The potential energy surface (PES) of the Cl(2P3/2) + C2H5I reaction is described by highly-accurate electronic structure computations, covering both hydrogen- and iodine-abstraction pathways and several substitution routes proceeding through either Walden inversion or front-side attack, including both atom- (H, I) and group-exchange (CH2I, CH3) mechanisms. Geometries and harmonic vibrational frequencies of all stationary points are determined at the MP2/aug-cc-pVDZ and CCSD(T)-F12b/aug-cc-pVDZ levels of theory, and single-point energies are further refined at the most accurate geometries using the coupled-cluster method with aug-cc-pVTZ and aug-cc-pVQZ basis sets. To target chemical accuracy, five additional energy corrections - accounting for core correlation, scalar relativistic, spin-orbit, and post-CCSD(T) effects - are incorporated into the CCSD(T)-F12b/aug-cc-pVQZ single-point energies. The resulting benchmark data allow for the detailed mapping of the reaction pathways, including the identification of transition states and pre- and post-reaction minima, which guide the system from the reactants toward the various product channels on the PES. Rate coefficients are determined using transition-state theory, including the Wigner tunneling correction, and compared to literature theoretical values.

Raman spectroscopic and theoretical investigation of polarity regulation and hydrogen bonding in DMSO binary systems.

This study explores a new category of excess electrons, ametalide complexes based on 15-crown-5, i.e., AM+(15-c-5)TM- (AM+ represents Li-K; TM- is Zn-Hg), to assess their nonlinear optical (NLO) features. All designed complexes exhibit electronic and thermodynamic stability as confirmed by their vertical ionization potential ranging 2.18 to 2.95 eV and interaction energies ranging -129.89 to -214.89kcal/mol. The metalide nature of complexes is validated through natural bond orbital, frontier molecular orbital, molecular electrostatic potential, and electron localization function mapping, showing negative charge, the position of HOMO, and excess electrons over TMs. The partial density of state spectra further validates the metalide nature. The UV-vis absorption spectra show that complexes are transparent in ultraviolet region. The polarizability (αo) and hyperpolarizability (βo) analysis indicates greater NLO response with highest βo of 1.08 × 106 a.u. and αo of 1022 a.u. for Li+(15-c-5)Hg- complex. Furthermore, application of an external electric field significantly enhances the NLO response. The largest enhancement is observed for Li+(15-c-5)Hg-, where β₀ increases from 1.08 × 106 to 2.56 × 106 a.u under a positive external electric field ((+ 0.1 × 10⁻2 a.u) applied along the AM to TM direction. These findings highlight the potential of metalides as a candidate for advanced NLO materials. All the density functional theory calculations were performed at ωB97X-D functional with 6-31 + G (d,p) and LANL2DZ basis set in Gaussian 16. The GaussView 6.0 and Multiwfn software were used to analyze electronic properties and view geometries of complexes.

Nuclear magnetic resonance (NMR) has unique strengths in metabolomics studies, particularly in quantifying mixtures and elucidating the structures of unknown molecules. One-dimensional (1D) proton (1H) NMR is the most common method; however, spectral overlap is significant, making analysis challenging. We present a new approach that utilizes chromatographically separated fractions from a pooled sample, henceforth called a metabolite fraction library (mFL). We developed an algorithm to extract highly correlated peaks from the mFL, collectively forming a metabolite basis set (mBS). The mBS can be fit to NMR profiling data, enabling comprehensive quantification. Applied to 10 mixtures of 53 metabolites, our approach accurately quantified 50 metabolites, quantified one impurity and one oxidation product, and described between 91 and 96% of the total spectral intensity. The method is demonstrated using the fungus Neurospora crassa, resulting in the identification of 45 metabolites with high confidence and 45 with medium confidence, accounting for 94% of the total spectral intensity.

