Joseph G. Fripiat

Non-empirical quantum mechanical calculations on pentasil-type zeolites

Abstract Molecular orbital calculations, using non-empirical SCF-MO techniques, are used to study monomer and dimer clusters having the geometrical structure of zeolites ZSM-5 (MFI) and ZSM-11 (MEL). The results indicate that the 12 T-sites of the MFI secondary building unit (SBU) are noticeably different from each other while the seven T-sites of the MEL SBU fall into two categories. Preferential locations of Al are proposed in both the MFI and MEL frameworks. Instabilities in the framework of MFI are identified and the validity of the Al avoidance principle and the acidic properties of these zeolites are discussed.

On the behaviour of exchange in restricted hartree-fock-roothaan calculations for periodic polymers

Abstract Analysis of the restricted Hartree-Fock (RHF) exchange potential is presented. In polymeric chains, a relation between the short-rangeness of the exchange and the energy gap is observed. In metallic systems, the exchange is of long-range and correlation corrections are expected to be important. The validity of RHF solutions in metallic situations is considered.

From uncoupled to coupled Hartree–Fock polarizabilities of infinite polymeric chains. Pariser–Parr–Pople applications to the polyacetylene chains

A general method is formulated to compute the asymptotic longitudinal polarizabilities of infinite systems. This procedure is an extension to infinite systems of the molecular random‐phase‐approximation method which provides coupled Hartree–Fock values and thus takes into account the field‐induced electron reorganizational effects. It is shown that the Genkin–Mednis uncoupled method corresponds to the drastic approximation of the coupled one where there is no electron reorganization. By looking at the asymptotic polarizabilities as the convergence values of the oligomeric results, the method is tested for polyacetylene chains in the Pariser–Parr–Pople approximation.

Quantum mechanical approach to the chemisorption of molecular hydrogen on defect magnesium oxide surfaces

AbstractQuantum mechanical theoretical calculations have been performed on the linear atomic chain

FOURIER REPRESENTATION METHOD FOR ACCURATE EVALUATION OF THE COULOMB AND EXCHANGE LATTICE SUMS IN EXTENDED CHAINS

(MgOHV_{\ddot Mg} HOMg)^{2 + }

Comparative Ab initio analysis of valence XPS data for acenes and paraphenyl oligomers: Application of the molecular-orbital intensity model

in order to simulate the interaction of molecular hydrogen with the defects present at the surface of activated MgO. The total energy of the system, the relative energy of the various molecular orbitals, and the electronic charge distribution have been computed for various lattice parameters (dO-O = 4.0–4.8 Å) as a function of the H-H (or O-H) separation. A symmetrical motion of the hydrogen nuclei with respect to the central Mg2+ vacancy was assumed. It is shown that chemisorption of hydrogen on surface O−ions sites results in the formation of pseudo-hydroxyl groups. For a small lattice parameter (4.0 Å), no stable state of molecular hydrogen has been found while an increase in the lattice parameter results in a uniform increase of the calculated activation energy for the molecular hydrogen activation process. A mechanism is proposed which is not so different from that put forward for the hydrogen activation by transition metal complexes. Molecular hydrogen is found to act as an electron donor.

Computational Aspects of Polymer Band Structure Calculations by the Fourier Space Restricted Hartree-Fock Method

Minimal basis set (STO‐3G) calculations on the metallic infinite chain of hydrogen atoms, (‐H‐)x, performed within the same computational framework as currently used for more realistic model systems, are reported to illustrate the dependence of the RHF energy bands on the summation of exchange contributions. The numerical results show the gradual decay of the density of states at the Fermi level as the number of terms in the exchange lattice sums is increased and rather convincingly verify the predictions of previous formal analyses.

Ewald-Type Formulas for Gaussian-Basis Bloch States in One-Dimensionally Periodic Systems

By considering chains of hydrogen and helium atoms with one Gaussian function centered on each atomic sites, i.e., the simplest possible metallic and insulating model systems retaining all the characteristics of the restricted Hartree–Fock–Roothaan method for extended chains, a scheme is proposed to carry out accurately and efficiently both Coulomb and exchange lattice sums which occur in a Gaussian basis set environment.