Vitamin-relevant molecular motifs combine heteroatom networks, hydrogen bonds, conjugated polyenes, saturated and fused rings, benzopyran units, and quinone cores. We present an integrated gas-phase validation strategy in which high-resolution rotational spectroscopy and infrared spectroscopy test complementary observables of the same computed structures. Ground-state rotational constants, after anharmonic vibrational correction, provide a quantitative structural anchor through the global mass distribution; gas-phase infrared spectra and tabulated vapor-phase band envelopes probe the local force fields responsible for diagnostic functional-group and fingerprint signatures. Compact fragments are benchmarked against explicitly correlated references, while larger motifs are treated with bond-corrected double-hybrid geometries and validated directly against experiment. The scope is deliberately reference-oriented. Rather than attempting to reproduce physiological condensed-phase environments, the work establishes isolatedmolecule benchmarks for tractable vitamins and transferable building blocks for larger, more flexible, or environment-dependent vitamin systems. The protocol is applied to complete vitamin molecules, nicotinic acid and ascorbic acid, and to vitamin-relevant fragments for the vitamin A, vitamin D, vitamin E, and vitamin K families: cyclohexanone, cis-1,3,5-hexatriene, 2-decalone, isochroman, benzoquinone, and naphthoquinone. The rotational benchmark confirms sub-percent accuracy for transferable non-hydrogen-bonded motifs, while the infrared comparisons validate carboxyl, ketone, polyene, and quinone fingerprints. For vitamin C, validation is structural and rotational because no isolated gas-phase infrared spectrum is available. Together, the rotational and infrared tests define a spectroscopic transferability map that separates intrinsic molecular structure from environmental perturbations and provides calibration targets for lower-cost quantum-chemical protocols and molecular-dynamics force fields.

Accurate anharmonic vibrational spectroscopy of medium-size molecules requires potential energy surfaces of near-spectroscopic quality, but the cost of direct high-level treatments rapidly becomes prohibitive when cubic and quartic force constants are needed. This limitation is especially severe for systems in the 30-50 atom range, where second-order vibrational perturbation theory (VPT2) remains one of the most attractive routes to quantitative vibrational analyses. Conventional dual-level schemes provide an effective compromise by combining high-level harmonic force fields with lower level anharmonic corrections for the full molecule. We introduce a multilevel fragmentation strategy based on the chemically meaningful subunits. When a single high-level fragment dominates the spectroscopic problem, the corresponding correction can be propagated directly through an anharmonic treatment. When two complementary fragments contribute equally, the harmonic correction can instead be assembled at the frequency level through a mode-resolved combination of the multilevel results. In both cases, the approach avoids the projection and Hessian reconstruction steps that often complicate fragment-based anharmonic treatments. The approach is further extended by a local harmonic correction designed for electronically delicate boundary bonds, where the standard link atom approximation may leave a residual error concentrated in the harmonic force field. The correction is formulated in internal coordinates and can be transferred from the compact auxiliary models to the target system. Thus, the purely harmonic approximation is confined mainly to the fragment boundaries rather than to the entire molecule. The benzonitrile application shows that treating the nitrile group at the high level while retaining the aromatic ring at the low level already yields an accurate global description, and that a propionitrile-based local correction further improves the C≡N stretching region. More challenging test cases are provided by tryptophan and uridine, which probe, respectively, chromophore-centered and genuinely dual-fragment multilevel corrections in larger vibrational manifolds. Overall, the resulting strategy offers a practical route to VPT2-quality vibrational analyses for molecules that admit natural partitions into chemically meaningful subunits. In this framework, mode localization should be viewed as a criterion for defining chemically meaningful fragmentations, rather than as an intrinsic limitation of the method.