It has recently been shown [J. Chem. Phys. 157, 074106 (2022)] that the propagator (P), step size (S) and total time (T) required by real-time time-dependent density functional theory (RT-TDDFT) simulation of X-ray absorptions (XAS) can be determined automatically (Auto) for whatever chemical systems described by whatever electronic Hamiltonians and basis sets, by making use only of the ground-state Kohn-Sham core orbital energies. The AutoPST algorithm is improved here in two aspects: (1) a universal bivariate linear relation is established for accurate predication of the time steps for both - and -edge XAS of any desired spectral accuracy; (2) an automated orbital selection scheme is introduced to pick up only those "active" core and virtual canonical molecular orbitals (CMO), thereby extending AutoPST to AutoSTOP. Such orbital selection is particularly necessary when an uncontracted basis set is used, which generates many high-lying CMOs that have no contributions to near-edge XAS but render the time step exceedingly small. The ratio of the number of active CMOs over the total number of CMOs decreases quickly as the increase of molecular size, thereby ensuring computational efficiency. It is also shown that both singlet and triplet core excited states of a closed-shell system can be obtained by RT-TDDFT with a weak spin-dependent external field. Spin-orbit couplings between the so-obtained singlet and triplet states can then readily be calculated to obtain the and spectra. Finally, the linearized variant of RT-TDDFT is briefly discussed.

Allyl-pyrocatechol and hydroxychavicol are catechol-type phenolic compounds from Piper betle that differ in the position of the allyl substituent, which may influence their electronic properties and antioxidant behavior. In this study, density functional theory (DFT) calculations were performed using the B3LYP functional with the 6-31 + G(2d,2p) basis set to investigate their frontier molecular orbital (FMO) characteristics and antioxidant mechanisms. Calculations were performed in gas and water phases using conductor-like polarizable continuum model (CPCM) solvation model. FMO analysis shows that hydroxychavicol exhibits a lower Eg in both phases (gas: 5.252eV; water: 5.273eV) compared to allyl-pyrocatechol (gas: 5.634eV; water: 5.808eV), suggesting relatively higher electronic reactivity and chemical softness. Solvent effects slightly increase the Eg of allyl-pyrocatechol, while hydroxychavicol remains relatively stable across phases. Thermodynamic evaluation indicates that formal hydrogen atom transfer (f-HAT) is more favorable in the gas phase based on bond dissociation enthalpy (BDE), whereas single electron transfer-proton transfer (SET-PT) is less competitive due to higher energetic requirements. In water phase, reduced ionization potential (IP) and proton affinity (PA) values suggest that solvation facilitates electron and proton transfer processes, making the sequential proton loss electron transfer (SPLET) pathway more relevant under polar conditions. Transition metal chelation (TMC), assessed via ΔHacidity, indicates position-dependent deprotonation tendencies that may contribute as a secondary antioxidant pathway. Overall, hydroxychavicol is more potential reactive, while allyl-pyrocatechol is predicted relatively more stable.

Semivolatile and intermediate-volatility organic compounds (S/IVOCs) are key but under-constrained precursors of urban secondary organic aerosol (SOA). We leveraged the COVID-19 lockdown-to-reopening transition in Beijing as a natural experiment to track molecularly resolved gaseous organics using comprehensive two-dimensional gas chromatography-mass spectrometry (GC × GC-MS). Across 305 quantified species, IVOCs accounted for ∼37% of the measured gas-phase loading yet dominated diagnosed incremental SOA formation (∼89%) in a simplified observation-constrained 0D diagnostic implementation based on two-dimensional volatility basis set (2D-VBS) concepts. Partial least squares discriminant analysis (PLS-DA) revealed compositional shifts, with n-C8, 2-butyl-1-octanol, and hexamethylcyclotrisiloxane (D3) enriched after reopening, indicating rebounds in traffic and volatile chemical product-related emissions; within the VMS mixture, D3 became dominant while lockdown-phase D3 remained near the low-end quantification regime. Meteorological normalization indicated that the decline in total gas-phase organics could not be explained by local meteorology alone and was consistent with a weakened regional background. Despite stronger anthropogenic tracers after reopening, diagnosed daily SOA yields decreased. This efficiency drop appears more strongly associated with thermodynamic constraints than with a broad reduction in atmospheric oxidation capacity. These results indicate that IVOC-enriched noncombustion and evaporative emissions may materially influence diagnosed urban SOA formation within the measured gas-phase precursor pool.

