Biomedical subjects

Andreas Görling

Publications and source records attributed to Andreas Görling.

9 recordsLinked to original sources

PubMed2006

Exact-exchange spin-current density-functional theory.

A spin-current density-functional theory (SCDFT) is introduced, which takes into account the currents of the spin density and thus currents of the magnetization in addition to the electron density, the noncollinear spin density, and the density current, which are considered in standard current-spin-density-functional theory. An exact-exchange Kohn-Sham formalism based on SCDFT is presented, which represents a general framework for the treatment of magnetic and spin properties. As an illustration, an oxygen atom in a magnetic field is treated with the new approach.

Journal Article↗

PubMed2006

Hydrazine nitrosation of a metal-bound nitric oxide: structural evidence for the formation of an ammine complex.

Hydrazine nitrosation of [Ru(NO)(py(si)S4)]Br.THF (1) (py(si)S4(2-) = 2,6-bis(3-triphenylsilyl-2-sulfanylphenylthiomethyl)pyridine2-) in methanol/DMF led to the formation of mononuclear ammine complex [Ru(NH3)(py(si)S4)] (2) and N2O, whereas the reaction performed in THF/CH2Cl2/toluene afforded thioether-bridged dinuclear ammine complex [(NH3)Ru(mu-py(si)S4)Ru(py(si)S4)] (3). Compound 2 dimerizes in solution at room temperature to form 3 and is regenerated upon treatment of 3 with NH3. A plausible mechanism for the hydrazine nitrosation of 1 has been proposed. The reaction of 1 with NH3 or N3- does not lead to a nucleophilic attack at the NO+ ligand but to a deprotonation that yields neutral nitrosyl complex [Ru(NO){py(si)S4(H+)}] (4), which is supported by density functional theory calculations.

Amines↗

PubMed2006

Rydberg states with quantum Monte Carlo.

Calculations on Rydberg states are performed using quantum Monte Carlo methods. Excitation energies and singlet-triplet splittings are calculated for two model systems, the carbon atom (3P and 1P) and carbon monoxide ((1Sigma and 3Sigma). Kohn-Sham wave functions constructed from open-shell localized Hartree-Fock orbitals are used as trial and guide functions. The fixed-node diffusion quantum Monte Carlo (FN-DMC) method depends strongly on the wave function's nodal hypersurface. Nodal artefacts are investigated for the ground state of the carbon atom. Their effect on the FN-DMC results can be analyzed quantitatively. FN-DMC leads to accurate excitation energies but to less accurate singlet-triplet splittings. Variational Monte Carlo calculations are able to reproduce the experimental results for both the excitation energies and the singlet-triplet splittings.

Journal Article↗

PubMed2005

Orbital- and state-dependent functionals in density-functional theory.

Shortcomings of present density-functional methods are considered. Kohn-Sham and time-dependent density-functional methods using orbital- and state-dependent functionals for exchange-correlation energies, potentials, and kernels are discussed as possible remedy for some of these shortcomings. A view on the Kohn-Sham formalism is presented which differs somewhat from the one conventionally taken. The crucial step of constructing local multiplicative exchange-correlation potentials in Kohn-Sham methods based on orbital- and state-dependent functionals is discussed. The description of open-shell systems via a symmetrized Kohn-Sham formalism employing state-dependent exchange-correlation functionals is elucidated. The generalized adiabatic connection Kohn-Sham approach for the self-consistent treatment of excited states within a density-functional framework is considered. In the latter approach orbital- and state-dependent exchange-correlation functionals occur in a density-functional framework which is no longer based on the Hohenberg-Kohn theorem but on a more general relation between electron densities and local multiplicative potentials.

Journal Article↗

PubMed2005

Open-shell localized Hartree-Fock method based on the generalized adiabatic connection Kohn-Sham formalism for a self-consistent treatment of excited states.