Chlorine-sulfur oxides represent potentially important yet largely uncharacterized intermediates in atmospheric and astrochemical environments, from explosive volcanic plumes to irradiated exoplanet atmospheres and the sulfur-rich exospheres of Io and icy moons. Here, we present the first comprehensive structural, spectroscopic, and energetic characterization of the ClSO3 and ClOSO2 radicals. Using a hierarchy of high-accuracy composite quantum-chemical approaches-including CCSD(T)-F12, core-valence corrections, dual-level anharmonic force fields, and multireference excited-state treatments-we determine equilibrium geometries, rotational and vibrational spectra, vertical excitation energies, and photodissociation thresholds. ClSO3 is found to be substantially more stable than the ClOSO2 isomer, whose dissociation into ClO + SO2 is nearly thermoneutral, indicating that it may act as a short-lived but chemically relevant intermediate. Strong IR features in the 600-1200 cm-1 region, together with broad electronic absorption, show that both radicals can affect UV/visible opacity and photochemistry in chlorine- and sulfur-bearing environments. The predicted dipole moments and rotational constants place several intense transitions within Atacama Large Millimeter/submillimeter Array and NOEMA frequency ranges, making these species promising targets for future astronomical searches. The benchmark dataset reported here provides the spectroscopic foundation required for laboratory identification and enables the incorporation of halogen-sulfur intermediates into atmospheric, planetary, and interstellar chemical networks.

The rigid-rotor/harmonic-oscillator model is often insufficient for unbiased comparison with high-resolution microwave data and infrared measurements of medium-sized molecules, but direct high-level anharmonic force fields are rarely affordable for targets with many tens of normal modes. Here, we examine a pragmatic quartic force field/second-order vibrational perturbation theory (QFF/VPT2) route, in which different electronic structure levels are used for different layers of the calculation. In the L1//L0 implementation, a low level (L0) supplies the cubic and semidiagonal quartic force constants, whereas a high level (L1) supplies the equilibrium geometry and either the complete or a mode-selected harmonic reference. In this protocol, an already available machine-learning (ML)-derived QFF can be upgraded selectively with new high-level harmonic information, and the lower-level anharmonic layer does not need to be rebuilt or retrained. Thus, all harmonic terms, or only selected ones, can be promoted to L1, provided that the correspondence between the L0 and L1 coordinates is controlled. The approach is tested on benzoic acid and on the two observed conformers of aspirin, where the molecules are large enough that analytical high-order derivatives and full high-level QFFs are not routine options. Microwave and infrared (IR) observables are used together because they test different parts of the model: rotational constants probe the equilibrium geometry and vibrational averaging, whereas IR spectra probe the anharmonic force field and intensity patterns. Benzoic acid shows that L1//L0 and L1//L1 are equivalent within about 10 reciprocal centimeters for the present spectroscopic purpose and identifies modes with large OH stretching content as the relevant targets for L1 harmonic repair. Aspirin preserves the same pattern, despite its larger size and conformational flexibility. For the diagnostic OH stretch of aspirin I, direct finite differences and a preexisting permutationally invariant polynomial machine-learned potential give identical L0//L0 and L1//L0 VPT2 fundamentals. The combined microwave/IR analysis therefore supports a controlled, mode-selective high-level correction of low-level or machine-learned anharmonic layers while also clarifying the limits of the strategy when a high-quality machine-learned potential is not already available.

The number of accurate equilibrium structures of organic radicals is still limited, despite the central role of these species in atmospheric and combustion chemistry, spectroscopy, and catalysis. Because equilibrium structures are experimentally inaccessible for all but the smallest radicals, reliable and predictive quantum-chemical protocols are essential. While double-hybrid density functionals can achieve remarkable accuracy for closed-shell molecules, their performance often deteriorates for delocalized open-shell systems, and they may require spin- and multiplicity-dependent bond-length corrections that compromise the smoothness of potential energy surfaces. Here, we show that a local correlation treatment based on pair natural orbitals (PNOs) provides a robust and practical solution to this long-standing problem. In particular, the PNO-LCCSD(F12b)(T*) approach, embedded in an efficient composite framework, enables near-spectroscopic equilibrium geometries for both closed- and open-shell molecular systems containing up to a few dozen atoms. To make this strategy broadly accessible, we extend a previously introduced external utility to PNO-based correlated methods, enabling the automated assembly of composite gradients. Starting from DFT geometries and Hessians, local correlation is combined with hierarchical optimization and an efficient driver in generalized internal coordinates, yielding the PPCS2 protocol. Benchmark tests on a diverse set of σ-, π-, aromatic, and heteroatom-centered radicals demonstrate uniform accuracy across the full data set and deliver high-precision equilibrium structures even for reactive radicals that remain challenging to characterize experimentally.

