Organic dicarboxylic acids are common atmospheric compounds and are important constituents of secondary organic aerosols (SOA) with implications for air quality and climate. Both fumaric acid (FA) and maleic acid (MA) are found in substantial amounts in aerosol particles and have widespread applications throughout industry. While there have been extensive studies of the isomerization and spectroscopic properties of FA and MA in aqueous solution, there has, to the best of our knowledge, not been any studies of the thermodynamic and spectroscopic properties of microhydrated gas-phase and solvated clusters of FA and MA together with Na+ and Cl- ions. Here, we present a study of the gas-phase and aqueous-phase clusters of FA and MA together with ions by probing cluster geometries, binding and solvation free energies, and infrared (IR) absorption spectra using quantum chemical methods. We performed a detailed configurational sampling protocol to obtain gas-phase structures calculated at the DLPNO-CCSD(T0)/aug-cc-pVTZ//omega B97X-D/6-31++G(d,p) level followed by a reoptimization in implicit solvent to obtain aqueous-phase structures. We find that only negligible shifts in geometry occur upon solvation of the gas-phase clusters containing FA and MA, while slightly larger rearrangements of the structures occur when solvating the ion-containing clusters. The binding free energies reveal that it is thermodynamically unfavorable to hydrate FA and MA in the gas phase but favorable to hydrate their conjugate bases (FA-, MA-, FA2-, and MA2-), while it is favorable to solvate all of the clusters. Furthermore, we find that it is both favorable to hydrate the ion-containing clusters in the gas phase and solvate them. Finally, in order to identify weakly bound clusters and guide future experimental work, our IR absorption analysis reveals that the harmonic frequencies of the FA and MA carboxylic O-H stretching modes of the microhydrated FA and MA clusters are red-shifted in the spectrum upon solvation. On the other hand, we find no clear trend in the spectra of the (FA)1(Na+)1(Cl-)1(w)0-5 and (MA)1(Na+)1(Cl-)1(w)0-5 systems, but interestingly we find consistent red-shifts in the spectra for (FA-)1(Na+)1(w)0-5, (MA-)1(Na+)1(w)0-5, (FA-)1(Na+)1(Cl-)1(w)0-5, and (MA-)1(Na+)1(Cl-)1(w)0-5.

Organic molecules contribute substantially to the formation of aerosols in the atmosphere, forming what is known as secondary organic aerosols (SOAs). The organic molecules are emitted as volatile organic compounds (VOCs) and undergo a number of reactions in the atmosphere. Due to the variety of both VOCs and reaction pathways, it has been difficult to elucidate the exact structure of an organic molecule that is able to drive new particle formation (NPF). Using quantum chemistry methods, we have studied the NPF ability of three different oxygenated organic molecules (OOMs): 3-methyl-1,2,3-butanecarboxylic acid (MBTCA), carboxyheptanoic acid (CHA), and pinyl diaterpenylic ester (PDPE). These all contain three carboxylic acids, which, as previous works suggest, are good candidates for driving NPF and have been observed in the atmosphere, as well as in lab experiments. Using computational methods, we studied the (OOM)1–2(SA)0–2(base)0–2 clusters, where SA is sulfuric acid, and the base is [ammonia (AM), methylamine (MA), dimethylamine (DMA), and trimethylamine (TMA)]. Geometry optimization and thermochemical parameters are calculated at the ωB97X-D/6-31++G(d,p) level of theory, and single-point energies are calculated at the DLPNO-CCSD(T0)/aug-cc-pVTZ level of theory. We found that PDPE was able to produce the most stable clusters, presumably due to its higher flexibility compared to MBTCA and CHA. Cluster formation potentials are simulated using the Atmospheric Cluster Dynamics Code (ACDC). We found that all three OOMs were able to enhance cluster formation for the (OOM)(SA)(base) systems by 2–3 orders of magnitude for most systems. In particular, the (OOM)(SA)(DMA) system has a high cluster formation potential, with similar trends in the enhancement across all three OOMs.

