Ulf Wahlgren

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.

Potential energy surface for the model unimolecular reaction HNC → HCN

Ab initio electronic structure theory has been used to determine the more important features of the potential energy surface for the simple isomerization reaction HNC → HCN. Extended basis sets were used in conjunction with both self−consistent−field (SCF) and configuration interaction (CI) wavefunctions. For nonlinear or Cs geometrical arrangements of the three atoms, the CI included 11735 configurations, i.e., all single and double excitations. This large scale CI reproduces the HCN ground state geometry quite accurately and has been used to tentatively identify HNC in the interstellar medium. The SCF calculations predict HNC to lie 9.5 kcal/mole above HCN, while CI yields 14.6 kcal/mole. Similarly, barrier heights of 40.2 and 34.9 kcal/mole are predicted by SCF and CI. Thus, the SCF approximation is qualitatively reasonable for HNC → HCN. If HNC is designated by a reaction angle of 180° and HCN by 0°, then the saddle point or transition state is predicted to lie at 73.7°, significantly closer to HCN. A...

On the cluster convergence of chemisorption energies

A new rule is suggested for calculating chemisorption energies using the cluster model. This rule is built on the realization that relatively large clusters (50 atoms and more) often need to be prepared for bonding by making an excitation to a proper bonding state (such a state will always be easily accessible in an infinite cluster). For hydrogen chemisorption, this bonding state must have a singly occupied orbital of the same symmetry as the hydrogen 1 s orbital. The new rule is applied to hydrogen chemisorption in the hollow sites of Ni(100) and Ni(111). When a cluster is prepared in a bonding state, even quite small clusters (≈ 10 atoms) give chemisorption energies in reasonable agreement with experimental surface results.

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.

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.

SCF ab‐initio ground state energy surfaces for CO2 and CO2−

SCF ab‐initio computations are performed for the ground states of CO2 and CO2−. The CO2 and CO2− potential surfaces have been obtained over a large region of space; in particular, the intersection of these two surfaces. Our results predict that the stability of CO2− depends strongly on whether it is formed near the equilibrium bond angle (135°), the most stable situation, or at significantly different angles. The calculations show that the 6a1 molecular orbital of CO2− is diffuse in character and that the computed equilibrium geometry (bond angle, 135.3°, bond length, 2.35 bohr) and electron affinity (−0.36 eV) are consistent with experiment.

A new mean-field and ECP-based spin-orbit method. Applications to Pt and PtH

Abstract All-electron spin-orbit mean-field integrals have been modified to be used with effective core potential valence wave-functions. Two different effective core potentials, one of the conventional Huzinaga type and the other an ab initio model potential, have been employed in our investigation. In all cases the full nodal structure of the valence orbitals has been kept. The applicability of the present approach has been tested for the 3D and 1D (d9s1) and the 3F (d8s2) electronic states of atomic platinum, the 2D d9 state of Pt+, and the low-lying 2 Δ , 2 Π , and 2 Σ + states of platinum hydride. Spin-orbit matrix elements evaluated for ab initio model potential wavefunctions are found to agree with all-electron results to within better than 3%, and the corresponding agreement for the spectroscopic constants of PtH is excellent.

A theoretical study of methyl chemisorption on Ni(111)

Calculations including electron correlation have been performed for methyl adsorbed on nickel clusters mimicking the Ni(111) surface. The chemisorption energies are evaluated following a recently developed scheme, in which the cluster is prepared for bonding. In this way cluster dependent oscillations of the chemisorption energies are largely eliminated. By also using calculated vibrational frequencies of the adsorbed methyl an almost certain assignment of the preferred chemisorption site is obtained. Methyl is found to adsorb in the threefold hollow site with a chemisorption energy in the range 50–55 kcal/mol. The origin of the soft C–H frequencies observed experimentally is a charge transfer from the metal into the C–H antibonding orbitals. The only weak sign of a direct metal–carbon–hydrogen interaction in the calculations is that the C–H frequency is slightly lower for an eclipsed compared to a staggered orientation of methyl in the threefold hollow. The present results are compared to previous experi...

Model studies of the chemisorption of hydrogen and oxygen on nickel surfaces. I. The design of a one-electron effective core potential which includes 3d relaxation effects

The present article is the first in a series describing investigations of the chemisorption and dissociation of hydrogen and oxygen on nickel surfaces. This paper deals with methodological questions, mainly concerning the form and properties of the crucial one-electron effective core potential (ECP) on nickel, which is used to describe chemisorption situations with sufficiently large nickel—adsorbate distances. Atomic chemisorption in the fourfold position on the Ni(100) surface is chosen as the model problem for testing and designing the nickel ECP. All-electron calculations on Ni4 and Ni5 with and without adsorbed atoms have been performed to produce reference results. Particularly noteworthy of the all-electron results is the large effect of 3d relaxation. To mimic cluster 3d relaxation effects a diffuse attractive 3d projection operator was added to the original standard form of the ECP.

The electronic structure of the azabenzenes an ab initio MO-SCF-LCAO study

Abstract Ab initio MO-SCF-LCAO calculations have been carried out for benzene and the azabenzenes pyridine, pyridazine, pyrimidine, pyrazine, s-triazine and s-tetrazine, and the results are presented. The general structure of the molecular orbitals is discussed and a classification scheme referring to an ideal case of cylindrical symmetry is described. Orbital energies, empirically corrected for reorganization and correlation, have been used to make assignments for the photoelectron spectra. A number of molecular properties have been computed and are discussed in relation to available experimental information. The electrostatic potential around each molecule is presented in the form of contour maps. Each nitrogen lone pair gives rise to a region of negative potential. The depths of these minima give some information about the relation between electronic structure and basicity. The potential in regions of high π-electron density is negative only in benzene and pyridine. This fact can be correlated with the increasing resistance against direct electrophilic substitution for the azabenzenes as compared to benzene itself. The calculations have been performed using contracted Gaussian functions as a basis. Four s- and two p-type atomic orbitals were used for carbon and nitrogen, whereas for hydrogen two s- and one pσ-type functions were used.