2,345 research outputs found
Optimized Unrestricted Kohn-Sham Potentials from Ab Initio Spin Densities
The reconstruction of the exchange-correlation potential from accurate ab
initio electron densities can provide insights into the limitations of the
currently available approximate functionals and provide guidance for devising
improved approximations for density-functional theory (DFT). For open-shell
systems, the spin density is introduced as an additional fundamental variable
in Spin-DFT. Here, we consider the reconstruction of the corresponding
unrestricted Kohn-Sham potentials from accurate ab initio spin densities. In
particular, we investigate whether it is possible to reconstruct the spin
exchange-correlation potential, which determines the spin density in
spin-unrestricted Kohn-Sham-DFT, despite the numerical difficulties inherent to
the optimization of potentials with finite orbital basis sets. We find that the
recently developed scheme for unambiguously singling out an optimal optimized
potential [J. Chem. Phys. 135, 244102 (2011)] can provide such spin potentials
accurately. This is demonstrated for two test cases, the lithium atom and the
dioxygen molecule, and target (spin) densities from Full-CI and CASSCF
calculations, respectively
, Calculation of nuclear magnetic resonance shieldings using frozen density embedding
We have extended the frozen-density embedding (FDE) scheme within density-functional theory [T. A. Wesolowski and A. Warshel, J. Phys. Chem. 97, 8050 (1993)] to include external magnetic fields and applied this extension to the nonrelativistic calculation of nuclear magnetic resonance (NMR) shieldings. This leads to a formulation in which the electron density and the induced current are calculated separately for the individual subsystems. If the current dependence of the exchange-correlation functional and of the nonadditive kinetic-energy functional are neglected, the induced currents in the subsystems are not coupled and each of them can be determined without knowledge of the induced current in the other subsystem. This allows the calculation of the NMR shielding as a sum of contributions of the individual subsystems. As a test application, we have calculated the solvent shifts of the nitrogen shielding of acetonitrile for different solvents using small geometry-optimized clusters consisting of acetonitrile and one solvent molecule. By comparing to the solvent shifts obtained from supermolecular calculations we assess the accuracy of the solvent shifts obtained from FDE calculations. We find a good agreement between supermolecular and FDE calculations for different solvents. In most cases it is possible to neglect the contribution of the induced current in the solvent subsystem to the NMR shielding, but it has to be considered for aromatic solvents. We demonstrate that FDE can describe the effect of induced currents in the environment accurately. © 2006 American Institute of Physics
Computational Insights into the Mechanism of the Selective Catalytic Reduction of NOx: Fe- versus Cu-Exchanged Zeolite Catalysts
We computationally investigate the mechanism of the reduction half-cycle of the selective catalytic reduction of nitrogen oxides with ammonia. We compare both Fe- and Cu-exchanged zeolite catalysts and aim at exploring all accessible reaction pathways. From our calculations, a comprehensive picture emerges that unifies several previous mechanistic proposals. We find that both for Fe and for Cu catalysts different reaction pathways are feasible but some of the possible reaction pathways differ in these two cases. Our computational results provide a basis for the interpretation of in situ spectroscopic investigations that can possibly distinguish the different mechanistic pathways
Electrocatalytic Activation of Donor–Acceptor Cyclopropanes and Cyclobutanes: An Alternative C(sp 3 )−C(sp 3 ) Cleavage Mode
We describe the first electrochemical activation of D–A cyclopropanes and D–A cyclobutanes leading after C(sp3)−C(sp3) cleavage to the formation of highly reactive radical cations. This concept is utilized to formally insert molecular oxygen after direct or DDQ-assisted anodic oxidation of the strained carbocycles, delivering β- and γ-hydroxy ketones and 1,2-dioxanes electrocatalytically. Furthermore, insights into the mechanism of the oxidative process, obtained experimentally and by additional quantum-chemical calculations are presented. The synthetic potential of the reaction products is demonstrated by diverse derivatizations
Environmental effects with Frozen Density Embedding in Real-Time Time-Dependent Density Functional Theory using localized basis functions
Frozen Density Embedding (FDE) represents a versatile embedding scheme to
describe the environmental effect on the electron dynamics in molecular
systems. The extension of the general theory of FDE to the real-time
time-dependent Kohn-Sham method has previously been presented and implemented
in plane-waves and periodic boundary conditions (Pavanello et al. J. Chem.
Phys. 142, 154116, 2015). In the current paper, we extend our recent
formulation of real-time time-dependent Kohn-Sham method based on localized
basis set functions and developed within the Psi4NumPy framework (De Santis et
al. J. Chem. Theory Comput. 2020, 16, 2410) to the FDE scheme. The latter has
been implemented in its "uncoupled" flavor (in which the time evolution is only
carried out for the active subsystem, while the environment subsystems remain
at their ground state), using and adapting the FDE implementation already
available in the PyEmbed module of the scripting framework PyADF. The
implementation was facilitated by the fact that both Psi4NumPy and PyADF, being
native Python API, provided an ideal framework of development using the Python
advantages in terms of code readability and reusability. We demonstrate that
the inclusion of the FDE potential does not introduce any numerical instability
in time propagation of the density matrix of the active subsystem and in the
limit of weak external field, the numerical results for low-lying transition
energies are consistent with those obtained using the reference FDE
calculations based on the linear response TDDFT. The method is found to give
stable numerical results also in the presence of strong external field inducing
non-linear effects
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