Oligomerization reactions from RO2 + R'O2 radicals, occurring via a triplet (RO···3O2···OR') cluster, are an important gas-phase reaction for the formation of low-volatile ROOR' accretion products. However, it remains unknown whether such reactions can occur at the interface of freshly nucleated particles (FNPs). For instance, FNPs coated with a shell of organic compounds could potentially form accretion products at the surface, further stabilizing the particle. Using quantum chemical methods, we here study how the RO2 + R'O2 reaction is influenced by interaction with FNP precursors such as sulfuric acid (SA), ammonia (AM), and dimethylamine (DMA). For the RO2's, we tested simple branched hydroxyl peroxy radicals (HO-RO2) as the tether to the FNP components. Cluster structures were obtained using a systematic conformational sampling approach based on the ABCluster program and CREST. We calculated the final structure and vibrational frequencies at the ωB97X-D/6-31++G-(d,p) level of theory. Energy levels for intersystem crossing calculations were carried out at the XMC-QDPT2/6-311++G-(d,p) level of theory, and spin-orbit coupling matrix elements were calculated using CASSCF-(6,4)/6-311++G-(d,p). Our calculations show that the rate of the intersystem crossing needed to form ROOR' accretion products at the FNP model clusters lies in the range of 106-109 s-1, similar to the rate previously computed in the gas phase. We also find that both the intermediate (RO···3O2···OR') clusters and the resulting ROOR' accretion products interact strongly with the FNP components, leading to suppressed evaporation if formed at the surface. Unfortunately, the formation is limited by the requirement of two RO2 radicals being involved. We hypothesize a pathway where the RO2/R'O2 are formed via oxidation reactions at the surface and recombine via a Langmuir-Hinshelwood mechanism. However, this must still be considered an extremely rare event.

Organic dicarboxylic acids are common atmospheric compounds and are important constituents of secondary organic aerosols (SOA) with implications for air quality and climate. Both fumaric acid (FA) and maleic acid (MA) are found in substantial amounts in aerosol particles and have widespread applications throughout industry. While there have been extensive studies of the isomerization and spectroscopic properties of FA and MA in aqueous solution, there has, to the best of our knowledge, not been any studies of the thermodynamic and spectroscopic properties of microhydrated gas-phase and solvated clusters of FA and MA together with Na+ and Cl- ions. Here, we present a study of the gas-phase and aqueous-phase clusters of FA and MA together with ions by probing cluster geometries, binding and solvation free energies, and infrared (IR) absorption spectra using quantum chemical methods. We performed a detailed configurational sampling protocol to obtain gas-phase structures calculated at the DLPNO-CCSD(T0)/aug-cc-pVTZ//ωB97X-D/6-31++G(d,p) level followed by a reoptimization in implicit solvent to obtain aqueous-phase structures. We find that only negligible shifts in geometry occur upon solvation of the gas-phase clusters containing FA and MA, while slightly larger rearrangements of the structures occur when solvating the ion-containing clusters. The binding free energies reveal that it is thermodynamically unfavorable to hydrate FA and MA in the gas phase but favorable to hydrate their conjugate bases (FA-, MA-, FA2-, and MA2-), while it is favorable to solvate all of the clusters. Furthermore, we find that it is both favorable to hydrate the ion-containing clusters in the gas phase and solvate them. Finally, in order to identify weakly bound clusters and guide future experimental work, our IR absorption analysis reveals that the harmonic frequencies of the FA and MA carboxylic O-H stretching modes of the microhydrated FA and MA clusters are red-shifted in the spectrum upon solvation. On the other hand, we find no clear trend in the spectra of the (FA)1(Na+)1(Cl-)1(w)0-5 and (MA)1(Na+)1(Cl-)1(w)0-5 systems, but interestingly we find consistent red-shifts in the spectra for (FA-)1(Na+)1(w)0-5, (MA-)1(Na+)1(w)0-5, (FA-)1(Na+)1(Cl-)1(w)0-5, and (MA-)1(Na+)1(Cl-)1(w)0-5.

