Helge Johansen

SCF LCAO MO calculations for the iso-electronic series ClO 4 − , SO 4 2− , and PO 4 3−

Ab initio SCF LCAO MO calculations have been performed for the ground states of the perchlorate, sulphate and phosphate ions. The basis sets used consisted of contracted gaussian-type orbitals having 12s, 10p and 2d-type primitive functions on the central atoms and 10s and 6p-type functions on the oxygens. The results are thought to be nearly at the Hartree-Fock limit.

Limited configuration interaction calculation of the optical spectrum for the permanganate ion

Abstract A series of limited configuration interaction calculations based upon an extended basis set is presented for the permanganate ion. The configurations have been selected so that they describe initial and final states equally well, and the size of the expansion has been increased until a reasonable convergence in the results has been assured. Both singlet and triplet states have been treated. Electron density changes occurring at the so-called charge-transfer transitions have been studied, and relaxation turns out to play an important role. The agreement with experimental spectra is satisfactory.

Analysis of a configuration interaction calculation for a charge transfer transition in MnO4

An analysis of the electron density distribution in the excited 1 1 T 2 state of the permanganate ion is presented. Based upon a configuration inter-action calculation, it is shown how the charge transfer excitations lead to transfer of density from the ligands to the central atom by diagonal density matrix elements and how density is moved in the opposite direction by the off-diagonal terms, leaving the atomic charges relatively unchanged. There is, however, a large internal redistribution of the 3d electrons. Only a few of the configurations turn out to be very important.

The spin density and the optical spectrum of CoCl2- 4

The CoCl2- 4 ion has been studied using ab initio methods at various levels of approximation (RHF, UHF, CI and MC SCF). The calculations have been performed both with and without the Madelung field from crystalline surroundings. One of the main purposes has been to investigate the spin density, and determine its dependence on both correlation and external fields. The spin density is found to be fairly dependent on correlation but rather independent of the surrounding crystal. The lower excited quartet states have been calculated using CI expansions based upon ground state RHF and MC SCF wavefunctions. The experimental spectrum is very well reproduced on the MC SCF/CI level, both with and without the Madelung field.

Ab initio SCF and limited CI calculations of the d-d transitions in cobalt oxide

Ab initio calculations using an extended set of gaussian-type orbitals have been performed for the CoO6 10- cluster in the Hartree-Fock approximation and with a limited configuration interaction. The results have been compared with those of crystal-field theory. CoO is found to be a very ionic system with localized d-electrons. The excited d-states may be described using a simple configuration interaction scheme, which is found to be almost equivalent to a crystal-field parameterized configuration interaction built on Hartree-Fock results. Localization and delocalization phenomena in openshell systems have been discussed.

On the stability of NCNO and its dissociation into CN and NO radicals as treated by ab initio methods

Abstract Ab initio calculations have been performed in order to investigate the ground state potential energy surface of the gas phase reaction NC + NO ⇌ NCNO. Both restricted (RHF) and unrestricted (UHF) Hartree-Pock approximations and direct configuration interaction (CI) methods were used. Up to 24067 configurations were included in the CI expansion. The RHF solution was found to be unstable for all investigated geometries and appreciably lower energies were obtained in the UHF approximation. The CI expansion based on the RHF solution as the reference state and including all single and double replacements, as a result of this instability, also proved to give an inadequate description of the potential energy surface even at the equilibrium configuration. Such a wavefunction obviously fails to describe the system at large internuclear separations. A more correct description of the surface was obtained when a UHF wavefunction was used as the reference state.

Analysis of the spin density for an excited triplet state of the permanganate ion

Abstract The spin density distribution for the 1 3 T 2 state of the permanganate ion has been investigated. It is shown that results from a limited configuration interaction calculation are very similar to those found from a simple orbital description. This is in great contrast to the large charge density relaxations which accompany the charge-transfer transitions.

A suggested revised conformation of methylthionitrite as seen by its spectra and preliminary ab initio calculations

Abstract A proof has recently been given that gaseous methylthionitrite, CH 3 SNO, occurs exclusively (or mainly) in its anti conformation [1]. The present paper claims that existing spectral evidence and ab initio calculations now performed suggest that gaseous methylthionitrite is mainly the syn conformer, the extra stability being of the order 1–2 kcal mol −1 . Methyl group rotation in the syn conformer is hindered by a three-fold barrier of height 689 cm −1 while the methyl group rotation in the anti conformer is hardly hindered. The syn/anti energy difference and the barriers hindering methyl group rotation closely parallel the corresponding measured quantities in methylnitrite, CH 3 ONO.

Ab initio RHF, CI, and MC SCF calculations on the vanadyl ion

Abstract The electronic structure of the vanadyl ion has been calculated in fields of monopoles and dipoles using RHF, CI and MC SCF methods. The MC SCF calculations use the complete active space formalism. The results show a strong participation of the vanadium 3d orbitals in both σ and π bonding. The 3d population is found to be between 2.3 and 2.9 - much higher than the formal d 1 characterization. The system becomes more ionic in the MC SCF description, and there is a significant mixing of configurations, in which excitations involving e to e * transitions are the most important. The electronic structures of some of the excited states have been calculated.

Molecular electrostatic potentials in transition-metal chemistry: an example

Abstract Using the protonation of Co(CO) 4 −1 as an example, the applicability of the electrostatic potential has been assessed by comparison with Hartree-Fock calculations. It is concluded that electrostatic potentials are useful for mapping the coordination sphere of a transition-metal atom, giving directions and distances, but that relative energies are less reliable.