We present WMS-Rot and its fitting companion WMS-FitRot as an integrated framework for the early stages of rotational spectral analysis, starting from spectroscopic parameters obtained from electronic-structure computations and progressing to assignment-aware local refinement driven by the same theoretical catalog used for prediction. The framework provides a practical and internally consistent route connecting modern composite quantum-chemical predictions to first-pass assignments and controlled refinement. More fundamentally, it reformulates the incorporation of theoretical information into the spectroscopic inverse problem: calculated parameters act not only as initial guesses but also as active constraints that stabilize assignments and guide early-stage refinement within a unified simulation-fit cycle. Applications to nicotinic acid and thiopronine show that accurate composite inputs markedly improve starting points compared to low-level models, enabling robust assignment, reliable conformer discrimination, and consistent refinement. The approach reproduces matched reduced-Hamiltonian fits while remaining fully compatible with standard SPCAT/SPFIT practice and provides diagnostic insight into parameter correlations, identifiability, and model conditioning.

Venus photochemistry couples SO2 variability, chlorine activation, sulfuric-acid cloud formation, and the still-unassigned near-UV absorber, yet several sulfur–chlorine intermediates in this network remain molecularly undefined. ClSO4 is one of them: it has been invoked as a chlorine-assisted route toward SO3 and H2SO4, but its structure and spectroscopy have not been established. We report a focused ab initio characterization of neutral doublet ClSO4 centered on explicit validation of its electronic structure. Multireference calculations with two independent active spaces locate the same O-bound chlorine sulfate radical and show that the relevant structure is not strongly multiconfigurational. An independent second-order perturbation-theory optimization recovers the same bond-localized topology, but the coupled-cluster minus second-order perturbation correction substantially refines the radical-bearing S–O distance, showing that second-order perturbation theory alone is not sufficient for final structural parameters. Final structural parameters are therefore obtained from a composite geometry, including explicit-correlation to account for basis-set extension, post-second-order correlation, and core–valence increments, whereas the vibrational analysis uses a second-order perturbation-theory force field and second-order vibrational perturbation treatment. The resulting vibrational spectrum contains a compact set of intense infrared markers, and the electronic absorption spectrum extends into the near-UV/visible region. These results establish ClSO4 as a chemically plausible and spectroscopically accessible intermediate in Venus sulfur–chlorine chemistry.

Intramolecular OH···O hydrogen bonds remain difficult to describe at spectroscopic accuracy. Electronic correlation, vibrational averaging, and conformational flexibility all influence the same rotational constants and the corresponding structural parameters, often on a comparable scale. This is especially critical for oxygen-rich molecules of astrochemical and biochemical interest, where OH···O(R), OH···O═C, and OH···OH contacts coexist and can approach quasi-symmetric proton sharing. Here we benchmark a hierarchy of quantum chemical methods against rotational spectroscopy for hydroxyaldehydes, hydroxyethers, hydroxyacids, polyols, and the limiting case of malonaldehyde. The analysis is organized according to the actual computational layers of the protocol: equilibrium geometries, empirical geometrical corrections, local-correlation approximations, vibrational corrections, and final comparison with experimental ground-state rotational constants. Direct comparison with parent and deuterated rotational constants is combined with semiexperimental equilibrium structures whenever isotopic information is sufficient, thereby separating equilibrium-geometry errors from vibrational contributions. The resulting picture is clear. Double-hybrid and bond-corrected models remain useful low-cost approximations, but their accuracy deteriorates as covalent bond-based transferability breaks down in hydrogen-bonded systems. By contrast, inclusion of local-correlation in the explicitly correlated coupled-cluster ansatz emerges as the most robust reduced-cost approximations in the present set and, especially at the equilibrium-geometry layer, often approaches the quality of the reference conventional model (PCS2). The comparison between these two variants therefore primarily tests the error introduced by the local-correlation approximation, whereas the comparison between bare DFT and its bond-corrected variant addresses the transferability of empirical structural corrections. The structural analysis shows that the dominant residual error is concentrated in the O···H contact, whose variation is typically 1 order of magnitude larger than that of individual covalent parameters. This shift is redistributed over the full hydrogen-bonded pseudocycle, explaining why direct spectroscopic benchmarks are more discriminating than inspection of isolated local coordinates. Because PCS2 already reproduces the rotational constants accurately, it can serve as an internal structural reference within the present accuracy target, and the semiexperimental analysis then identifies the OH···O contact as the natural target for extending local-regression ideas from covalent bonds to hydrogen-bond interactions. At the same time, the present OH···O-specific mapping is intentionally preliminary and should be regarded as a proof of concept based on a limited set of localized systems, not yet as a generally validated correction framework. Malonaldehyde is treated separately in this respect, because its experimental rotational constants are tunneling-averaged dynamical observables rather than direct observables of a single localized equilibrium structure. These results define a practical hierarchy for extending spectroscopic-accuracy structural predictions to larger carbohydrates and related hydrogen-bonded systems.

