Journal of Chemical Theory and Computation最新文献

We assess the accuracy and limitations of the grouped-bath configuration interaction (GBCI), formerly termed the grouped-bath ansatz for the spin-flip nonorthogonal configuration interaction (SF-GNOCI), by calculating L-edge X-ray absorption spectroscopy (XAS) spectra and 2p3d resonant inelastic X-ray scattering (RIXS) spectra of transition-metal complexes. We compare the computed GBCI spectra with those from the spin-flip complete-active-space (SF-CAS) method and experiment. These comparisons demonstrate that the bath-orbital relaxation in GBCI is crucial for accurately describing both the energies and wave functions of charge-transfer states. For L-edge XAS, GBCI accounts for the core-hole relaxation neglected in SF-CAS, leading to a uniform energy shift across the spectral bands. For 2p3d RIXS, GBCI produces an energy shift in the charge-transfer band that corrects the overestimated energy in SF-CAS. Moreover, GBCI improves the wave functions of charge-transfer states, thereby correcting the RIXS intensities overestimated by SF-CAS. Nevertheless, discrepancies with experiment indicate that additional electronic correlation remains necessary. We expect that the X-ray spectral comparison presented in this study will serve as a useful benchmark for validating electronic-structure theories of transition-metal complexes.

The discovery of transition pathways to unravel distinct reaction mechanisms and, in general, rare events that occur in molecular systems is still a challenge. Recent advances have focused on analyzing the transition-path ensemble using the committor probability, widely regarded as the most informative one-dimensional reaction coordinate. Consistency between transition pathways and the committor function is essential for accurate mechanistic insight. In this work, we propose an iterative framework to infer the committor and, subsequently, to identify the most relevant transition pathways. Starting from an initial guess for the transition path, we generate biased sampling, from which we train a neural network to approximate the committor probability. From this learned committor, we extract dominant transition channels as discretized strings lying on isocommittor surfaces. These pathways are then used to enhance sampling and iteratively refine both the committor and transition paths until convergence. The resulting committor enables accurate estimation of the reaction rate constant. We demonstrate the effectiveness of our approach on benchmark systems, including a two-dimensional model potential, peptide conformational transitions, a Diels-Alder reaction, and the reversible folding of the Trp-cage.

Within the framework of a functional integral formalism incorporating ionic charge and hard-core (HC) interactions on an equal footing, we formulate a unified theory of equilibrium thermodynamics and ion association in charged solutions. Via comparison with recent Monte-Carlo (MC) simulation results (J. Forsman et al., PCCP 26, 19921 (2024)), it is shown that our approach is able to predict with quantitative precision the pair distributions of monovalent ions with the typical hydrated sizes d = 3.0 Å and 4.0 Å up to the molar concentration n_i ≈ 2.0 M. Moreover, comparison with additional simulation data from the literature indicates that within the characteristic regime of ionic packing fraction η ≲ 0.1, the theory can accurately account for the ion size dependence of the excess energy and pressure from d = 14.3 Å down to d = 1.6 Å. Via the adjustment of the hydration radius, our electrostatic formalism can also reproduce the nonmonotonic salt dependence of the experimentally measured osmotic coefficients of various aqueous and nonaqueous solutions. In accordance with AFM experiments involving weakly polar nonaqueous electrolytes, the underlying sharp competition between the particularly strong opposite charge attraction and the excluded volume constraint is shown to limit the occurrence of substantial ionic pair formation to the submolar concentration regime n_i ≲ 50 mM; at larger concentrations, HC repulsion hinders ion association and results in the quasi-saturation of the pair fraction curves.

We introduce the qumode subspace variational quantum eigensolver (QSS-VQE), a hybrid quantum-classical algorithm for computing molecular excited states using the Fock basis of bosonic qumodes in circuit quantum electrodynamics (cQED) devices. This approach harnesses the native universal gate sets of qubit-qumode architectures to construct highly expressive variational ansatze, offering potential advantages over conventional qubit-based methods. In QSS-VQE, the electronic structure Hamiltonian is first mapped to a qubit representation and subsequently embedded into the Fock space of bosonic qumodes, enabling efficient state preparation and reduced quantum resource requirements. We demonstrate the performance of QSS-VQE through simulations of molecular excited states, including dihydrogen and a conical intersection in cytosine. Additionally, we explore a bosonic model Hamiltonian to assess the expressivity of qumode gates, identifying regimes where qumode-based implementations outperform purely qubit-based approaches. These results highlight the promise of leveraging bosonic degrees of freedom for enhanced quantum simulation of complex molecular systems.

