaction of yarn composition, stitch architecture, and structural configuration rather than from isolated descriptors. Twelve knitted samples differing in stitch type and yarn linear density, and incorporating photoluminescent and reflective yarns, were analysed. Fabric thickness and air permeability were selected as representative structural and comfort-related parameters, while optical response was characterised using a dimensionless reflectance ratio under multiple illumination conditions. All parameters were normalised to enable comparative representation within a unified design space. The resulting maps reveal visual clusters, structurally isolated cases, and illumination-dependent optical equivalence between structurally different configurations. The findings demonstrate that similar optical performance can be achieved through alternative structural solutions, depending on the illumination context. The proposed approach provides a qualitative, design-oriented framework that supports engineering decision-making without implying optimisation or ranking, while revealing alternative design pathways and context-dependent equivalence.

Biexcitonic states govern singlet fission and triplet-triplet and exciton-exciton annihilation, yet a unified understanding of how these processes compete within a shared electronic manifold remains elusive. We outline a conceptual framework based on fragment-based configuration interaction that systematically constructs diabatic Hamiltonians spanning the full one-particle (LE, CT) and two-particle (LELE, CTCT, TT, and CTX with X = LE or T) manifolds from monomer-local building blocks, preserving physical interpretability throughout. SymbolicCI provides analytic Hamiltonian matrix elements for efficient large-scale calculations; NOCI-F delivers benchmark-quality couplings. The resulting diabatic Hamiltonians can be coupled to quantum dynamics simulations. Applications to ethylene aggregates and the anthracene crystal reveal CTX configurations as electronic gateways bridging excitonic manifolds, with CT-mediated relaxation pathways competing with conventional annihilation. In H-type aggregates, the LECT admixture stabilizes a "biexcimer" analogous to one-particle excimers. By providing first-principles access to biexciton formation, separation, and transport, we hope to stimulate further exchange between electronic-structure and quantum dynamics communities toward a predictive understanding of multiexcitonic photophysics.