Odd Gropen

A mean-field spin-orbit method applicable to correlated wavefunctions

Abstract Starting from the full microscopic Breit-Pauli or no-pair spin-orbit Hamiltonians, we have devised an effective one-electron spin-orbit Hamiltonian in a well defined series of approximations by averaging the two-electron contributions to the spin-orbit matrix element over the valence shell. In addition the two-electron integrals were restricted to comprise only one-centre terms. The validity of these approximations has been tested on several palladium containing compounds. Excellent agreement of the matrix elements of the mean-field operator with corresponding full results is observed; deviations amount to a few cm −1 in absolute value or at most 0.2% on a relative scale. The newly defined mean-field operator can thus safely be employed to evaluate spin-orbit effects in transition metal containing compounds.

Effective core potential calculations using frozen orbitals. Applications to transition metals

Abstract The effective core potential (ECP) method is modified to include frozen orbitals in order to improve the description of the outer core-valence interactions. Applications are made to the Sc, Ni and Pd atoms and several compounds containing these. It is shown that the new ECP resolves the earlier problems concerning the bond distance in ScO( 2 Σ − ) and the rotational barrier in [NiH 4 ] 2− . At the MC and CI levels the 2 Δ and 2 Π states of NiH are accurately described using the new type of ECP.

Principles of direct 4-component relativistic SCF: application to caesium auride

A theory of 4-component direct SCF calculations is presented using a quaternion formulation of the Dirac–Fock equations. Screening of integral batches is supplemented with screening on individual integrals and with separate screening of Coulomb and exchange contributions to the Fock matrix. The direct SCF method is applied to study bonding in caesium auride. The caesium–gold bond is found to be highly ionic, with gold present as the anion. A relativistic bond contraction of 41 pm is observed.

Effective core potential parameters for first- and second-row atoms

An improved effective core potential (ECP) technique is described and used to give ECP parameters for the atoms of the first two rows of the periodic table. A given basis set is parametrized which allows for a direct comparison with all‐electron calculations. Extensive test calculations on first‐ and second‐row molecules using the ECP have been performed, giving excellent agreement with the all‐electron results at the SCF, CASSCF, and CI levels of treatment. Correlating and diffuse functions may be added without modifying the ECP parameters. The present ECP descriptions result in CPU time reductions of the order of 50% in addition to reduced disk storage.

THE ELECTRONIC-STRUCTURE OF THE PTH MOLECULE - FULLY RELATIVISTIC CONFIGURATION-INTERACTION CALCULATIONS OF THE GROUND AND EXCITED-STATES

Fully relativistic all‐electron self‐consistent field calculations based on the Dirac–Coulomb Hamiltonian have been performed on the three lowest lying states of the PtH molecule. The resulting four‐component Dirac–Hartree–Fock (DHF) molecular spinors are subsequently used in relativistic configuration interaction (CI) calculations on the five lower states of PtH. Spectroscopic properties are obtained by fitting the potential curve to a Morse function and show good agreement with experimental data. The effect of relativistic corrections to the Coulomb electron–electron interaction is investigated at the DHF level and is found to be insignificant for the molecular spectroscopic properties investigated by us. The CI wave functions are found to have only one dominant configuration, indicating a lack of static correlation. Dynamic correlation in the d shell is, however, important for the spectroscopic properties of PtH. The results conform with a bonding scheme in which the three lower and two upper states of...

Reduction of uranyl by hydrogen: an ab initio study

Abstract We present in this paper a systematic investigation of the accuracy of different theoretical approaches to uranyl reduction. All-electron and RECP results are compared at the SCF and different correlated levels, including density functional methods. The comparison is done for geometries and reaction energies. The influence of spin-orbit interaction on energies is also investigated.

RELATIVISTIC EFFECTS ON THE BONDING OF HEAVY AND SUPERHEAVY HYDROGEN HALIDES

Abstract The bonding in the hydrogen halides HI, HAt and HUus (Uus = element 117) has been studied using four-component Dirac-Hartree-Fock calculations and finite basis sets. The calculations show that the effect of spin-orbit splitting on the valence p-orbital dominates the bonding for the compound of the superheavy element, and even for the sixth row the spin-orbit interaction should be treated self-consistently for an accurate description of the electronic structure.

A theoretical study of the chemisorption of methane on a Ni(100) surface

Abstract The dissociative chemisorption of methane at an on-top site on a Ni(100) surface has been studied using a cluster model at the ab initio CASSCF-CI level. In all cases the Ni atom interacting directly with the adsorbate was described at the all-electron level, while the remaining metal atoms were described by one-electron effective core potentials. The calculations yield an activation energy barrier of 15-17 kcal/mol. The binding mechanism at the transition state is discussed.

Bonding and electronic structure in diatomic ThO: Quasirelativistic effective core potential calculations

Abstract CASSCF and CCI calculations have been performed on several low-lying electronic states of ThO, using quasirelativistic (one-component) effective core potentials. The relativistic effects are incorporated using the “no-pair” equation with free-particle projection operators. The three lowest electronic states can be identified with experimentally detected ω levels. The bond is formed by thorium dσ, and dπ electrons interacting with oxygen pσ and pπ, while the 7s electrons are essentially lone pair. The availability of 5f orbitals for hybridization increases the overlap between the 6d and 2p orbitals and strengthens the bond considerably. The Th populations for the X1Σ+ state at Re are 7s1.707p0.226d1.125fO.35. Iterative relativistic extended Huckel (REX) calculations give a qualitatively similar picture of the bonding.

On the combination of ECP-based CI calculations with all-electron spin-orbit mean-field integrals

Abstract Spin-orbit interaction in ab initio calculations using effective core potentials is usually treated with pseudo-spin-orbit-operators, which are fitted on relativistic all-electron data. This procedure has been applied successfully for many systems. However, in combination with all-electron calculations, a recently developed atomic mean-field spin-orbit code succeeded in reproducing spectroscopic data with high accuracy. We propose a new way to combine these approaches in order to benefit from their conceptual and computational advantages. We present applications on two experimentally and/or theoretically well-known atomic systems, platinum and thallium, to prove the validity and precision of this new ansatz.