Olav Vahtras

Response Theory and Calculations of Spin-Orbit Coupling Phenomena in Molecules

We review response theory and calculations of molecular properties involving spin-orbit interactions. The spin-orbit coupling is evaluated for reference states described by single- or multi-configuration self-consistent field wave functions. The calculations of spin-orbit related properties rest on the formalism of linear and quadratic response functions for singlet and triplet perturbations when no permutational symmetry in the two-electron operators is assumed and from which various triplet as well as singlet response properties are derived. The spin-orbit coupling matrix elements between singlet and triplet states are evaluated as residues of (multi-configuration) linear response functions, and are therefore automatically determined between orthogonal and non-interacting states. Spin-forbidden radiative transition intensities and lifetimes are determined from the spin-orbit coupling induced dipole transitions between two electronic states of different multiplicity and are obtained as residues of quadratic response functions. The potential of the theory and its range of applications is illustrated by a selection of recent investigations covering different molecular phenomena. The applications include second-order energy contributions, intensity rearrangement in electron spectra, calculation of predissociative lifetimes of dicationic states, assignment of triplet bands in absorption spectra, intersystem crossings and reactivity, external heavy atom effects on S-T transitions, phosphorescence spectra and radiative lifetimes of triplet states. We give an outlook on spin-orbit interaction induced phenomena in extended systems and on applications to general spin catalysis phenomena.

Ab initio calculations of electronic g-factors by means of multiconfiguration response theory

Abstract A response theory algorithm is derived for calculations of the second-order contribution to the electronic g -tensor, that is the combined perturbation of the orbital Zeeman effect and spin–orbit coupling, for single- and multiconfiguration reference states. Sample calculations are carried out for the NH and O 2 molecules, comparing with experimental data and testing different parametrizations of the wavefunctions.

Non-linear electric and magnetic properties obtained from cubic response functions in the random phase approximation

We derive cubic response functions in the Random Phase Approximation for calculations of non-linear frequency-dependent properties of molecules, and demonstrate an implementation of these functions ...

Magnetic phosphorescence of molecular oxygen. A study of the b1Σg+-X3Σg− transition probability using multiconfiguration response theory

Abstract Multiconfigurational response function calculations of the magnetic dipole transition moment for the red atmospheric emission band b 1 Σ g + -X 3 Σ g − have been performed at different internuclear distance. Spin and orbital angular momentum contributions are calculated as residues of the linear and quadratic response functions using the full Breit-Pauli spin-orbit coupling operator. Vibronic band intensities are calculated by vibrational averaging of the magnetic dipole transition moments and compared with experimental data. The Einstein coefficient for the 0,0 transition probability (0.079 s − ) is obtained 10% smaller than the most recent experimental result of Ritter and Wilkerson (0.089 s − ) but in fair agreement with previous experimental data (0.077–0.0825 s −1 ), while an excellent agreement for all other known vibronic transition probabilities for the red atmospheric band is obtained.

Cubic response functions in the random phase approximation

Cubic response functions in the random phase approximation have been derived and their use for computations of static and dynamic second hyperpolarizabilities is demonstrated. The performance of im ...

SUBSTITUTED BENZENES AS BUILDING BLOCKS IN NEAR-EDGE X-RAY ABSORPTION SPECTRA

Abstract Calculations of NEXAFS (Near-Edge X-ray Absorption Fine Structure) spectra of fluorine substituted benzenes, C 6 H 6− n F n , n = 1–6, and of aniline and phenol have been carried out for the purpose of elucidating systematic trends of these spectra on number, position and strength of substituents. It is found that the differential and average shifts of π ∗ NEXAFS spectra for the unconnected carbon atoms are very small, which make these spectra excellent fingerprints for a benzene-ring building block. The excitation energy for the connected carbon π ∗ spectra fingerprints the substitution strength and the relative intensity fingerprints the multiplicity of substitution, while the positional dependency of either the connected or the unconnected carbon spectra is too small to fingerprint the isomers. Previous assignments of the spectra are critically examined and in some cases revised. critically examined and in some cases revised.

CORRELATED RESPONSE CALCULATIONS OF THE SPIN-ORBIT INTERACTION CONTRIBUTION TO NUCLEAR SPIN-SPIN COUPLINGS

We present ab initio perturbation calculations of the electronic spin–orbit interaction contributions to indirect spin–spin coupling tensors using linear and quadratic response methods. The effects of both the Fermi‐contact and the spin–dipole interactions as well as the one‐ and two‐electron spin–orbit interactions are considered for the 1JHX and 2JHH couplings in the H2X (X=O, S, Se, and Te) and HX (X=F, Cl, Br, and I) systems using nonrelativistic multiconfiguration self‐consistent‐field reference states. Novel second‐order spin–orbit‐induced contributions to the J tensors due to the magnetic‐field dependence of the spin–orbit Hamiltonian are introduced and formulated as linear response functions. Calculations of these contributions are reported for the 2JHH coupling tensors in the H2X systems. ©1999 John Wiley & Sons, Inc. J Comput Chem 20: 1314–1327, 1999

Linear response calculations of electronic g-factors and spin-rotational coupling constants for diatomic molecules with a triplet ground state

Electronic g-factors for ESR spectra of a number of diatomic molecules with a ground X3I£- state and their electronic spin-rotational coupling constants have been calculated by a linear response me ...

Some recent developments of high-order response theory

We review some recent developments of high-order response theory and illustrate the utility of this theory for a selection of nonlinear properties.

THEORETICAL STUDY OF X-RAY CIRCULAR DICHROISM OF AMINO ACIDS

Abstract Ordinary and rotatory X-ray absorption intensities are computed for chiral amino acids (alanine, cysteine, serine and valine) in order to explore in what way near-edge X-ray absorption and X-ray circular dichrosim (CD) can fingerprint such compounds. It is predicted that ordinary X-ray absorption spectra are quite alike for the different compounds, which is in line with that they contain similar building blocks, of which only one is changed by substitution. X-ray CD spectra are more sensitive and pose better prospects to be used as fingerprints. This seems to hold especially for spectra of unique atoms, like the nitrogen spectra of amino acids, while spectra with contributions from several chemically shifted non-unique atoms, like the carbon atoms, may appear too scrambled to be useful for experiments at moderate resolution.