The initial formation of secondary aerosols, a large cause of uncertainty in modern radiative forcing modeling, can be simulated using quantum chemical methods. When based on quantum chemistry, the simulations have an exponential dependence on the free energy, requiring a high-accuracy description. In this study, we have computed harmonic frequencies and quasi-harmonic free energies for a set of 10 monomers and 29 dimers relevant for atmospheric molecular clusters at the DF-CCSD(F12b)(T*)/cc-pVDZ-F12 level of theory. The set is used to benchmark the M06-2X, PW91, ωB97X-D3BJ functions with the Jensen, Karlsruhe, and Pople style basis sets. The composite methods B97-3c, r2SCAN-3c, and ωB97X- 3c are also tested. We find ωB97X-D3BJ/ma-def2-SVP to be an optimal choice as it has low errors (mean absolute error of 0.13 kcal/mol) and few outliers in the thermal contribution. For calculations on larger clusters, B97-3c stands out. Using the ωB97X-D3BJ/ma-def2-SVP and B97-3c level of theories, we compute anharmonic frequencies using the VPT2 method and determine an anharmonic scaling factor of 0.961 and 0.953 for ωB97X-D3BJ/ma-def2-SVP and B97-3c frequencies, respectively. The scaling factor together with incorporating multi-conformer entropy effects and a high-level single-point correction at the Normal LNO-CCSD(T)/CBS(aug- 3,aug-4) are used to compare to experimental determined free energies of the 11 hydrogen-bonded systems. We find that we obtain sub 1 kcal/mol errors when incorporating the scaling factor, the multi-conformer entropy effects, and the single point correction.

The gas-phase hydrogen abstraction reaction kinetics of atmospheric volatile organic compounds (VOCs) have been investigated using multiconformer transition state theory (MC-TST) as part of the development of the Jammy Key for Transition States (JKTS), an automated tool developed to address the vast number of organic species in the atmosphere that constantly undergo reactions with radicals. The rate constants for OH-initiated reactions with several short-chain compounds─methane, ethane, propane, and their corresponding alcohols and carbonyls─were computationally determined and compared to experimental data. Additionally, the OH abstraction kinetics of pinonaldehyde, a key oxidation product of biogenic VOCs, were studied in detail. Tunnelling effects were evaluated using Wigner and Eckart tunnelling corrections to ensure accurate prediction of reaction rates. JKTS yielded rate constants within a factor of ∼2-3 of experimental data across all systems studied, with branching ratios for pinonaldehyde showing significant contributions from aldehydic and tertiary hydrogen abstraction pathways. The calculated rate constants for pinonaldehyde, 1.739 × 10-11 cm3 molecule-1 s-1 (Eckart) and 1.847 × 10-11 cm3 molecule-1 s-1 (Wigner), align well with the experimental values of (4-9) × 10-11 cm3 molecule-1 s-1 at room temperature. These results demonstrate the capability of JKTS to automate the computation of reaction kinetics and support its application in atmospheric chemistry for accurate modeling of VOC oxidation mechanisms.

Organic acids are important atmospheric compounds that affect the aerosol physicochemical properties and the formation of secondary organic aerosols (SOA) with implications for air quality and climate. Pyruvic acid (PA) is ubiquitous in the atmosphere, biosphere, and hydrosphere. While the pure gas-phase and aqueous-phase chemistry of PA has been extensively studied, its simultaneous interactions with water and ions in the particle phase remains elusive. Here, we present a study on the solvation of PA and its structurally similar analogs─lactic acid (LA), propionic acid (ProA), and 2,2-dihydroxypropionic acid (diol)─by probing geometries, solvation free energies, and infrared (IR) absorption spectra using quantum chemical methods. We performed a refinement of structures in the aqueous phase based on an elaborate configurational sampling scheme in the gas phase, which we have reported previously. The aqueous phase is modeled using explicit microhydration within an implicit polarizable continuum model. We find that the solvated organic acid clusters have a high conservation of geometry when transitioning from the gas phase to the aqueous particle phase, while the solvated ion-containing clusters show significantly larger structural rearrangements. Solvation of organic acids is found to be thermodynamically favorable in the aqueous particle phase─both with and without ions─unlike in the gas phase. Finally, in order to identify weakly bound clusters and guide future experiments, our IR absorption analysis shows that the harmonic frequencies of PA carboxylic O-H stretching of the microhydrated PA clusters are red-shifted in the spectrum in the aqueous phase compared to the gas phase. Conversely, we find no clear trends in the spectrum obtained with our qualitative approach for the O-H frequencies of the microhydrated ion-containing PA clusters.

