Thierry Leininger

AB INITIO CALCULATION OF ROVIBRONIC TRANSITION SPECTRA OF CAH

The ground (1u20092Σ+,X) and excited electronic states, 1u20092Π (A), 2u20092Σ+ (B,B′), 1u20092Δ, 2u20092Π (E), 3u20092Σ+ (D), 4u20092Σ+ (C), 5u20092Σ+ (K), 3u20092Π (L), and 2u20092Δ, of CaH are studied by ab initio calculation, with extensive configuration interactions. Calculated spectroscopic constants, in particular, the excitation energies, are more accurate than in previous calculations and agree well with the currently available experimental data. This work confirms the existence of a single vibrational level, v′=4, bound to the second well of the B state calculated from the experimental data by Martin [J. Chem. Phys. 88, 1797 (1988)]. Calculated oscillator strengths for the B–X and D–X show irregular band structures because of multiple avoided crossings of the potential energy curves. The avoided crossing between the D and C states should be taken into account to explain the apparent perturbations observed in the experimental spectra. The wave functions, dipole moments, and transition dipole moments of these states are discussed in detail.

Comparison of the widely used HF pseudo-potentials: MH+ (M = Fe, Ru, Os)

Abstract We compared the molecular characteristics of the cation of diatomic metal hydrides of the group-8 atoms (FeH + , RuH + , OsH + ) calculated with the HF pseudo-potentials of Christiansen , Hay and Wadt, Stoll and Preuss , and others. Atomic basis functions of comparable quality optimized for each pseudo-potential were used, and extensive configuration interactions were done. Although similar numbers were obtained for the spectroscopic constants and the wavefunctions in most cases, substantial deviations were shown for some.

Ab initio calculations for electron attachment to Cl2

Ab initio calculations are performed for the two species involved in the attachment of an electron to Cl2 : the neutral ground state (1 g + ) and the four anionic states (2 u + , 2 g , 2 u , 2 g + ). Four approaches are used: CASPT2, CASPT3, AQCC and CCSD(T). The good agreement with the known spectroscopic constants for Cl2 gives confidence in the present results. The neutral-anionic crossings have been carefully located. For the most reliable method, CCSD(T), we found the crossings at 95, 441, 955 and 1276 meV above the neutral ground vibrational level, for 2 u + , 2 g , 2 u and 2 g + anionic curves respectively. The recently observed resonance around 100 meV in electron scattering is thus assigned to the lowest crossing only, in contradiction with the previously suggested mechanism which also involved the states of Cl2 - .

Correlated and Non-Correlated Wave Functions for Organometallics

Electron correlation is the everlasting concern of people involved in ab initio Hartree-Fock calculations. If the investigation is restricted to the structure and properties of the ground state, the importance of correlation depends i) on the type of bonding in the molecule under scrutiny, and ii) on the level of accuracy requested for the calculation, that is, on the sensitivity of the investigated property. This sensitivity may be highly interrelated with the nature of the bonding: in the specific area of organometallic complexes, it has been recognized since more than a decade that no realistic description of multiple, direct metal-metal bonds can be obtained without an adequate treatment of the left-right correlation.1,2 Accidental near degeneracies connected for instance with the sd hybridization in complexes of Ni(0) should also be accounted for in a systematic way.3 Other cases where correlation is susceptible to qualitatively modify the description of the ground state can be detected from the occurrence of Hartree-Fock instability which makes the energy of the considered system symmetry-dependant.4,5 Introducing a systematic treatment of correlation for large systems in the frame of the Hartree-Fock methodology would lead to technical problems and to an intolerable computer cost. Our experience in the computation of the ground state electronic structure and properties of dimetallic complexes and of clusters with higher nuclearity argues for a flexible approach of the correlation problem.

Reply to comment on “comparison of the widely used HF pseudo-potentials: MH+ (M = Fe, Ru, Os)”

Abstract Table 1 of our paper showing calculated ionization energies of Fe, Ru and Os is given with correction. It is emphasized that the energy characteristics calculated using a pseudo-potential should be compared with other pseudo-potential or all-electron values calculated with a comparable basis set.