In this study, a comparative quantum-chemical investigation of 2-chloro-N-(4-methoxyphenyl)acetamide (p-acetamide), 2-(4-methoxyphenylamino)-2-oxoethyl acrylate (MPAEA),and 2-(4-methoxyphenylamino)-2-oxoethyl methacrylate (MPAEMA) was carried out to elucidate the effects of progressive structural modification on their electronic, spectroscopic, thermochemical, and non-covalent interaction properties. Geometry optimizations and electronic-structure calculations were performed within the framework of density functional theory using the 6-311G basis set. Electronic properties were analyzed through natural bond orbital (NBO) analysis, frontier molecular orbital (FMO) distributions, and global reactivity descriptors. The calculated HOMO-LUMO energy gaps revealed that MPAEA exhibits enhanced charge-transfer capability because of its conjugated acrylate structure, whereas MPAEMA shows a larger gap, suggesting higher electronic stability. Time-dependent density functional theory (TD-DFT) calculations were used to predict UV-Vis absorption features, revealing that structural modification significantly influences excitation energies and optical responses. Molecular electrostatic potential (MEP) maps and density of states (DOS/tDOS) analyses provided further insight into charge distribution and orbital contributions, highlighting increased electron delocalization in conjugated systems. Thermochemical analysis showed that thermal energy, heat capacity, and entropy increased systematically with temperature for all molecules, with MPAEMA exhibiting the highest thermodynamic values because of its extended molecular framework. Non-covalent interaction (NCI), density overlap regions indicator (DORI), and reduced density gradient (RDG) analyses revealed distinct weak-interaction patterns, confirming that structural complexity enhances interaction diversity and electron-density distribution. Overall, the results indicate that the transformation from the acetamide framework to acrylate and methacrylate derivatives significantly modifies the electronic structure, optical behavior, thermodynamic response, and interaction topology of methoxyphenyl-based molecular systems.

The van der Waals (VdW) region of the potential energy surface (PES) of the collision complex of NO(A2Σ+) and N2(X1Σg+) was explored using explicitly correlated coupled-cluster methods. The Rydberg character of the NO(A2Σ+) state necessitated a sufficiently large and diffuse basis set. Basis set superposition error endemic to such systems was mitigated by the use of an explicitly correlated method in addition to a counterpoise correction, subsequent benchmarking was performed via both energetics and molecular properties such as the dipole and quadrupole moment. The method ultimately used was CCSD(T)-F12a/t-aug-cc-pVTZ, with the full VdW PES constructed through a series of three-dimensional cuts exploring the space between key orientations. A broad main minimum well was observed surrounding the linear nitrogen-to-nitrogen ("LN") orientation with a maximum depth of -239.51 cm-1 at an intermolecular separation of . This new PES is useful for interpretation of recent molecular beam scattering experiments.

Using the multi-reference configuration interaction method with the aug-cc-pV(Q/5)Z basis set, 7762 ab initio energy points were computed and used to construct an analytical potential energy surface (PES) for HCO(12A″). The fitted PES has an overall root-mean-square deviation of 0.833 kcal/mol. Based on this PES, the geometry, energy, and harmonic frequencies of stationary points were analyzed in detail, showing good agreement with other theoretical data. Subsequently, quantum time-dependent wave packet (TDWP) and quasi-classical trajectory (QCT) calculations were performed on this new PES to study the reaction C(3P) + OH(X2Π) → CO(a3Π) + H(2S). The reaction probabilities, integral cross sections, differential cross sections, product rot-vibrational distributions, and rate constants were obtained. TDWP results show a rich resonance structure, and the QCT calculations provide a qualitatively correct description of the reaction cross section. Discrepancies between the dynamical information derived from the two methods indicate pronounced quantum effects in this reaction. The reaction is predominantly governed by a complex-forming mechanism. Although increasing collision energy shortens the complex lifetime, the indirect mechanism remains operative across the studied energy range. This study provides fundamental insights into the microscopic reaction mechanisms and dynamics of carbon chemistry in interstellar environments and provides theoretical support for future experimental results.