Recent advances in machine learning interatomic potentials have enabled the simulation of cluster formation from precursor vapor at a high level of theory. However, performing these simulations requires verifying that the models accurately describe cluster formation dynamics, particularly collision processes. In this work, we study the performance of two distinct machine learning (ML) architectures, AIMNet2 and PaiNN, against GFN1-xTB and ωB97X-3c reference data for atmospherically relevant collision systems (H2SO4–H2SO4, H2SO4–HSO4-, and H2SO4–NH(CH3)2).We evaluate the models' ability to reproduce one-dimensional potentials of mean force (PMFs) and collision probabilities. Both models achieve excellent agreement with reference PMFs, yielding RMSEs at least an order of magnitude lower than chemical accuracy (1 kcal mol-1). Notably, PaiNN achieves lower errors in the binding region.However, we observe significant differences in collision probabilities. While AIMNet2 accurately reproduces these probabilities, PaiNN fails to capture long-range interactions beyond its local cutoff (10 Å). For the charged H2SO4–HSO4- system, this leads to a complete loss of collision probability beyond 14 Å and an underestimation at shorter distances.Our results demonstrate a clear trade-off: while PaiNN offers superior accuracy for equilibrium properties and binding energies, its local nature makes it unsuitable for collision kinetics in systems with strong long-range interactions. Conversely, AIMNet2's ability to model these long-range interactions makes it the necessary choice for simulating collisions in such systems.

The interaction between liquid carbon dioxide and material surfaces plays an important role for the function and durability of carbon capture and utilization (CCU) systems. Experimental data on this interaction are scarce due to the difficulty in tracking interfacial layers of CO2 in contact with solid surfaces. In this work, we use surface-sensitive sum-frequency generation (SFG) spectroscopy combined with molecular dynamics simulations and DFT calculations to measure the geometry and orientation of the first molecular layer of liquid CO2 in contact with a sapphire surface. The results show that the CO2 molecules preferentially adopt a tilted orientation with an angle of 14 ± 8° with respect to the surface normal. Furthermore, the electronic structure of CO2 is perturbed by the interface leading to symmetry breaking: one C═O bond is elongated along the molecular axis while the other is shortened. These results pave the way for using SFG spectroscopy to measure the interactions of liquid CO2 with functional surfaces, catalysts, and CCU relevant materials.

Understanding how atmospheric molecular clusters form and grow is key to resolving one of the biggest uncertainties in climate modeling: the formation of new aerosol particles. While quantum chemistry offers accurate insights into these early-stage clusters, its steep computational costs limit large-scale exploration. In this work, we present a fast, interpretable, and surprisingly powerful alternative: the k-nearest neighbor (k-NN) regression model. By leveraging chemically informed distance metrics, including a kernel-induced metric and one learned via metric learning for kernel regression (MLKR), we show that simple k-NN models can rival more complex kernel ridge regression (KRR) models in accuracy while reducing computational time by orders of magnitude. We perform this comparison with the well-established Faber-Christensen-Huang-Lilienfeld (FCHL19) molecular descriptor; however, other descriptors (e.g., FCHL18, MBDF, and CM) can be shown to have similar performance. Applied to both simple organic molecules in the QM9 benchmark set and large data sets of atmospheric molecular clusters (sulfuric acid-water and sulfuric-multibase-base systems), our k-NN models achieve near-chemical accuracy, scale seamlessly to data sets with over 250,000 entries, and even appears to extrapolate to larger unseen clusters with minimal error (often nearing 1 kcal/mol). With built-in interpretability and straightforward uncertainty estimation, this work positions k-NN as a potent tool for accelerating discovery in atmospheric chemistry and beyond.

