Resonating holes vs molecular spin-orbit coupled states in group-5 lacunar spinels.

The valence electronic structure of magnetic centers is one of the factors that determines the characteristics of a magnet. This may refer to orbital degeneracy, as for jeff = 1/2 Kitaev magnets, or near-degeneracy, e.g., involving the third and fourth shells in cuprate superconductors. Here we explore the inner structure of magnetic moments in group-5 lacunar spinels, fascinating materials featuring multisite magnetic units in the form of tetrahedral tetramers. Our quantum chemical analysis reveals a very colorful landscape, much richer than the single-electron, single-configuration description applied so far to all group-5 GaM4X8 chalcogenides, and clarifies the basic multiorbital correlations on M4 tetrahedral clusters: while for V strong correlations yield a wave-function that can be well described in terms of four V4+V3+V3+V3+ resonant valence structures, for Nb and Ta a picture of dressed molecular-orbital jeff = 3/2 entities is more appropriate. These internal degrees of freedom likely shape vibronic couplings, phase transitions, and the magneto-electric properties in each of these systems.

Quantum interference effects elucidate triplet-pair formation dynamics in intramolecular singlet-fission molecules.

Quantum interference (QI)-the constructive or destructive interference of conduction pathways through molecular orbitals-plays a fundamental role in enhancing or suppressing charge and spin transport in organic molecular electronics. Graphical models were developed to predict constructive versus destructive interference in polyaromatic hydrocarbons and have successfully estimated the large conductivity differences observed in single-molecule transport measurements. A major challenge lies in extending these models to excitonic (photoexcited) processes, which typically involve distinct orbitals with different symmetries. Here we investigate how QI models can be applied as bridging moieties in intramolecular singlet-fission compounds to predict relative rates of triplet pair formation. In a series of bridged intramolecular singlet-fission dimers, we found that destructive QI always leads to a slower triplet pair formation across different bridge lengths and geometries. A combined experimental and theoretical approach reveals the critical considerations of bridge topology and frontier molecular orbital energies in applying QI conductance principles to predict rates of multiexciton generation.

Benchmarking Gaussian Basis Sets in Quantum-Chemical Calculations of Photoabsorption Spectra of Light Atomic Clusters.

The choice of Gaussian basis functions for computing the ground-state properties of molecules and clusters, employing wave function-based electron-correlated approaches, is a well-studied subject. However, the same cannot be said when it comes to the excited-state properties of such systems, in general, and optical properties, in particular. The aim of the present study is to understand how the choice of basis functions affects the calculations of linear optical absorption in clusters, qualitatively and quantitatively. For this purpose, we have calculated linear optical absorption spectra of several small charged and neutral clusters, namely, Li2, Li3, Li4, B2 +, B3 +, Be2 +, and Be3 +, using a variety of Gaussian basis sets. The calculations were performed within the frozen-core approximation, and a rigorous account of electron correlation effects in the valence sector was taken by employing various levels of configuration interaction (CI) approach both for the ground and excited states. Our results on the peak locations in the absorption spectra of Li3 and Li4 are in very good agreement with the experiments. Our general recommendation is that for excited-state calculations, it is very important to utilize those basis sets which contain augmented functions. Relatively smaller aug-cc-pVDZ basis sets also yield high-quality results for photoabsorption spectra and are recommended for such calculations if the computational resources are limited.

Why Does a B12H12 Icosahedron Need Two Electrons to be Stable: A First-Principles Electron-Correlated Investigation of B12Hn (n = 6, 12) Clusters.

In this work, we present large-scale electron-correlated computations on various conformers of B12H12 and B12H6 clusters to understand the reasons behind the high stability of dianion icosahedron (Ih) and cage-like B12H6 geometries. Although the B12 icosahedron is the basic building block in some structures of bulk boron, it is unstable in its free form. Furthermore, its H-passivated entity, i.e., a B12H12 icosahedron, is also unstable in the free form. However, dianion B12H12 has been predicted to be stable as a perfect icosahedron in the free-standing form. To capture the correct picture for the stability of B12H122- and B12H6 clusters, we optimized these structures by employing the coupled-cluster singles and doubles (CCSD) approach and the cc-pVDZ basis set. We also performed the vibrational frequency analysis of the isomers of these clusters using the same level of theory to ensure the stability of the structures. For all of the stable geometries obtained from the vibrational frequency analysis, we additionally computed their optical absorption spectra using the time-dependent density functional theory (TDDFT) approach at the B3LYP/6-31G* level of theory. Our calculated absorption spectra could be probed in future experiments on these clusters.

Systematic First-Principles Configuration-Interaction Calculations of Linear Optical Absorption Spectra in Silicon Hydrides: Si2H2n (n = 1-3).

We have performed first-principles electron-correlated calculations employing large basis sets to optimize the geometries and to compute linear optical absorption spectra of various low-lying conformers of silicon hydrides: Si2H2n, n = 1, 2, 3. The geometry optimization for various isomers was carried out at the coupled-cluster singles-doubles-perturbative-triples [CCSD(T)] level of theory, while their excited states and absorption spectra were computed using a large-scale multireference singles-doubles configuration-interaction approach, which includes electron-correlation effects at a sophisticated level. Our calculated spectra are the first ones for Si2H2 and Si2H4 conformers, while for Si2H6, we obtain excellent agreement with the experimental measurements, suggesting that our computational approach is reliable. Our calculated absorption spectra exhibit a strong structure-property relationship, suggesting the possibility of identifying various conformers based on their optical absorption fingerprints. Furthermore, we have also performed geometry optimization for the selected optically excited states, providing insights into their character.