Ionization potentials and electron affinities provide the energetic basis for several conceptual density functional theory descriptors, but their use in donor–acceptor maps requires careful distinction between physically bound anions, weak or borderline electron-affinity cases, and formally computed diagnostic states. In this work, a periodic donor–acceptor descriptor map was constructed for main-group atoms from H to Kr using a ΔSCF-DFT framework. Neutral atoms, monocations, and formally defined monoanionic states were evaluated to obtain ionization potentials, electron affinities, and global reactivity descriptors, including electronegativity, chemical hardness, chemical potential, electrophilicity, electrodonating power, and electroaccepting power. The production dataset was calculated at the ωB97X-D4/def2-QZVPPD level and benchmarked against reference atomic data. This protocol reproduced ionization potentials with a mean absolute error of 0.134 eV and electron affinities with a mean absolute error of 0.116 eV for the reference EA set, including the weak calcium case. A functional and basis-set sensitivity analysis using ωB97X-D4/def2-TZVPPD, PBE0/def2-QZVPPD, and PBE0/def2-TZVPPD showed that ionization potentials are comparatively robust, whereas electron affinities are strongly affected by the quality of the diffuse basis set. The normalized donor–acceptor map reproduces chemically intuitive periodic trends, with alkali metals occupying the strong-donor region and halogens defining the strong-acceptor region. The analysis explicitly separates core validation atoms from weak or borderline electron-affinity cases and diagnostic finite-basis anionic states, emphasizing that formally computed negative electron affinities for unbound anions should not be interpreted as physical bound states. The resulting nonrelativistic dataset provides a reproducible atomic descriptor reference for interpreting donor–acceptor behavior in atoms, clusters, superatoms, doped materials, and charge-transfer systems.

DFT calculations are increasingly combined with molecular docking to rank drug candidates, yet most studies report the interaction energy (ΔEint), computed at the complex geometry, as a surrogate for binding affinity. This quantity omits the deformation energy (ΔEdef): the thermodynamic penalty of distorting both partners from their free-state geometries into their bound conformations. Because ΔEdef is always positive (typically 2-20 kcal mol-1) and molecule-dependent, its omission systematically overestimates binding strength and can reverse predicted rank-orderings. We present the energetic decomposition ΔEbind = ΔEint + ΔEdef, demonstrate using published crystallographic strain data from over 3,000 protein-ligand complexes that deformation energies do not cancel between structurally distinct ligands, and propose a minimal five-step correction protocol applicable to any DFT-based drug design study. The protocol requires only two additional geometry optimizations beyond the standard workflow, adding only modest additional computational cost. This work does not introduce new computational data; it highlights an energetic inconsistency in common computational practice and provides a straightforward correction to enable more consistent electronic binding energy evaluation and improved candidate comparison. METHODS: The analysis is based on the supramolecular energy decomposition framework and the activation strain model (ASM), in which binding energy is partitioned into interaction and deformation (strain) components using standard variational principles. No new DFT calculations are reported. The argument draws on published conformational strain datasets obtained at various DFT levels and molecular mechanics force fields from crystallographic analyses of the PDBBind database. The proposed correction protocol is general and can be applied with any DFT functional, basis set, and quantum chemistry software package (e.g., Gaussian, ORCA, or equivalent).