Molecular collisions and subsequent clustering events are fundamental to atmospheric cluster formation. Accurately modeling these processes requires interatomic potentials that simultaneously capture the long-range forces governing collision kinetics and the short-range quantum effects driving reactivity. In this work, we evaluate the AIMNet2 and PaiNN machine learning architectures trained on GFN1-xTB and omega B97X-3c quantum chemical data for molecular collisions involving sulfuric acid.The models exhibit low mean absolute errors in energies and forces and accurately reproduce potentials of mean force relative to the GFN1-xTB reference. However, discrepancies are observed for the collision dynamics. While AIMNet2 accurately reproduces reference collision rate coefficients across all systems, PaiNN underestimates the rate coefficient for the charged sulfuric acid-bisulfate system by similar to 50 %. This error originates from the model's local atomic environment approximation, which neglects the strong long-range attractive forces at large intermolecular distances. Simulations with the OPLS-AA classical force field demonstrate that simple fixed partial charges are sufficient to describe these interactions.Comparing models trained on GFN1-xTB and omega B97X-3c data reveals that while increasing the level of electronic structure theory significantly alters the potential energy surface in the short-range binding region, it generally has less impact on the long-range shoulder and the resulting collision rate coefficients.Our results highlight that while local equivariant models like PaiNN offer exceptional accuracy for thermodynamics, correctly simulating collision kinetics in systems with strong long-range interactions requires models that explicitly account for forces beyond the local environment, such as AIMNet2.

Aerosol nucleation accounts for the majority of secondary aerosols, yet it is unclear how biogenic and anthropogenic chemical precursors contribute to nucleation in mixed atmospheric environments. Here, we show laboratory experiments and quantum calculations, which demonstrate, for the first time to our knowledge, that highly oxygenated organosulfates (OOS) formed in the gas phase contribute to aerosol nucleation independently of sulfuric acid and oxygenated organic molecules. More than 200 different gas-phase OOS were detected with the nitrate CI-APi-TOF from a mixture of alpha-pinene, ozone, and SO2. The gas-phase OOS concentrations were strongly correlated with sulfuric acid. Quantum chemical modeling simulations showed that OOS forms in the gas phase from alpha-pinene diols and SO3 via a barrierless process. Nucleation rates increased much more rapidly with increasing alpha-pinene concentrations in the mixed system than in the pure biogenic system, clearly demonstrating that OOS are effective nucleation precursors in mixed biogenic and anthropogenic systems.

Sulfuric acid (SA), ammonia (AM), and dimethylamine (DMA) are believed to be key contributors to new particle formation (NPF) in the atmosphere. NPF happens through gas-to-particle transformation via cluster formation. However, it is not obvious how small clusters grow to larger sizes and eventually form stable aerosol particles. Recent experimental measurements showed that the presence of mixtures of bases enhanced the nucleation rate by several orders of magnitude. Using quantum chemistry methods, this study explores this base synergy in the formation of large clusters from a mixture of SA, AM, and DMA. We calculated the binding free energies of the (SA)n(AM)x(DMA)n−x clusters, with n from 1 to 10, where x runs from 0 to n. The cluster structures were obtained using our recently developed comprehensive configurational sampling approach based on multiple ABCluster runs and meta-dynamic sampling via the Conformer–Rotamer Ensemble Sampling Tool (CREST). The structures and thermochemical parameters are calculated at the B97-3c level of theory. The final single point energy of the clusters is calculated at the ωB97X-D3BJ/6-311++G(3df,3pd) level of theory. Based on the calculated thermochemistry, we found that AM, despite being a weaker base, forms more intermolecular interactions than DMA and easily becomes embedded in the cluster core. This leads to the mixed SA–AM–DMA clusters being lower in free energy compared to the pure SA–AM and SA–DMA clusters. We find that the strong base DMA is important in the very first steps in cluster formation, but for larger clusters an increased ammonia content is found. We also observed that the cluster-to-particle transition point for the mixed SA–AM–DMA clusters occurs at a cluster size of 14 monomers, which is notably smaller than the transition points for the pure SA–AM (16 monomers) or pure SA–DMA (20 monomers) systems. This indicates a strong synergistic effect when both AM and DMA are present, leading to the formation of stable freshly nucleated particles (FNPs) at smaller cluster sizes. These findings emphasize the importance of considering several base molecules when studying the formation and growth of FNPs.

