MOLCAS/OpenMolcas is an ab initio electronic structure program providing a large set of computational methods from Hartree–Fock and density functional theory to various implementations of multiconfigurational theory. This article provides a comprehensive overview of the main features of the code, specifically reviewing the use of the code in previously reported chemical applications as well as more recent applications including the calculation of magnetic properties from optimized density matrix renormalization group wave functions.

Relativistic multireference ab initio wave function calculations within the restricted active space (RAS) framework were performed to calculate metal and ligand X-ray absorption (XAS) near-edge spectroscopy (XANES) intensities for the metal M edges of [PuO(HO)], [AnO] (An = U, Np, Pu), and [AmCl] and the Cl K edge of the Am complex. The extent of An(5f)-ligand bonding was determined via natural localized molecular orbital analyses of the relevant spin-orbit coupled multireference states. The calculated spectra are in good agreement with experiments and allow a detailed assignment of the observed spectral features. The XANES M-edge spectra are representative of the actinide orbital covalency in the probed core-excited states, which may be different from the ground-state covalency. An assignment of ground-state An orbital covalency based on XAS spectra should therefore be made with caution.

Unsupported non-bridged uranium–carbon double bonds have long been sought after in actinide chemistry as fundamental synthetic targets in the study of actinide-ligand multiple bonding. Here we report that, utilizing Ih(7)-C80 fullerenes as nanocontainers, a diuranium carbide cluster, U=C=U, has been encapsulated and stabilized in the form of UCU@Ih(7)-C80. This endohedral fullerene was prepared utilizing the Krätschmer–Huffman arc discharge method, and was then co-crystallized with nickel(II) octaethylporphyrin (NiII-OEP) to produce UCU@Ih(7)-C80·[NiII-OEP] as single crystals. X-ray diffraction analysis reveals a cage-stabilized, carbide-bridged, bent UCU cluster with unexpectedly short uranium–carbon distances (2.03 Å) indicative of covalent U=C double-bond character. The quantum-chemical results suggest that both U atoms in the UCU unit have formal oxidation state of +5. The structural features of UCU@Ih(7)-C80 and the covalent nature of the U(f1)=C double bonds were further affirmed through various spectroscopic and theoretical analyses.

The first isolable molecular thorium nitride, [(NR2)3Th(μ-N)Th(NR2)3]−, was synthesized by reaction of [Th{N(R)(SiMe2)CH2}(NR2)2] with NaNH2 and characterized by X-ray crystallography, 15N NMR spectroscopy, and DFT calculations.

In benzophenone, intersystem crossing occurs efficiently between the S1(nπ*) state and the T1 state of dominant nπ* character, leading to excited triplet states after photoexcitation. The transition mechanism between S1(nπ*) and T1 is still a matter of debate, despite several experimental studies. Quantum mechanical calculations have been performed in order to assess the relative efficiencies of previously proposed mechanisms, in particular, the direct S1→ T1 and indirect S1→ T2(ππ*) → T1 ones. Multiconfigurational wave function based methods are used to discuss the nature of the relevant states and also to determine minimum energy paths and conical intersections. It is found that the T1 state has a mixed nπ*/ππ* character and that the T2(ππ*) state acts as an intermediate state between the S1 and T1 states. This result is in line with recent experiments, which suggested a two-step kinetic model to populate the phosphorescent state after photoexcitation [Aloïse et al., J. Phys. Chem. A, 2008, 112, 224-231].

Molecular cerium complexes are a new class of tunable and energy-efficient visible- and UV-luminophores. Understanding and controlling the emission brightness and color are important for tailoring them for new and specialized applications. Herein, we describe the experimental and computational analyses for series of tris(guanidinate) (1-8, Ce{(RN)C(N Pr)}, R = alkyl, silyl, or phenyl groups), guanidinate-amide [GA, A = N(SiMe), G = (MeSi)NC(N Pr)], and guanidinate-aryloxide (GOAr, OAr = 2,6-di- tert-butylphenoxide) cerium(III) complexes to understand and develop predictive capabilities for their optical properties. Structural studies performed on complexes 1-8 revealed marked differences in the steric encumbrance around the cerium center induced by various guanidinate ligand backbone substituents, a property that was correlated to photoluminescent quantum yield. Computational studies revealed that consecutive replacements of the amide and aryloxide ligands by guanidinate ligand led to less nonradiative relaxation of bright excited states and smaller Stokes shifts. The results establish a comprehensive structure-luminescence model for molecular cerium(III) luminophores in terms of both quantum yields and colors. The results provide a clear basis for the design of tunable, molecular, cerium-based, luminescent materials.

It is generally assumed that astatide (At(-) ) is the predominant astatine species in basic aqueous media. This assumption is questioned in non-complexing and non-reductive aqueous solutions by means of high-pressure anion-exchange chromatography. Contrary to what is usually believed, astatide is found to be a minor species at pH=11. A different species, which also bears a single negative charge, becomes predominant when the pH is increased beyond 7. Using competition experiments, an equilibrium constant value of 10(-6.9) has been determined for the formation of this species from AtO(OH) with the exchange of one proton. The identification of this species, AtO(OH)2 (-) , is achieved through relativistic quantum mechanical calculations, which rule out the significant formation of the AtO2 (-) species, while leading to a hydrolysis constant of AtO(OH) in excellent agreement with experiment when the AtO(OH)2 (-) species is considered. Beyond the completion of the Pourbaix diagram of astatine, this new information is of interest for the development of (211) At radiolabeling protocols.

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