Bond dissociation energies (BDEs) have been calculated for the set of actinide halides AnX with An = Ac, Pa, and Np-Lr and X = F-I. Two composite thermochemistry methods based on the Feller-Peterson-Dixon (FPD) approach have been utilized, one involving spinor-based relativistic CCSD(T) calculations where spin-orbit (SO) was included at the orbital level and another using scalar relativistic CCSD(T) with a posteriori SO contributions based on 2-component multireference configuration interaction calculations. The method that was chosen for a given actinide halide was based on which representation yielded the best single determinant reference determinant for the coupled cluster calculation. The spinor-based method was chosen for all cases except for AmX, CmX, and BkX. Both composite approaches included contributions accounting for basis set truncation, outer-core correlation, the Gaunt interaction, and QED. The scalar FPD results, as well as the spinor-based calculations for AcF, also included higher order electron correlation up through CCSDT(Q). In addition to BDEs, CCSD(T) equilibrium bond lengths, harmonic frequencies, and vibrational anharmonicity constants are reported for all species. Last, the FPD BDEs for the fluorides were used to confirm the trend across the actinide series previously predicted by Gibson using bonding models based atomic promotion energies that provide a single 6d electron for bonding. In particular the local minimum in the BDEs at AmF is confirmed in the present calculations. The BDEs for LrX are predicted to be slightly larger than those of AcX, making them the largest in the actinide halide series.

Deep-lying core electrons carry highly localized, site-specific information that forms the basis of X-ray photoelectron spectroscopy. Accurately predicting their associated core ionization potentials (IPs) is a demanding theoretical task, requiring a balanced treatment of strong orbital relaxation, electron correlation, and relativistic effects. Over the years, a variety of approaches have been developed, ranging from state-specific wave function methods to linear-response formalisms and Green's function techniques. However, their assessment has often relied on comparisons with experiments, where multiple sources of error (basis set incompleteness, relativistic corrections, and vibrational effects) are entangled, making it difficult to isolate the performance of correlation treatments. In the present work, we establish a consistent, theory-based benchmark for core IPs by computing 84 nonrelativistic values (73 second-row and 11 third-row IPs) at the full configuration interaction level within the core-valence separation approximation, using large correlation-consistent basis sets augmented with tight-core and diffuse functions (aug-cc-pCVXZ). These results define theoretical best estimates within a fixed finite basis set, providing a chemically accurate reference for method development and validation. Importantly, our data set allows for systematic, theory-versus-theory comparisons that disentangle correlation and relaxation effects from other physical contributions. On this basis, we assess the performance of widely used approximate methods, including equation-of-motion coupled-cluster approaches up to the inclusion of quadruple excitations, the one-shot G0W0 scheme, as well as state-specific methods.

A defining quantity of a physical system is its energy which is represented by the Hamiltonian.In closed quantum mechanical or/and coherent wave-based systems the Hamiltonian is introduced as a Hermitian operator which ensures real energy spectrum and secures the decomposition of any state over a complete basis set spanning the space where the states live. Pseudo-Hermitian, or P T symmetric, systems are a special class of non-Hermitian ones. They describe open systems but may still have real energy spectrum. The eigenmodes are however not orthogonal in general.This qualitative difference to Hermitian physics has a range of consequences for the physical behaviour of the system in the steady state or when it is subjected to external perturbations. This overview reviews the recent progress in the field of pseudo-Hermitian physics as it unfolds when applied to low-energy excitations of magnetically ordered materials. The focus is mainly on long wave length spin excitations (spin waves) with magnons being the energy quanta of these excitations. Various setups including ferromagnetic, antiferromagnetic, magnonic crystals, and hybride structures with different types of coupling to the environments as well as spatio-temporally engineered systems will be discussed with a focus on the particular aspects that are brought about by the pseudo-Hermiticity such as mode amplifications, non-reciprocal propagation, magnon cloaking, non-Hermitian skin effect, PT-symmetric assisted Floquet engineering, topological energy transfer, and field-induced enhanced sensitivity.