It is important to investigate formation, composition and properties of secondary organic aerosol (SOA) from monoterpenes in order to develop an accurate understanding of their atmospheric chemistry, impact on the aerosol budget and the effects of climate change. Δ3-Carene is one of the monoterpenes emitted in highest amounts in the boreal forest, yet only few studies have investigated the atmospheric chemistry and aerosol formation of Δ3-carene.In this work, we have investigated aerosol formation and composition of SOA from ozonolysis of Δ3-carene at different concentration levels in the AURA atmospheric simulation chamber at Aarhus University, Denmark. At low concentrations of Δ3-carene (about 10 ppb), SOA formation shows minimal temperature dependence under dry conditions. This contrasts with results from studies of Δ3-carene at higher concentrations (about 50 ppb) and studies of the structurally quite similar monoterpene a-pinene. Furthermore, we observed increased particle nucleation at higher relative humidity (about 80% RH, 10°C). Chemical analysis of the SOA found a series of carboxylic acids, in line with previous studies, with different concentration profiles over time, depending on experiment temperature. In experiments with ozonolysis of mixtures of Δ3-carene and a-pinene, we were able to identify a mixed dimer composed of molecular units from each of the precursors.

Pyruvic acid is an omnipresent compound in nature and is found both in the gas phase and in the particle phase of the atmosphere as well as in aqueous solution in the hydrosphere. Despite much literature on the photochemical degradation and stability of pyruvic acid in different chemical environments, the study of simultaneous interactions between gas-phase pyruvic acid or similar carboxylic acids with water and ions is not well-understood. Here, we present a study of microhydrated molecular clusters containing pyruvic acid and the structurally analogous carboxylic acids lactic acid, propionic acid, and 2,2-dihydroxypropanoic acid by probing geometries, binding free energies, hydrate distributions, as well as their infrared (IR) absorption spectra. We performed a meticulous configurational sampling protocol for the various hydrated clusters ranging from low level of theory to high level of theory to identify the lowest free energy structure. We find that cluster geometries and especially their water structure are highly sensitive to the presence and character of ions. We show that the hydration of the studied organic acids is thermodynamically unfavorable in the gas phase and ions are necessary for mediating interactions between organic acids and water thus stabilizing the clusters. Finally, we find a clear correlation between decreasing pyruvic acid carboxylic O-H stretching frequencies, increasing intensity when adding more water to the clusters, and a correlation between increasing redshifting of the O-H frequencies upon addition of ions to the clusters. The observations done in this study could pave the way to unravel the mechanisms behind the transitioning of organic acids from the gas phase to the particle phase.

Iodine-driven nucleation is thought to be a significant source of new particle formation, especially in marine and polar regions. Despite numerous studies, the mechanism is still not fully understood. To shed further light on this, we apply ZORA-DLPNO-CCSD-(T0)/TZVPP//ωB97X-D3BJ/aug-cc-pVTZ-PP to calculate the thermochemistry of iodine-containing clusters up to tetramers and simulate the cluster formation potential for several nucleation paths using the atmospheric cluster dynamics code (ACDC). We find that iodine oxyacid-amine nucleation can be competitive with sulfuric acid-amine nucleation if iodic acid is present in a 10:1 ratio compared to sulfuric acid. Therefore, the importance of the iodine-driven pathway is regionally dependent. Likewise, we find that increasing the relative humidity from 34 to 73% only changes the cluster formation potential by a factor of 2. Nucleation pathways consisting of only iodic and iodous acid are unable to explain the relative nucleation rates previously observed in experiments. In contrast, the simultaneous nucleation of iodine oxides, assisted by iodine oxyacids, is better able to describe the trend. This indicates that a nucleation pathway starting with iodine oxides is more likely to be able to explain observed particle numbers. However, this current model does not include all of the hydrates of the clusters and does not account for the hydrolysis reactions of the iodine oxides. This would need to be incorporated in future studies.

