Rishu Khurana

Iron–sulfur clusters play central roles in biological photosynthesis, respiration, and catalysis. The [Fe4S4(SCH3)4]2– cluster possesses a dense manifold of low-lying electronic states arising from strong magnetic interactions among the Fe centers, making its electronic structure challenging to describe theoretically. Previous multireference studies have generally optimized excited states individually while neglecting the interactions among low-lying states. In this work, we investigate the low-lying electronic states of [Fe4S4(SCH3)4]2– using localized active space state interaction singles (LASSIS) combined with pair-density functional theory (PDFT). By treating all states within a common model-space Hamiltonian, LASSIS generates a single set of orthogonal low-lying states. The resulting spin-state energies organize into distinct manifolds. The same overall pattern is obtained for active spaces ranging from 22 electrons in 20 orbitals to 54 electrons in 36 orbitals. To enable these calculations, we also implement GPU acceleration for the most computationally demanding components of the LASSIS workflow.

Methane C-H activation at transition-metal sites often involves electronic structures that challenge conventional single-reference electronic structure descriptions. Although Kohn-Sham density functional theory (DFT) is widely used to study catalytic trends, its reliability for reactions involving strongly correlated species remains uncertain. Here we present a systematic multireference investigation of methane activation at metal-organic framework (MOF) node catalysts across the 3d transition-metal series. We introduce an automated workflow for active space selection to enable consistent application of multireference methods, including multiconfiguration pair-density functional theory and n-electron valence state perturbation theory, to these catalytic systems. These calculations show substantial static correlation in the C-H activation reaction step and predict activation barriers that differ from DFT by 30-70 kJ mol^-1, with DFT often qualitatively disagreeing in barrier height trends across transition metals. Analysis of multireference wave functions shows that reactivity is governed by the electronic structure of the M-O moiety along a continuum from metal-oxo to oxyl radical and O biradical character. Increased oxygen-centered spin density and weakened M-O bonding are identified as descriptors of catalytic activity which correlate with lower activation barriers.

Spectroscopy plays a central role in modern materials science and catalysis by enabling direct insight into electronic structure, oxidation states, coordination environments, and dynamic transformations under reaction conditions

M-edge X-ray absorption spectroscopy (XAS), which probes 3p→3d transitions in first-row transition metals, provides detailed insights into oxidation states, spin-states, and local electronic structure with high element and orbital specificity. Operating in the extreme ultraviolet (XUV) region, this technique provides sharp multiplet-resolved features with high sensitivity to ligand field and covalency effects. Compared to K- and L-edge XAS, M-edge spectra exhibit significantly narrower full widths at half maximum (typically 0.3–0.5 eV versus >1 eV at the L-edge and >1.5–2 eV at the K-edge), owing to longer 3p core-hole lifetimes. M-edge measurements are also more surface-sensitive due to the lower photon energy range, making them particularly well-suited for probing thin films, interfaces, and surface-bound species. The advent of tabletop high-harmonic generation (HHG) sources has enabled femtosecond time-resolved M-edge measurements, allowing direct observation of ultrafast photoinduced processes such as charge transfer and spin crossover dynamics. This review presents an overview of the fundamental principles, experimental advances, and current theoretical approaches for interpreting M-edge spectra. We further discuss a range of applications in catalysis, materials science, and coordination chemistry, highlighting the technique’s growing impact and potential for future studies.

Modeling multimetallic systems efficiently enables faster prediction of desirable chemical properties and the design of new materials. This work describes an initial implementation for performing multireference wave function method localized active-space self-consistent field (LASSCF) calculations through the use of multiple graphics processing units (GPUs) to accelerate time-to-solution. Density fitting is leveraged to reduce memory requirements, and we demonstrate the ability to fully utilize multi-GPU compute nodes. Performance improvements of 5-10x in total application runtime were observed in LASSCF calculations for multimetallic catalyst systems up to 1200 AOs and an active space of (22e,40o) using up to four NVIDIA A100 GPUs. Written with performance portability in mind, a comparable performance is also observed in early runs on the Aurora exascale system using Intel Max Series GPUs.

Modeling multimetallic systems efficiently enables faster prediction of desirable chemical properties and design of new materials. This work describes an initial implementation for performing multireference wave function method localized active space self-consistent field (LASSCF) calculations through the use of multiple graphics processing units (GPUs) to accelerate time-to-solution. Density fitting is leveraged to reduce memory requirements, and we demonstrate the ability to fully utilize multi-GPU compute nodes. Performance improvements of 5-10x in total application runtime were observed in LASSCF calculations for multimetallic catalyst systems up to 1200 AOs and an active space of (22e,40o) using up to four NVIDIA A100 GPUs. Written with performance portability in mind, comparable performance is also observed in early runs on the Aurora exascale system using Intel Max Series GPUs.

A single unpaired electron in an organic molecule residing in the singly occupied molecular orbital (SOMO) renders it an organic radical. It incorporates exchange splitting in the frontier occupied and unoccupied orbitals, separating the α- and β-orbitals. This fact enormously impacts the electron transport properties in organic radicals by promoting spin-polarized current and significantly enhancing conductance compared to their closed-shell counterparts. Exploring these phenomena, several monoradicals have been investigated through molecular spintronic experiments and theories. In this work, we addressed the impact of an increasing number of radical centers on the transport properties of multiradical molecular species by considering di- and triradicals based on a stable Blatter's radical. With an increasing number of radical centers, the number of SOMOs increases. Does the increased number of frontier SOMOs provide larger exchange splitting and better transport properties? Here, we observed that the spatial distributions of SOMOs and their coupling with electrodes play a decisive role compared with the presence of multiple unpaired electrons in the molecular systems.

A single unpaired electron in an organic molecule residing in the singly occupied molecular orbital (SOMO) renders it an organic radical. It incorporates exchange splitting in the frontier occupied and unoccupied orbitals, separating the alpha and beta-orbitals. This fact enormously impacts the electron transport properties in the organic radicals by promoting spin-polarized current and significantly enhanced conductance as compared to their closed shell counterparts. Exploring these phenomena, several mono-radicals have been investigated by molecular spintronic experiments and theories. In this work, we addressed the impact of increasing number of of radical centers on the transport properties of the multi-radical molecular species by considering di- and tri-radicals based on a stable Blatter’s radical. With the increasing number of radical centers, the number of SOMOs increases. Does the increased number of frontier SOMOs provide larger exchange splitting and better transport properties? Here, we have observed that the spatial distributions of SOMOs and their coupling with electrodes play a decisive role compared to the presence of multiple unpaired electrons in the molecular systems.

X-ray absorption near-edge spectroscopy (XANES) is an advanced technique for probing the local electronic structure of catalysts, effectively identifying the noninnocent nature of ligands in transition-metal complexes. Metallocorroles with noninnocent corrole rings exhibit unusual electronic structures that challenge traditional density functional theory (DFT) methods, necessitating more rigorous approaches to describe electron correlation accurately. We explored K-edge XANES spectra of Fe, Mn, and Co metallocorroles using TDDFT and wave function-based methods. This is the first investigation employing multireference methods, specifically RASSCF, RASPT2, and MC-PDFT, to analyze the redox noninnocent nature of metallocorroles reflected in their XANES spectra. We quantified the noninnocent character of the corrole and the oxidation states of the metals, capturing more than singly excited excitations responsible for the pre-edge peak. Our findings demonstrate the importance of these advanced computational techniques for accurately predicting XANES spectra, providing a reliable understanding of the electronic properties of such complexes. This study offers a new strategy for investigating ligand redox noninnocence via integrated experimental and computational XANES.