According to the “Technical Specification for Air Quality Testing in Archives Repositories,” air pollutants in archives can be categorized into exogenous and endogenous pollutants. Common exogenous pollutants include sulfur dioxide (SO2), nitrogen dioxide (NO2), ozone (O3), and hydrogen sulfide (H2S), while endogenous pollutants mainly consist of formaldehyde (HCHO) and acetic acid (CH3COOH). This study combines external electric field technology with density functional theory (DFT) and the B3LYP method to theoretically analyze the spectral characteristics and degradation mechanisms of these six pollutant gases. Molecular models of the six gases were constructed using Gaussian software. The configurations of five pollutant gas molecules (SO2, NO2, O3, H2S, and HCHO) were optimized using the B3LYP/6-31G(d) basis set, while the configuration of acetic acid was optimized using the B3LYP/3-21G basis set, yielding their stable structures and spectral information. The study found that characteristic peaks in the spectra shifted under the influence of an electric field. Additionally, by scanning the potential energy surfaces of selected molecular bonds under varying electric field strengths along specific directions, the required external electric field strengths for the degradation of the six common pollutant gases in archives were determined as follows: 0.1050 a.u. for SO2, 0.0975 a.u. for NO2, 0.0925 a.u. for O3, 0.1000 a.u. for H2S, 0.1500 a.u. for HCHO, and 0.0705 a.u. for CH3COOH. The results clarify the degradation thresholds of these six pollutant gases under an external electric field. The findings indicate that acetic acid (0.0705 a.u.) and ozone (0.0925 a.u.) are highly sensitive to electric fields, while formaldehyde requires the strongest electric field (0.1500 a.u.) for degradation. These results provide a reference and theoretical foundation for electric field-assisted degradation technology targeting pollutant gases in archives.

Silicon and nitrogen co-doped carbon quantum dots (SiN@CQDs) showed the strongest potential as a sensing platform for pathogen mycotoxins. Frontier molecular orbital analysis showed that SiN@CQD had the smallest intrinsic HOMO-LUMO gap (1.51eV) compared to N@CQD and Si@CQD, indicating enhanced chemical softness, charge mobility, and reactivity. Adsorption of aflatoxin (AFT), dothistromin (DTM), and versicolorin (VCR) on SiN@CQD was spontaneous and thermodynamically favorable, with energies of - 2.62, - 2.81, and - 2.95eV, respectively. Natural bond orbital analysis revealed strong bidirectional charge transfer between toxins and the quantum dot surface, with the VCR-SiN@CQD complex exhibiting the highest stabilization energy and smallest donor-acceptor gap. Density of states, atomic charge, and molecular electrostatic potential analyses further demonstrated pronounced charge redistribution near the Fermi level following adsorption. Quantum theory of atoms in molecules (QTAIM) and non-covalent interaction (NCI) results confirmed adsorption via hydrogen bonding, π-π stacking, and electrostatic interactions. Overall, these findings provide molecular-level evidence supporting SiN@CQD as a promising candidate to guide subsequent experimental design of toxin detection systems. All calculations were performed using Gaussian 09 software. Density functional theory (DFT) computations were carried out using the PBE0 hybrid functional with Grimme's D3 dispersion correction (PBE0-D3) and the Def2-SVP basis set. Natural bond orbital (NBO) analysis was carried out with NBO 3.0 to quantify charge transfer and donor-acceptor interactions. Quantum theory of atoms in molecules, NCI, and density of states (DOS) analyses were conducted using Multiwfn 3.8. Molecular orbitals, topological features, and isosurfaces were visualized using visual molecular dynamics (VMD). Finite CQD surface models were used to represent local adsorption environments.

