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Abstract This work provides a detailed multi‐component analysis of aromaticity in monosubstituted (X = CH₃, C, C, NH₂, NH⁻, NH⁺, OH, O⁻, and O⁺) andpara‐homodisubstituted (X = CH₃, CH₂, NH₂, NH, OH, and O) benzene derivatives. We investigate the effects of substituents using single‐reference (B3LYP/DFT) and multireference (CASSCF/MRCI) methods, focusing on structural (HOMA), vibrational (AI(vib)), topological (ELF_π), electronic (MCI), magnetic (NICS), and stability (S₀–T₁splitting) properties. The findings reveal that appropriateπ‐electron‐donating andπ‐electron‐accepting substituents with suitable size and symmetry can interact with theπ‐system of the ring, significantly influencingπ‐electron delocalization. While the charge factor has a minimal impact onπ‐electron delocalization, the presence of ap_zorbital capable of interacting with theπ‐electron delocalization is the primary factor leading to a deviation from the typical aromaticity characteristics observed in benzene. 

Abstract The chemical stability and the low‐lying singlet and triplet excited states of BN‐n‐acenes (n = 1–7) were studied using single reference and multireference methodologies. From the calculations, descriptors such as the singlet‐triplet splitting, the natural orbital (NO) occupations and aromaticity indexes are used to provide structural and energetic analysis. The boron and nitrogen atoms form an isoelectronic pair of two carbon atoms, which was used for the complete substitution of these units in the acene series. The structural analysis confirms the effects originated from the insertion of a uniform pattern of electronegativity difference within the molecular systems. The covalent bonds tend to be strongly polarized which does not happen in the case of a carbon‐only framework. This effect leads to a charge transfer between neighbor atoms resulting in a more strengthened structure, keeping the aromaticity roughly constant along the chain. The singlet‐triplet splitting also agrees with this stability trend, maintaining a consistent gap value for all molecules. The BN‐n‐acenes molecules possess a ground state with monoconfigurational character indicating their electronic stability. The low‐lying singlet excited states have charge transfer character, which proceeds from nitrogen to boron. 

This study explores open shell biradical and polyradical molecular compounds based on extended multireference (MR) methods (MR-configuration interaction with singles and doubles (CISD) and MR-averaged quadratic coupled cluster (AQCC) approach) using the numbers of unpaired densities NU. These results were used to guide the analysis of the fractional occupation number weighted density (FOD) calculated within the finite temperature (FT) density functional theory (DFT) approach. As critical test examples, the dissociation of carbon-carbon (CC) single, double and triple bonds, and a benchmark set of polycyclic aromatic hydrocarbons (PAHs) has been chosen. By examining single, double, and triple bond dissociations, we demonstrate the utility and accuracy but also limitations of the FOD analysis for describing these dissociation processes. In significant extension of previous work (Phys Chem Chem Phys 25: 27380-27393) the assessment of FOD applications for different classes of DFT functionals was performed examining the range-separated functionals ωB97XD, ωB97M-V, CAM-B3LYP, LC-ωPBE, and MN12-SX, the hybrid (M06-2X) functional and the double hybrid (B2P-LYP) functional. In all cases, strong correlations between NFOD and NU values are found. The major task was to develop a new linear regression formula for range-separated functionals allowing a convenient determination of the optimal electronic temperature Tel for the FT-DFT calculation. We also established an optimal temperature for the semi-empirical extended tight-binding GFN2-xTB method. These findings significantly broaden the applicability of FOD analysis across various DFT functionals and semi-empirical methods. 

Molecular π-magnets based on single organic molecules have attracted increasing attention for their potential applications in optoelectronics and spintronics. Global aromaticity in conjugated macrocyclic polyradicaloids is still an open question that has only been tackled in molecules with an even number of electrons. Here, we report the on-surface synthesis of a cyclopenta-ringfused oligo(m-phenylene) macrocycle, 9MC, with an odd number of electrons. The generated polyradicaloid undergoes a surface-induced distortion to a D3h symmetry with a fully delocalized doublet ground state. Interestingly, 9MC exhibits two aromatic annulene-within-an-annulene (AWA) ring currents in the inner and outer rings. 

The effect of water on gold-supported chiral graphene nanoribbons has been studied. The results show a spontaneous hydrogenation of the ribbons with a well-defined periodic pattern, even at room temperature and with no other external activation. 

The phenalene (triangulene) and olympicene molecules belong to the polycyclic aromatic hydrocarbons (PAHs) class, which attracted substantial technological interest due to their unique electronic properties. Electronic structure calculations serve as a valuable tool in investigating the stability and reactivity of these molecular systems. In the present work, the multireference calculations, namely the complete active space second-order perturbation theory (CASPT2) and multireference averaged quadratic coupled cluster (MR-AQCC), were employed to study the reactivity and stability of phenalene and olympicene isomers, as well as their modified structures where the sp3-carbon at the borders were removed. The harmonic oscillator model of aromaticity (HOMA) and the nucleus-independent chemical shift (NICS) as geometric and magnetic indexes calculated with density functional theory were utilized to assess the aromaticity of the studied molecules. These indexes were compared with properties such as the excitation energy and natural orbitals occupation. The reactivity analyzed using the HOMA index combined with MR-AQCC revealed the radical character of certain structures, as well as the weakening of their aromaticity. Moreover, the results suggest that the removal of sp3-carbon atoms and the addition of hydrogen atoms did not alter the π network and the excitation energies of the phenalene molecules. 

The biradicaloid character of different types of polycyclic aromatic hydrocarbons (PAHs) based on small band gaps is an important descriptor to assess their opto-electronic properties. 

The nonplanar character of graphene with a single carbon vacancy defect (SV) is investigated utilizing a pyrene-SV model system by way of complete active space self-consistent field theory (CASSCF) and multi-reference configuration interaction singles and doubles (MRCISD) calculations. Planar structures were optimized with both methods showing the 3B1 state as the ground state with three energetically close states within an energy range of 1 eV. These planar structures constitute saddle-points. However, upon following the out-of-plane imaginary frequency yields more stable (by 0.22 to 0.53 eV), but non-planar structures of CS symmetry. Of these, the 1A’ structure is the lowest in energy and is strongly deformed into an L-shape. Following a further out-of-plane imaginary frequency in the non-planar structures leads to the most stable, but most deformed singlet structure of C1 symmetry. In this structure a bond is formed between the carbon atom with the dangling bond and a carbon of the cyclopentadienyl ring. This bond stabilizes the structure by more than 3 eV compared to the planar 3B1 structure. Higher excited states were calculated at MR-CISD level showing a grouping of four states low in energy and higher states starting around 3 eV. 

In this article the recent developments of the open-source OpenMolcas chemistry software environment, since spring 2020, are described, with the main focus on novel functionalities that are accessible in the stable branch of the package and/or via interfaces with other packages. These community developments span a wide range of topics in computational chemistry, and are presented in thematic sections associated with electronic structure theory, electronic spectroscopy simulations, analytic gradients and molecular structure optimizations, ab initio molecular dynamics, and other new features. This report represents a useful summary of these developments, and it offers a solid overview of the chemical phenomena and processes that OpenMolcas can address, while showing that OpenMolcas is an attractive platform for state-of-the-art atomistic computer simulations.