Osvaldo Goscinski

Moment-conserving decoupling of green functions via pade approximants

A generalization of the Pade approximant technique obtained by inner projections leads to approximate Green functions whose spectral densities satisfy a certain number of frequency moments. This approach contains decoupling schemes previously given and their explicit representation in terms of the moments.

Dimensional scaling in chemical physics

I: Basic Aspects. 1. Introduction. 2. Tutorial. II: The Research Frontier. 3. large-D Limit for N-Electron Atoms. 4. Low D Regime. 5. Hyperspherical Symmetry. 6. Hypercylindrical Symmetry. 7. General Computational Strategies. 8. Two-Electron Excited States. 9. Large-D Limit for Metalic Hydrogen. 10. D-Interpolation of Virial Coefficients. III: Related Methods. 11. Nonseparable Dynamics. 12. Pseudomolecular Electron Correlation in Atoms. Index.

Dispersion forces, second- and third-order energies☆

Abstract The properly symmetrized second order energy, ϵ Q 2 , has been observed to approach one half of the correct dispersion energy ϵ 2 for large interatomic distances. We contend that the proper quantity to be examined is ϵ Q 2 λ Q . λ Q approaches ϵ Q 2 /(ϵ Q 2 -ϵ Q 3 ) ≈ 2 at R = 8 a o for H 2 and ϵ Q 2 /ϵ Q 3 ≈ 2. Hence E o -- ϵ Q 1 + ϵ Q 2 is a bad approximation.

Transition operators for molecular ΔESCF calculations: Ionization in water and furan

Abstract Ab initio calculations of binding energies with a transition operator method are performed. They are demonstrated to include relaxation, which is essential for the prediction of deep-hole chemical shifts.

Role of localization in the prediction of core ESCA lineshapes of N2, O2 and NO

Abstract Hartree—Fock ab initio calculations have been performed on electronic energy surfaces of the ground and the core hole state species of N 2 , O 2 and NO in order to investigate vibrational structure of their high resolution ESCA spectra. The importance of relaxation effects in predicting geometry changes following core hole photoionization is known and was verified here. As a new feature the influence of relaxation on lineshapes is studied. Both the conventional Franck—Condon analysis and a method for calculating linewidths of photoelectron spectra from the gradient of the hole state energy curve are employed. Two major conclusions are made: (i) Agreement with experiment using the Hartee—Fock scheme can be obtained only if the core holes are treated localized. (ii) The vibrational effects for the two multiplet split components in the paramagnetic core hole ions NO + and O 2 + are different, and these effects are reflected in the ESCA linewidths. Ab initio calculations within the Hartree—Fock formalism have been carried out on the electronic energy surfaces of the ground and the core hole states of N 2 , O 2 and NO. The differences in geometries and force constants between the ground and the ionized species follow a systematic pattern [4–7,15]. The results of these studies are used to compute high resolution ESCA spectra. Two interesting features occur. 1. (i)The agreement with the experimental spectra can only be achieved if the core holes are treated as localized in the Hartree—Fock scheme. This is analogous to recent results on angular distributions of molecular-photoelectrons obtained by Dill et a. [45]. Even though correlation corrections to symmetry restricted calculations can in principle yield good answers, limited configuration interaction treatments can be misleading for transition energies [46,47] and presumably for predicting lineshapes. 2. (ii) The linewidths of the two multiplet-split components in the paramagnetic molecules O 2 and NO should not be expected to be the same a priori. This can play an important role in interpretation of high resolution ESCA spectra of some paramagnetic transition metal compounds which also exhibit a substantial exchange induced splitting [47–51]. However, in general, to obtain a better agreement between theory and experiment both more advanced ab initio calculations and more detailed account of anharmonicity in the Franck—Condon analysis are required. Coupled with the development of more advanced instrumentation, such calculations are presently in progress.

On transition energies and probabilities by a transition operator method

A transition operator method is presented for calculations of energies and probabilities of radiative and non-radiative transitions. It is based on the optimization of the initial and final states simultaneously and provides a common set of orthonormal spin orbitals for both states. No exchange approximations are made. It is shown both analytically and numerically that it accounts accurately for relaxation energies even though it avoids the nonorthogonality problem inherent in the self-consistent field methods that optimize the initial and final states separately. Extensions and limitations are discussed and applications are pointed out.

Core hole localization in N+2

Abstract The question of localized treatments of hole states of diatomics is considered. The effective interaction between localization sites is discussed in relation to system parameters such as internuclear distance. Computations are performed on those states of N + 2 involving holes in the molecular orbitals arising from the 1s and 2s.

Chemical shifts of auger electron lines and electron binding energies in free molecules. Sulfur compounds

The first paper describing a series of systematic investigations of the chemical shifts in Auger electron spectra from various free molecules is presented. Excitation is performed by means of a fine focus electron beam. The Auger electrons are retarded in a four component lens system and recorded at high resolution in a new multidetector system placed in the focal plane of the ESCA instrument. A calibration procedure against the KL2L3 Auger electron line of Ne is described. The first study concerns Auger electron line shifts for sulfur in some small molecules and the results are compared to the corresponding chemical shifts in the core photoelectron spectra. A formalism based on a transition potential model is briefly presented which takes account of the relaxation energies involved in Auger transitions as well as in single photoionization.

Relaxation effects on ESCA chemical shifts by a transition potential model

Abstract A transition operator method for describing ionization of inner-core levels is implmented at the semi-empirical level. It involves one calculation per molecule. The results compare favourably with experiment and more elaborate treatments.

Treatment of resonances with the dilated electron propagator: The 2P shape resonance in e‐Mg scattering

The dilated electron propagator technique based on an underlying bivariational SCF and a second order self‐energy is applied to study the 2P shape resonance in e‐Mg scattering. Basis set effects are studied and comparisons with the results from an alternative construction of the second order dilated electron propagator are made.