This is the first theoretical investigation that systematically analyzes the interactions between the hexachlorophosphazene, [PCl2N]3, and small molecule impurities H2O and HCl in a 1 : 1 stoichiometric ratio. Utilizing both ab initio methods and seven DFT functionals in conjunction with the triple-ζ basis set, the pivotal structures in proposed reaction mechanisms are fully characterized and the energy change for each step was determined at the CCSD(T)/aTZ‖MP2/aTZ level of theory. Our QM calculations show that [PCl2N]3 can be hydrolyzed via a single-step mechanism with an activation energy of ca. 180 kJ mol−1, or be ring-opened by HCl through a two-step mechanism, in which the rate-determining step has an activation energy of ca. 120 kJ mol−1. Because the activation energy of these two reactions is notably lower than that of the ring-opening polymerization and the ring–ring expansion equilibrium (which requires ca. 240 kJ mol−1 of energy determined at a comparable DFT level of theory), our study indicates that even trace amount of H2O and HCl can significantly interfere with the polymerization process. Beyond revealing new mechanistic details, our calculations also indicate that all selected functionals can provide reasonable electronic structures to describe the reaction progress. On the other hand, while each of the functionals investigated here excels in closely matching the CCSD(T)/aTZ‖MP2/aTZ energy barriers for certain steps in the reaction, the B3LYP functional is capable of providing the most consistent results. This establishes that the B3LYP functional can be suitable for investigating phosphazene reactions as a computationally efficient and robust quantum mechanical approach while maintaining near–ab initio accuracy.

A scalable framework is introduced for the calculation of valence and core ionization energies within the second-order algebraic-diagrammatic construction [ADC(2)] formalism. The approach is based on the construction of state-specific orbital domains that are determined entirely by the underlying electronic structure and ionization process. As a result, the procedure adapts automatically to the character of the ionized state and can be applied in a genuine black-box manner without system-specific tuning. The methodology is tested for conventional ADC(2), its spin-opposite-scaled variant, and an ADC(2)-based double-hybrid functional. Benchmark calculations show that the errors introduced by the local approximation remain far below the intrinsic uncertainties of the underlying correlated methods. For both valence and core ionization energies, the deviations are typically on the order of a few hundredths of an electronvolt. At the same time, substantial reductions of the orbital space are achieved, leading to the significant acceleration of the most expensive steps of the correlated treatment. The efficiency and robustness of the approach are demonstrated for extended molecular systems of practical relevance. Once the reference orbital set is obtained, the valence ionization energy of a 132-atom thermally activated delayed fluorescence emitter can be determined within approximately 20 min using a triple-ζ basis set, while the 4 N K-edge core ionization energies of a 372-atom porphyrin derivative are obtained within about 2 h. In both cases, the corresponding ADC(2) eigenvalue problem itself requires only about 1 min per state. The proposed framework therefore enables routine applications of ADC(2)-based methods to molecular systems that are beyond the reach of conventional implementations.

Unitary Coupled Cluster (UCC) theory is a promising variational method for electronic structure calculations, particularly for systems that exhibit strong electronic correlation and for implementation on quantum computers. However, its practical application is limited to small chemical systems with small basis sets due to its steep computational scaling, which results from its nonterminating Baker-Campbell-Hausdorff expansion. Here, we introduce an active space UCCSD(4)/MP2 approach that leverages a fourth-order many-body perturbation theory truncation of UCCSD within a selected active space while treating external excitations at the MP2 level. We explore two variants: a composite method that sums separate internal and external contributions and an interacting method that couples the amplitudes for potentially greater accuracy. We test our approach on a range of systems, including molecules from the GW100 data set in their equilibrium geometries, a moderately correlated metaphosphate hydrolysis reaction, and the strongly correlated torsion of ethylene. Our results suggest that the interacting method with canonical orbitals is robust and stable for both weakly and moderately correlated systems and accurately reproduces the full UCCSD(4) potential energy curves, including only 15-25% of the virtual orbitals in its active space. In comparison, the composite formulation exhibits greater sensitivity to the choice of orbital basis and active space size, leading to less systematic behavior across the benchmark set. For ethylene torsion, a system dominated by strong static correlation, both the composite and interacting formulations employing canonical orbitals closely track the full UCCSD(4) reference while preserving the qualitative behavior of the parent method, including the breakdown in strongly multireference regimes. This active space framework offers a computationally tractable approach for modeling correlated molecules and reactions on classical computers and provides a viable path for scaling UCC calculations for resource-constrained quantum hardware.