We present a formally exact adiabatic connection (AC) framework for the correlation energy of multireference wave functions, derived using the fermionic operator algebra. This framework can be formulated in both the particle–hole (ph) and particle–particle (pp) representations by exploiting the corresponding decompositions of the two-electron reduced density matrix. While the phAC formalism is well established, here we derive the ppAC formula and provide explicit working expressions based on the extended random-phase approximation. The second-order multireference pp correlation energy introduced by Tucholska et al. [J. Phys. Chem. Lett. 2024, 15, 12001] emerges naturally as the lowest-order approximation in this context. Exploiting the equivalence of ph and pp correlation amplitudes, we propose a combined ffAC method that incorporates both contributions and avoids double counting of correlation. We test all the considered AC methods, i.e., ph- and ppAC, their linearized variants ph- and ppAC0, and the ph-pp combined methods ffAC and ffAC0, on a variety of challenging cases, including multiple bond dissociations, atomic excitations, singlet–triplet gaps of organic biradicals, and singlet and triplet excitation energies of organic chromophores. Across all systems, the linearized ffAC0 method consistently provides the most accurate results. Its accuracy matches or exceeds that of NEVPT2, yet it is significantly more efficient, requiring only one- and two-electron reduced density matrices.

Implementation of effective core potentials (ECPs) into the molecular scattering suite UKRmol+ is presented together with a set of calculations for a range of targets relevant for plasma modeling. Continuum description in scattering and photoionization calculations for large targets or high-energy electrons often requires the use of numerical continuum functions and the associated molecular integrals. We derive expressions for ECP integrals over B-spline-type orbitals using their momentum-space representation and describe their implementation. Sample calculations are presented for electron collision with ethylene (C₂H₄), bromine (Br₂), silicon tetrabromide (SiBr₄), and tungsten hydride (WH), as well as photoionization of methyl iodide (CH₃I).

The spin–orbit (SO) coupling is a key topic in lanthanide chemistry and physics since it is directly relevant to essential physicochemical properties of rare-earth materials, especially in spectroscopic, optical and magnetic domains. In this work, we systematically study different SO operators of relativistic Hamiltonians within both all-electron and pseudopotential (i.e., effective core potential) frameworks, and their impact on the microstate energy levels of the 4f-multiplets of trivalent lanthanide ions (Ln³⁺, Ln = Ce–Yb). It is found that some SO operators adapted for the relativistic all-electron Hamiltonian can also replicate the experimental SO splitting of 4f-multiplets for Ln³⁺ with fairly reasonable accuracy when combined with the small-core pseudopotential method, such as Breit-Pauli and screened-nuclear SO operators. The feasibility of the perturbative SO coupling scheme for lanthanides is further discussed. We anticipate that these SO coupling methodologies will be applicable to extended lanthanide-doped material systems, enabling comprehensive investigations of their spectroscopic, optical, and magnetic properties.

Self-consistent-field (SCF) in the grand-canonical (GC) ensemble faces convergence difficulties with significant fractional occupation at low temperatures. By recognizing the orbital coefficient matrix and the fractional occupation vectors as two independent variables, this work provides a new viewpoint upon GC-SCF as an optimization problem on a product manifold of a flag manifold and a Euclidean space. During the optimization process, the manifold is automatically adjusted to the orbitals, which are divided into three partitions based on the occupation numbers, so that the occupation numbers are optimized properly, avoiding notorious gradient explosion and vanishing. Important concepts in manifold optimization are discussed and their specific expressions in GC-SCF are given. Via numerical benchmarks on various examples, our algorithms are shown to be more efficient than the conventional direct inversion in the iterative subspace (DIIS). Among them, we recommend the augmented Roothaan–Hall method, which reaches the balance between time consumption and convergence rate.