Andreas Görling

A transparent interpretation of the relativistic contribution to the N.M.R. ‘heavy atom chemical shift’

The N.M.R. chemical shift problem is formulated so that a correct nonrelativistic limit is obtained for the paramagnetic term using Extended Huckel level wavefunctions. Then the relativistic contributions are identified from a REX-EHT comparison in second-order perturbation theory. The results obtained for hydrogen halides, HX, agree with the observed experimental trend. The main mechanisms are the spin-orbit induced proton spin density in the π1/2 MO and the spin-orbit induced Zeeman term in the 3σ MO. The results obtained for 13C shifts in haloacetylenes and methyl halides are qualitatively correct, but too small. Possible reasons are discussed.

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.

Exact Kohn-Sham exchange kernel for insulators and its long-wavelength behavior

We present an exact expression for the frequency-dependent Kohn-Sham exact-exchange (EXX) kernel for periodic insulators, which can be employed for the calculation of electronic response properties within time-dependent (TD) density-functional theory. It is shown that the EXX kernel has a long-wavelength divergence behavior as the exact full exchange-correlation kernel and thus rectifies one serious shortcoming of the adiabatic local-density approximation and generalized gradient approximations kernels. A comparison between the TDEXX and the GW approximation Bethe-Salpeter-equation approach is also made.

Excitation energies of terthiophene and its dioxide derivative: a first-principles study

Abstract The excitation spectra of terthiophene and terthiophene-S,S-dioxide are investigated by time-dependent density-functional theory (TD-DFT) calculated data are compared with recent experimental results. The second singlet–triplet excitation energy (T2) is calculated above the first singlet–singlet (S1) one. In terthiophene-S,S-dioxide the formation of a bonding interaction in the lowest unoccupied molecular orbital decreases its kinetic energy and explains the red-shift of the excitation spectrum. The inter-system crossing (ISC) rate in terthiophene-S,S-dioxide is expected to be lower than in terthiophene due to the increased T2–S1 energy gap, which is also found to not decrease with inter-ring torsion as in terthiophene.

The Kohn–Sham treatment of anions via the localized Hartree–Fock method

Abstract Kohn–Sham calculations of small anions with the localized Hartree–Fock (LHF) method, a recently developed effective exact exchange Kohn–Sham (KS) method, are presented. In contrast to conventional KS methods employing density-functionals from the local density or from generalized gradient approximations, the LHF approach yields bound anions, and thus, is found to be suitable for the treatment of anions. The superiority of the LHF method over conventional KS approaches originates in the fact that the former, in contrast to the latter, is free of Coulomb self-interaction. This is demonstrated by a discussion of the involved exchange and effective KS potentials.

Orbitals from a self-interaction free Kohn-Sham potential as a single electron basis for ab initio methods

Abstract In the present work we study how well orbitals obtained from either an effective exact exchange Kohn–Sham approach or from a BLYP computation are suited as basis functions for ab initio approaches. These orbitals are compared with CASSCF and canonical Hartree–Fock orbitals. The usefulness of the various orbitals is discussed by means of the effort needed to push the error of a MR-CI computation below a given threshold. We consider as a test case the excited states of ethene and find that orbitals from the effective exact exchange Kohn–Sham approach perform very well.

The effects of oxygenation on the optical properties of dimethyl-dithienothiophenes: Comparison between experiments and first-principles calculations

Modifications of the optical properties of dimethyl-dithienothiophenes due to the oxygen functionalization of the central sulfur atom are investigated. We have measured the absorption, photoluminescence (PL) and PL excitation spectra, the PL quantum efficiencies, and the PL decay times. These experimental results are interpreted and compared with first-principles time-dependent density-functional theory calculations, which predict, for the considered systems, excitation and emission energies with an accuracy of 0.1 eV. It is found that the oxygenation strongly changes optical and photophysical properties. These effects are related to the modifications of the energetically lowest-unoccupied molecular orbital and the energetically second highest occupied one, which change the relative position of the two lowest singlet and triplet excited states.

Norm-conserving pseudopotentials in the exact-exchange Kohn-Sham formalism

We present a rigorous approach for constructing norm-conserving pseudopotentials within the exact-exchange (EXX) Kohn-Sham density functional formalism. The approach is based on the relativistic EXX scheme within the framework of the optimized potential method. We derive an integral equation that yields the exact local Kohn-Sham exchange potential due to valence electrons. This valence exchange potential is used for constructing ionic pseudopotentials and replaces the standard procedure of unscreening atomic pseudopotentials that is shown to be not applicable in the EXX formalism. The EXX pseudopotentials excellently reproduce the experimental atomic ionization energies and they show a better transferability than the standard pseudopotentials based on the local density approximation (LDA). The relativistic EXX pseudopotentials are shown to lead to the correct ordering of conduction band minima in diamond-structure Ge and reduce the LDA errors in the energetic positions of d bands in solids.

Optical properties of functionalized thiophenes: a theoretical and experimental study

Abstract The optical properties of ter-thiophene, diphenyl-thiophene, dithieno-thiophene and their analogues with the central sulphur atom functionalized with two oxygen atoms are investigated by first-principles time-dependent density-functional theory: absorption and emission energies and singlet–triplet energy-gaps are reported and compared with experimental results. Photoluminescence quantum yield measurements in solvents of different viscosity for the functionalized systems are reported. The non-radiative decay channels are discussed.

Analysis of the Quality of Kohn–Sham Orbitals for Subsequent MRSD-CI Calculations of Excitation Energies

Abstract In the present work we study the properties of Kohn–Sham orbitals used in subsequent CI methods for the computation of electronic excitation energies in terms of orbital relaxation, static and dynamical correlation effects. As model systems, we use the water and the ethene molecule. As sets of orbitals, we use orbitals from an effective exact exchange Kohn–Sham method, the localized Hartree–Fock (LHF) method, as well as BLYP and HF orbitals. Our study shows that the use of LHF orbitals leads to negligible static correlation effects and thus facilitates assignments of excitation spectra, but it also shows that the remaining correlation contributions have to be taken into account carefully since error cancellation is only found in some fortitious cases. The comparison of LHF and BLYP orbitals shows that the BLYP orbitals are less suited for the description of the cationic system and as a consequence for the computation of Rydberg states.