136 research outputs found
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 O<sub>2</sub> and N<sub>2</sub> 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 O<sub>2</sub> and N<sub>2</sub> 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
O<sub>2</sub> binding in terms of redox activity with the metalated
catecholate. O<sub>2</sub> is predicted to bind more strongly than
N<sub>2</sub> for all cases except Cu<sup>2+</sup>, with general agreement
in the binding trends among all methods
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 H<sub>2</sub> and N<sub>2</sub> 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
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