Theoretical Investigation of Plutonium-Based Single-Molecule Magnets

The electronic structure of a plutonium-based single-molecule magnet (SMM) was theoretically examined by means of multiconfigurational electronic structure theory calculations, including spin–orbit coupling effects. All Pu 5f to 5f transitions for all possible spin states were computed, as well as ligand to metal charge transfer and Pu 5f to 6d transitions. Spin–orbit coupling effects were included <i>a posteriori</i> to accurately describe the electronic transitions. The spin–orbit coupled energies and magnetic moments were then used to compute the magnetic susceptibility curves. The experimental electronic structure and magnetic susceptibility curve were reproduced well by our calculations. A compound with a modified electron-donating ligand (namely a carbene ligand) was also investigated in an attempt to tune the electronic properties of the plutonium SMM, revealing a higher ligand field splitting of the 5f orbitals of Pu, which could in turn enhance the barrier against magnetic relaxation

Air Separation by Catechol-Ligated Transition Metals: A Quantum Chemical Screening

The separation of O₂ and N₂ from air is of great importance in a variety of industrial contexts, but the primary means of accomplishing the separation is cryogenic distillation, an energy-intensive process. A material that could enable air separation to occur at conventional temperatures would be of great economic and environmental benefit. Metalated catecholates within metal–organic frameworks have been considered for other gas separations and are shown here to have significant potential for air separation. Calculations of interaction energies between catecholates with first-row transition metals and guests O₂ and N₂ were performed using density functional theory and multireference complete active space self-consistent field followed by second-order perturbation theory. A general recipe is offered for active space selection for metalated catecholate systems. The multireference results are used to rationalize O₂ binding in terms of redox activity with the metalated catecholate. O₂ is predicted to bind more strongly than N₂ for all cases except Cu²⁺, with general agreement in the binding trends among all methods

A Tribute to Vincenzo Barone

A Tribute to Vincenzo Baron

Intramolecular Charge Transfer and Local Excitation in Organic Fluorescent Photoredox Catalysts Explained by RASCI-PDFT

We investigate the electronically excited states of two recently synthesized organic fluorescent photoredox catalysts of the dihydrophenazine family. The mixed charge transfer and local excitation behavior of dark and bright transitions is unveiled by multiconfiguration pair-density functional theory (MC-PDFT) based on a restricted active space configuration interaction (RASCI) wave function (RASCI-PDFT). The RASCI-PDFT calculations give an accurate description of the experimental optical absorption spectra with active spaces too large for conventional complete active space self-consistent-field calculations. These results were achieved by the inclusion of many valence orbitals in the active space and their optimization within a cost-effective restricted active space self-consistent field framework without a RAS2 subspace, followed by calculations at the RASCI level including orbitals in RAS2. This novel strategy can be extended to systems that need a large number of orbitals in the active space

Intramolecular Charge Transfer and Local Excitation in Organic Fluorescent Photoredox Catalysts Explained by RASCI-PDFT

We investigate the electronically excited states of two recently synthesized organic fluorescent photoredox catalysts of the dihydrophenazine family. The mixed charge transfer and local excitation behavior of dark and bright transitions is unveiled by multiconfiguration pair-density functional theory (MC-PDFT) based on a restricted active space configuration interaction (RASCI) wave function (RASCI-PDFT). The RASCI-PDFT calculations give an accurate description of the experimental optical absorption spectra with active spaces too large for conventional complete active space self-consistent-field calculations. These results were achieved by the inclusion of many valence orbitals in the active space and their optimization within a cost-effective restricted active space self-consistent field framework without a RAS2 subspace, followed by calculations at the RASCI level including orbitals in RAS2. This novel strategy can be extended to systems that need a large number of orbitals in the active space

Multiconfiguration Pair-Density Functional Theory and Complete Active Space Second Order Perturbation Theory. Bond Dissociation Energies of FeC, NiC, FeS, NiS, FeSe, and NiSe

We investigate the performance of multiconfiguration pair-density functional theory (MC-PDFT) and complete active space second-order perturbation theory for computing the bond dissociation energies of the diatomic molecules FeC, NiC, FeS, NiS, FeSe, and NiSe, for which accurate experimental data have become recently available [Matthew, D. J.; Tieu, E.; Morse, M. D. <i>J. Chem. Phys</i>. <b>2017</b>, 146, 144310–144320]. We use three correlated participating orbital (CPO) schemes (nominal, moderate, and extended) to define the active spaces, and we consider both the complete active space (CAS) and the separated-pair (SP) schemes to specify the configurations included for a given active space. We found that the moderate SP-PDFT scheme with the tPBE on-top density functional has the smallest mean unsigned error (MUE) of the methods considered. This level of theory provides a balanced treatment of the static and dynamic correlation energies for the studied systems. This is encouraging because the method is low in cost even for much more complicated systems

Intramolecular Charge Transfer and Local Excitation in Organic Fluorescent Photoredox Catalysts Explained by RASCI-PDFT

We investigate the electronically excited states of two recently synthesized organic fluorescent photoredox catalysts of the dihydrophenazine family. The mixed charge transfer and local excitation behavior of dark and bright transitions is unveiled by multiconfiguration pair-density functional theory (MC-PDFT) based on a restricted active space configuration interaction (RASCI) wave function (RASCI-PDFT). The RASCI-PDFT calculations give an accurate description of the experimental optical absorption spectra with active spaces too large for conventional complete active space self-consistent-field calculations. These results were achieved by the inclusion of many valence orbitals in the active space and their optimization within a cost-effective restricted active space self-consistent field framework without a RAS2 subspace, followed by calculations at the RASCI level including orbitals in RAS2. This novel strategy can be extended to systems that need a large number of orbitals in the active space

Active Space Dependence in Multiconfiguration Pair-Density Functional Theory

In multiconfiguration pair-density functional theory (MC-PDFT), multiconfiguration self-consistent-field calculations and on-top density functionals are combined to describe both static and dynamic correlation. Here, we investigate how the MC-PDFT total energy and its components depend on the active space choice in the case of the H₂ and N₂ molecules. The active space dependence of the on-top pair density, the total density, the ratio of on-top pair density to half the square of the electron density, and the satisfaction of the virial theorem are also explored. We find that the density and on-top pair density do not change significantly with changes in the active space. However, the on-top ratio does change significantly with respect to active space change, and this affects the on-top energy. This study provides a foundation for designing on-top density functionals and automatizing the active space choice in MC-PDFT

Correction to “Multiconfiguration Pair-Density Functional Theory Spectral Calculations Are Stable to Adding Diffuse Basis Functions”

Correction to “Multiconfiguration Pair-Density Functional Theory Spectral Calculations Are Stable to Adding Diffuse Basis Functions

Self-Interaction Error in Density Functional Theory: An Appraisal

Self-interaction error (SIE) is considered to be one of the major sources of error in most approximate exchange-correlation functionals for Kohn–Sham density-functional theory (KS-DFT), and it is large with all local exchange-correlation functionals and with some hybrid functionals. In this work, we consider systems conventionally considered to be dominated by SIE. For these systems, we demonstrate that by using multiconfiguration pair-density functional theory (MC-PDFT), the error of a translated local density-functional approximation is significantly reduced (by a factor of 3) when using an MCSCF density and on-top density, as compared to using KS-DFT with the parent functional; the error in MC-PDFT with local on-top functionals is even lower than the error in some popular KS-DFT hybrid functionals. Density-functional theory, either in MC-PDFT form with local on-top functionals or in KS-DFT form with some functionals having 50% or more nonlocal exchange, has smaller errors for SIE-prone systems than does CASSCF, which has no SIE