Geometry optimization at the coupled-cluster level remains a major bottleneck in high-accuracy quantum chemistry, particularly for explicitly correlated and local-correlation approaches for which analytical gradients are not available. Here we present a general and scalable framework that enables routine geometry optimizations at the PNO-F12 level by combining numerical gradients with a dual-coordinate strategy: geometry updates are performed in generalized internal coordinates, while gradients are evaluated via finite differences along normal modes. A key aspect of the present development is that the optimization of mode-dependent displacement step sizes is essential to control not only the usual truncation and noise errors of finite differences, but also additional numerical effects arising from explicit correlation and from the geometry dependence of pair-specific orbital domains. This allows numerical gradients to reach the precision required for submilliangstrom structural refinements. The resulting protocol delivers equilibrium geometries of near-spectroscopic accuracy for both benchmark molecules and challenging large systems such as cytosine tautomers, corannulene, and coronene, while retaining affordable computational times and excellent parallel scalability. Comparison with near-canonical composite schemes shows that the use of PNO-F12 numerical gradients preserves coupled-cluster structural accuracy. The present work demonstrates that geometry optimization is no longer a limiting factor for bringing explicitly correlated coupled-cluster accuracy from energies to equilibrium structures, opening the way to routine high-accuracy structural and spectroscopic studies for medium-to-large molecular systems.

We present an integrated computational protocol for the accurate prediction of temperature- and pressure-dependent kinetics of unimolecular reactions and apply it to the competing HCl elimination channels of 2-chloropropene yielding propyne and allene. Reaction pathways and stationary points are characterized within the Pisa Composite Schemes (PCS) framework by combining PCS2-optimized geometries with PCS3 single-point energies augmented by TQλ contributions, thus achieving subchemical-accuracy performance (better than 0.25 kcal mol-1) at a feasible computational cost. A key element of the workflow is the availability of analytical gradients for composite methods, enabled by an interoperable tool based on generalized internal coordinates, which renders composite-level geometry optimizations and IRC calculations straightforward and robust. Vibrational effects are refined beyond the harmonic approximation by including anharmonic corrections through second-order vibrational perturbation theory. Final pressure-dependent rate constants and branching ratios are obtained by solving the 1-D master equation with the Master Equation System Solver software. In the high-pressure limit, the computed activation barrier for the dominant propyne-forming channel is ΔH0⧧ = 65.54 kcal mol-1, and the corresponding Arrhenius activation energy agrees with shock-tube measurements within 0.3%. Overall, the proposed framework offers a robust and systematically improvable strategy for predictive pressure-dependent kinetics, enabling quantitative support to detailed combustion modeling and shock-tube kinetic analyses at an affordable computational cost.

Spectroscopic techniques play a central role in the structural characterization and identification of molecular systems. In this context, the combined use of complementary spectroscopic methods is becoming increasingly important, as different observables probe distinct aspects of the molecular structure and dynamics. Achieving accurate and internally consistent theoretical predictions across multiple spectroscopic domains remains computationally demanding for medium-sized molecular systems, including polycyclic aromatic hydrocarbons (PAHs) and their derivatives. Here, we present a standardized computational protocol designed to deliver accurate rotational and vibrational spectra at a computational cost that remains accessible. The approach integrates double-hybrid functionals, one-parameter bond-length corrections, dual-level treatments of harmonic and anharmonic effects, and automated spectral simulations within a single, internally consistent framework. Starting from a SMILES string or a preliminary molecular scaffold, the workflow enables the determination of a comprehensive set of parameters, including equilibrium geometries, dipole moments, rotational constants (with vibrational corrections), centrifugal distortion constants, anharmonic vibrational frequencies, and infrared intensities, with accuracy close to the spectroscopic level. Applications to phenalene and the 1,2- and 1,4-dihydronaphthalene isomers confirm the expected level of accuracy and allow direct comparison of the simulated spectra to their experimental counterparts. Overall, this approach provides a practical tool for the interpretation of laboratory infrared and microwave spectra and the structural characterization of molecular systems of an intermediate size.