An effective exact-exchange Kohn-Sham approach for the treatment of excited electronic states, the generalized adiabatic connection open-shell localized Hartree-Fock (GAC-OSLHF) method is presented. The GAC-OSLHF method is based on the generalized adiabatic connection Kohn-Sham formalism and therefore capable of treating excited electronic states, which are not the energetically lowest of their symmetry. The method is self-interaction free and allows for a fully self-consistent computation of excited valence as well as Rydberg states. Results for atoms and small- and medium-size molecules are presented and compared to restricted open-shell Hartree-Fock (ROHF) and time-dependent density-functional results as well as to experimental data. While GAC-OSLHF and ROHF results are quite close to each other, the GAC-OSLHF method shows a much better convergence behavior. Moreover, the GAC-OSLHF method as a Kohn-Sham method, in contrast to the ROHF approach, represents a framework which allows also for a treatment of correlation besides an exchange by appropriate functionals. In contrast to the common time-dependent density-functional methods, the GAC-OSLHF approach is capable of treating doubly or multiply excited states and can be easily applied to molecules with an open-shell ground state. On the nodal planes of the energetically highest occupied orbital, the local multiplicative GAC-OSLHF exchange potential asymptotically approaches a different, i.e., nonzero, value than in other regions, an asymptotic behavior which is known from exact Kohn-Sham exchange potentials of ground states of molecules.

Journal Article↗

PubMed2005

Density functional calculation of the electronic circular dichroism spectra of the transition metal complexes [M(phen)3]2+ (M = Fe, Ru, Os).

The circular dichroism spectra of the tris-bidentate metal complexes Lambda-[M(phen)3]2+, with M = Fe, Ru, Os and phen = 1,10-tris-phenanthroline, are investigated computationally, employing time-dependent density functional theory. Good agreement with experimental spectra is obtained for Ru and Os. The Lambda-[Os(phen)3]2+ spectrum is analyzed in detail. It is shown how relativistic effects red shift CD bands where the Os 5d-orbital participates to a large extent in the excitations. Further, the participation of the metal in the ligand pi --> pi exciton CD is determined to be of the order of 10%. Though solvent effects can have a noticeable effect on individual transitions and rotatory strengths, they are demonstrated to have only a very small overall effect on the resulting simulated CD spectra. For Lambda-[Fe(phen)3]2+, the results are shown to be rather sensitive to the choice of the applied hybrid and nonhybrid density functionals, and the optimized geometries based thereupon. In particular, the sign pattern of the lower-energy part (up to 33 x 10(3) cm(-1)) of the Lambda-[Fe(phen)3]2+ CD spectrum is difficult to reproduce. Some combinations of functionals and geometries yield good agreement with experiment, but no "best" approach can be devised based on the available results. Possible sources of errors in the spectrum of Lambda-[Fe(phen)3]2+ due to deficiencies in the functionals and the exchange-correlation kernels are investigated.

Circular Dichroism↗

PubMed2004

Quantum and thermal fluctuation effects on the photoabsorption spectra of clusters.

A first principles approach for the calculation of photoabsorption cross sections of clusters and molecules is presented which includes both quantum and thermal fluctuations. The method relies on an ab initio path integral representation of the nuclear quantum motion in conjunction with time-dependent density-functional theory for electronic excitations. It is shown that quantum fluctuations of cold lithium clusters, such as Li8 below 50 K, affect significantly their photoabsorption spectra.

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PubMed2002

Excitonic optical spectrum of semiconductors obtained by time-dependent density-functional theory with the exact-exchange kernel.

Applying a novel exact-exchange (EXX) approach within time-dependent density-functional theory, we obtained the optical absorption spectrum of bulk silicon in good agreement with experiments including excitonic features. Analysis of the EXX kernel shows that inclusion of the Coulomb coupling of electron-hole pairs and the correct long-wavelength behavior in the kernel is crucial for the proper description of excitonic effects in semiconductors.

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PubMed2002

Asymptotic behavior of the Kohn-Sham exchange potential.

The Kohn-Sham exchange potential of finite systems is shown to approach different asymptotic limits on nodal surfaces of the energetically highest-occupied orbital than in other regions. This leads to barrier-well structures in the near asymptotic region, which have a strong influence on virtual orbitals and thus on excitation energies. Common approximations for the exchange potential do not exhibit these features. These asymptotic structures, however, can be correctly described by effective exact-exchange methods. Conditions for the presence of an asymptotic barrier well in the full exchange-correlation potential are discussed.

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