Aerosols are the largest source of uncertainty in modern global radiative forcing modeling. Atmospheric molecular clusters are important intermediates in atmospheric new particle formation (NPF). The evaporation rate of clusters can be calculated using quantum chemical methods, with an exponential dependence on the free energy. Hence, for simulating accurate NPF rates, high-accuracy calculations are needed. We have constructed a versatile benchmark set of 218 conformers of atmospheric molecular dimer clusters consisting of sulfuric acid (SA), formic acid (FA), nitric acid (NA), methanesulfonic acid (MSA), water (W), ammonia (AM), methylamine (MA), dimethylamine (DMA), trimethylamine (TMA), and ethylenediamine (EDA) molecules. Using this test set, we benchmark the local coupled cluster methods, DLPNO-CCSD-(T0) and LNO-CCSD-(T), using different basis sets and locality settings, and test extrapolation procedures to the complete basis set (CBS), local approximation free (LAF), and complete PNO space (CPS) limits. The extrapolations are tested against the binding energies of high-level CCSD-(F12*)-(T+)/cc-pVTZ-F12 reference calculations. We find that the LNO-CCSD-(T) methods offer a better accuracy-to-cost ratio for atmospheric molecular clusters than the usually employed DLPNO-CCSD-(T0) method. Furthermore, the CBS limit extrapolation using the aug-cc-pVTZ and aug-cc-pVQZ basis sets should be readily attainable for the LNO-CCSD-(T) method on the usually studied cluster sizes (4-8 monomers). Simulating the new particle formation rate of the (SA)1-4(AM)1-4 and (SA)1-4(DMA)1-4 systems using the Atmospheric Cluster Dynamics Code, we find an increased sensitivity to the locality settings for larger clusters, but the basis set error is still the most dominant. Hence, simulated cluster formation rates would also benefit from doing LAF extrapolation. Finally, we illustrate the calculations of LNO-CCSD-(T)/CBS binding energies of a large (SA)15(TMA)15 cluster (300 atoms). Hence, the application of LNO-CCSD-(T) allows for significantly more accurate binding energies of much larger clusters than previously possible.

[This corrects the article DOI: 10.1021/acsomega.3c06794.].

Information about the optical properties of atmospheric molecular clusters is scarce as they are challenging to measure using current experimental techniques. Here we explore the absorption and Rayleigh scattering properties of acid-base molecular clusters using quantum chemical methods. We studied 127 small (acid)1-2(base)1-2 cluster systems, with the acids sulfuric acid (SA), methanesulfonic acid (MSA), nitric acid (NA), and formic acid (FA) in all combinations of the bases ammonia (AM), methylamine (MA), dimethylamine (DMA), trimethylamine (TMA), and ethylenediamine (EDA). To further explore the effect of cluster size on the optical properties, we studied the large (SA)n(AM)n cluster systems, with n up to 15 acid-base pairs. We calculated the polarizability tensors and the 10 lowest excitation energies at the CAM-B3LYP/aug-cc-pVTZ level of theory. We find that the isotropic polarizability is almost linearly dependent on the cluster size, with small variations depending on the cluster composition. The anisotropic polarizability is plateauing as a function of cluster size. The larger the cluster, the more dominant the isotropic contribution becomes in the calculation of the Rayleigh light scattering activity. As a consequence, the Rayleigh scattering activity will increase quadratically as a function of cluster size. We stress that future studies on the scattering properties should be evaluated as effective scattering, taking the concentrations of the clusters into account. We find that the clusters absorb infrared (IR) radiation in the atmospheric spectral window region but speculate that their lifetime is too short to be competitive with common greenhouse gases. Due to the lack of strong chromophores in the studied acid-base clusters, the ultraviolet-visual (UV-vis) absorption is found to occur in the deep UV. Hence, clusters with more organic content should be studied in the future. Finally, we outline several directions in which the field of studying the optical properties of clusters and aerosols using response theory methods could evolve.

We report measurements of absolute saturation vapor pressures around room temperature for three fatty acid methyl esters (methyl octanoate, methyl decanoate, and methyl dodecanoate) using a recently developed experimental method in which the saturation vapor pressures are determined from the vaporization dynamics of a cooled sample during thermalization to a higher chamber temperature.