Vibrational spectra convey a wealth of structural and dynamical information; however, their reliable assignment and interpretation often benefit from the integration of complementary spectroscopic techniques and require the support of accurate quantum chemical calculations. The harmonic approximation is frequently insufficient for quantitative spectroscopy, while fully anharmonic treatments rapidly become computationally prohibitive for large and flexible molecular systems, in particular, for biomolecules. In this framework, we introduce a general perturb-then-diagonalize approach that relies on a three-class partitioning of normal modes into primary, auxiliary, and spectator subsets and combines numerical strategies based on analytical Hessians and analytical gradients. Accurate anharmonic contributions are explicitly included for the modes of primary interest, while the influence of external modes is accounted for through finite differences of analytical gradients, avoiding the much more expensive evaluation of Hessians. Several case studies demonstrate the robustness, ease of use, and accuracy of the proposed approach across a broad range of molecular systems, including situations in which vibrational and rotational spectroscopic data provide complementary information. When combined with a dual-level strategy in which accurate methods are employed for harmonic terms and less expensive methods for anharmonic contributions, the present framework enables vibrational spectra of near-spectroscopic accuracy for biomolecules and other chemically rich systems. More complex environments can be addressed by coupling the method with multilayer approaches.

Chlorine-sulfur oxides (Cl2SOn, n = 1-4) are key intermediates in the oxidative and photochemical cycles that couple sulfur and chlorine chemistry in the atmosphere of Venus. These compounds are thought to participate in catalytic processes converting SO2 to SO3 and, ultimately, to H2SO4, thereby contributing to the formation and maintenance of the planet's characteristic cloud system. While thionyl and sulfuryl chloride have been experimentally characterized, the higher oxidation members of the series remain largely unobserved and their structures and spectroscopic signatures are essentially unknown. In this work, high-accuracy equilibrium geometries, rotational constants, and anharmonic vibrational frequencies are determined for the Cl2SOn series using explicitly correlated ab initio methods combined with second-order vibrational perturbation theory. This approach enables a balanced description of electron correlation and anharmonic effects, providing spectroscopic parameters that closely reproduce available experimental data and extend predictive accuracy to the uncharacterized higher oxides. The resulting dataset furnishes the first consistent reference for these reactive species, facilitating their potential identification in laboratory spectra and planetary observations. The predicted rotational and vibrational features are expected to assist in the interpretation of forthcoming high-resolution measurements from ESA's EnVision and NASA's VERITAS missions, offering new insight into the complex chlorine-sulfur-oxygen chemistry shaping the Venusian atmosphere.

The rigid-rotor/harmonic-oscillator model is often insufficient for unbiased comparison with high-resolution microwave data and infrared measurements of medium-sized molecules, but direct high-level anharmonic force fields are rarely affordable for targets with many tens of normal modes. Here, we examine a pragmatic quartic force field/second-order vibrational perturbation theory (QFF/VPT2) route, in which different electronic structure levels are used for different layers of the calculation. In the L1//L0 implementation, a low level (L0) supplies the cubic and semidiagonal quartic force constants, whereas a high level (L1) supplies the equilibrium geometry and either the complete or a mode-selected harmonic reference. In this protocol, an already available machine-learning (ML)-derived QFF can be upgraded selectively with new high-level harmonic information, and the lower-level anharmonic layer does not need to be rebuilt or retrained. Thus, all harmonic terms, or only selected ones, can be promoted to L1, provided that the correspondence between the L0 and L1 coordinates is controlled. The approach is tested on benzoic acid and on the two observed conformers of aspirin, where the molecules are large enough that analytical high-order derivatives and full high-level QFFs are not routine options. Microwave and infrared (IR) observables are used together because they test different parts of the model: rotational constants probe the equilibrium geometry and vibrational averaging, whereas IR spectra probe the anharmonic force field and intensity patterns. Benzoic acid shows that L1//L0 and L1//L1 are equivalent within about 10 reciprocal centimeters for the present spectroscopic purpose and identifies modes with large OH stretching content as the relevant targets for L1 harmonic repair. Aspirin preserves the same pattern, despite its larger size and conformational flexibility. For the diagnostic OH stretch of aspirin I, direct finite differences and a preexisting permutationally invariant polynomial machine-learned potential give identical L0//L0 and L1//L0 VPT2 fundamentals. The combined microwave/IR analysis therefore supports a controlled, mode-selective high-level correction of low-level or machine-learned anharmonic layers while also clarifying the limits of the strategy when a high-quality machine-learned potential is not already available.

High-resolution rotational and vibrational spectroscopies now probe structural effects at a scale where the residual error of practical electronic-structure models is no longer a negligible background. For medium-sized molecules, this issue is especially evident in equilibrium structures, rotational constants, and vibrational frequencies derived from density-functional theory treatments. We introduce a topology-dependent local refinement scheme in which the residual error of a double-hybrid functional reference is corrected in the representation where it is most compact. A transferable core-valence layer, evaluated either empirically or using second-order Møller-Plesset perturbation theory, is kept separate from the valence hierarchy and can be applied from level 0 onward. At level 1, the bond field level 1 refinement contains the conjugation term, whereas pair-local level 1 generalizes the valence correction to additional selected atom-pair classes. Selected vibrational residuals are represented in normal-mode or fragment space. The correction layer is continuous along the potential-energy surface and anchors the residual term to a chemically defined local environment. The approach is assessed for polycyclic aromatic hydrocarbons, intramolecular OH⋯O systems, carbohydrates, and a nucleoside. In rigid covalent molecules, rotational-constant errors near 1% at the hybrid functional level are reduced by more than an order of magnitude. In hydrogen-bonded carbohydrates, the pair-local refinement lowers the discrepancies to below 0.3%. The OH⋯O(H) class provides an explicit case in which a transferable pair-local correction can be parameterized within an accessible high-level reference window. The results indicate that residual structural and vibrational errors are not best treated as uniform empirical remainders. They can be organized in low-dimensional representations selected by the molecular topology and by the observable being refined, providing an economical route to improved equilibrium structures and localized vibrational treatments within standard electronic-structure workflows.

Polycyclic aromatic hydrocarbons (PAHs) and their curved buckybowl derivatives are key motifs in combustion, atmospheric, and astrochemical environments, as well as fundamental building blocks of graphene-based materials. Here, we show that the Pisa composite schemes provide an accurate and affordable framework for predicting equilibrium structures, rotational constants, and vibrational spectra across representative planar (indene and azulene) and curved (corannulene and sumanene) PAHs. Accurate equilibrium geometries are combined with a fully anharmonic treatment based on generalized second-order vibrational perturbation theory, including both mechanical and electrical contributions to infrared intensities. This enables reliable prediction not only of fundamental frequencies but also of overtone and combination-band intensities, which are strictly absent in the harmonic approximation. Indene and azulene validate the structural and spectroscopic accuracy of the protocol for planar π-systems, whereas corannulene and sumanene quantify the effects of bowl curvature on mode mixing, symmetry breaking, and activation of otherwise forbidden transitions. Across all systems, the proposed computational approach yields fundamental frequencies with typical deviations of 5-10 cm-1, reproduces curvature-dependent shifts in C-H stretching and ring-deformation modes, and captures characteristic splitting patterns and intensity redistribution induced by both mechanical and electrical anharmonicity.

Abstract We present a multilevel strategy for harmonic vibrational spectroscopy in which a lower cost calculation supplies the complete coupled force field and higher level energy calculations refine selected curvatures. Symmetry-oriented non-redundant internal coordinates retain chemical identity and coupling topology, while concordant normal-mode directions provide an efficient frequency end point. Their sequential use improves mode shapes without requiring a higher level Hessian. Across four validation fixtures, the local-coordinate models recover unsupplied coupling correlations of 0.987–0.999; the complete sequence uses at most 2N independent higher level curvatures for N normal modes.

Sulfur-chlorine-oxygen chemistry is central to Venus-relevant reducing environments and has been widely studied, but the oxygen-free sulfur-chlorine compounds accessible within that chemical space remain much less characterized beyond the smallest members. Here, Cl2S, Cl2S2, Cl2S3, and Cl2S4 are treated as a calibrated progression of chlorine-capped, oxygen-free sulfur chains. The experimentally anchored Cl2S and Cl2S2 systems validate the local S-Cl and S-S descriptions before extension to the higher homologues, with good agreement with experiment in both cases; for Cl2S2, the calculations also confirm the high-energy pyramidal minimum proposed in a previous study. For Cl2S3, two open helical minima are separated by a torsional barrier of about 9.0 kcal mol-1, and a pyramidal local minimum is also characterized. For Cl2S4, the open chain remains more stable than the compact cyclic/transannular and pyramidal alternatives. Across the series, intense computed IR fundamentals concentrate in the 490-520 cm-1 terminal/backbone stretching region, while the experimentally assigned Raman bands near 440-450 cm-1 are consistent with substantial combination/overtone contributions, by analogy with S2Cl2. Excited-state calculations give weak low-energy ultraviolet onsets and place most oscillator strength deeper in the ultraviolet, defining a molecular reference frame for reduced sulfur-chlorine chemistry under Venus-relevant conditions.

An affordable and scalable computational strategy rooted in the Pisa Composite Schemes (PCS) framework is applied to the challenging case of substituted aromatic nitriles, delivering accurate molecular structures together with rotational and vibrational spectroscopic parameters at a computational cost comparable to that of standard density functional theory. Two semiexperimental (SE) equilibrium structures are derived from high-resolution rotational spectroscopy combined with computed vibrational corrections. These structures are found to be significantly more accurate than literature substitution (rs) geometries, thus providing stringent reference data for method validation. For one system, the fully parameter-free PPCS2 approach also offers an independent and consistent structural cross-check. Building on these benchmarks, a cost-effective dual-level strategy combining double-hybrid equilibrium geometries and harmonic force fields with hybrid-level anharmonic contributions is identified as an efficient operative level of theory (DPCS3//HPCS2). Further refinement of selected bond lengths through one-parameter effective corrections (BDPCS3 model) yields ground-state rotational constants in excellent agreement with the experiment, with typical deviations well below 0.1%. The validated protocol is applied to ethynylbenzonitrile (EBN) and hydroxybenzonitrile (HBN) isomers, yielding anharmonic infrared spectra in outstanding agreement with the experiment for the ortho and para HBN species and providing predictive, high-confidence reference data for the remaining isomers. Overall, this work demonstrates that near-spectroscopic accuracy for both rotational and vibrational observables can be achieved at affordable computational cost by treating electronic correlation and vibrational effects on an equal footing, thereby enabling reliable multispectroscopic characterization of substituted aromatic nitriles of astrochemical relevance.

Structural interpretation of accurate experimental data becomes increasingly challenging when nonlocal effects become important, particularly in delocalized open-shell systems and flexible hydrogen-bonded molecules. In these regimes, density functional methods, even when augmented by bond corrections, become insufficient because the transferable unit of the structural error is defined at an inappropriate scale. Here, we show that transferable corrections can instead be formulated by shifting from individual bonds to larger interaction-driven building blocks. These can be identified automatically and assigned, in a black-box fashion, to the appropriate rung of a general accuracy ladder, thereby establishing a direct correspondence between the scale at which errors arise and the level of electronic-structure theory required to describe them, while retaining an affordable computational cost. Across radicals, nonplanar aromatic systems, and flexible hydrogen-bonded networks, quantitative agreement between theory and experiment is recovered only when the physically relevant transferable unit of the error is matched to a commensurate level of theory and vibrational averaging is treated consistently. Accurate structures are therefore obtained not by uniformly increasing the level of theory but by matching the scale of error transferability with the appropriate level of description. Within this framework, multilevel strategies naturally emerge for large systems in which different regions are treated at different rungs of the ladder according to their structural complexity. This enables available experimental rotational constants to be reproduced within ∼0.1-0.2% at affordable cost, corresponding to root-mean-square deviations of atomic positions on the order of 1-2 × 10-3 Å.