R. Moccia

Magnetic Properties of Polyatomic Molecules. I. Magnetic Susceptibility of H2O, NH3, CH4, H2O2

The magnetic susceptibility of H2O, NH3, CH4, and H2O2 molecules has been evaluated by the perturbed HF scheme. Several wavefunctions, all of LCAO SCF MO type, have been considered. By using a double perturbation technique we derive some useful quantities which allow one to evaluate diamagnetic and paramagnetic contributions to the susceptibility for any gauge. The results corresponding to the largest basis sets are in satisfactory agreement with experimental data. The dependence of the evaluated quantities upon the gauge transformation corresponding to a change of the origin of the vector potential has been examined in detail.

Magnetic Properties of Polyatomic Molecules. II. Proton Magnetic Shielding Constants in H2O, NH3, CH4, and CH3F

Proton magnetic shielding constants in H2O, NH3, CH4, and CH3F molecules have been evaluated by the perturbed HF scheme, by employing extended basis sets of Slater orbitals. Explicit formulas for both diamagnetic and paramagnetic contributions to the observable are given, for a gauge transformation corresponding to any change of the origin of the external vector potential. The calculated results are in fairly good agreement with the experimental data, when the chosen gauge is coincident with the central nucleus of the molecule.

SCF‐MO's and Molecular Properties of Methane

SCF‐MOs of the CH4 molecule have been calculated for the experimental geometry by using bases of up to 39 STOs. The best molecular energy obtained is −40.20452 a.u. The ground‐state wavefunctions have been utilized to compute some one‐electron properties, viz., the electric octupole moment, the electric field gradient at the protons, and the diamagnetic susceptibility. In addition, the electric polarizability and the paramagnetic susceptibility have been evaluated using a perturbed H–F calculation.

The calculation of photoionisation cross sections of simple polyatomic molecules by L2 methods

Abstract This article is concerned with the calculation of the photoionisation cross section of molecules due either to the one-photon or, in view of the increasing interest in the nonlinear optical processes, to the two-photon absorption. The emphasis is put upon the approaches which may exploit the extensive wealth of know-how for the calculation of the electronic bound states of molecules to gain useful and detailed information about the electronic continuum. Thus the main attention of this report is devoted to the methods which permit the extraction of properties pertaining to the electronic continuum from L 2 basis-set calculations. The first part outlines the features characterizing the electronic continua required to calculate the differential ionisation cross section, and a method, amenable to systematic improvements, based upon the reaction K -matrix to obtain all the required transition matrix elements either from an L 2 energy-variational or from an L 2 basis-set RPA calculation. Then, following an overview of the problems connected with the implications of the Born-Oppenheimer approximation and with the choice of the gauge, a brief review is presented of the Stieltjes moment theory which permits the evaluation of the oscillator-strength density in the continuum from a discretized representation. The remaining part reports some specific examples of calculations for the one-photon and the two-photon processes.

Electric polarizability of polyatomic molecules

Abstract Electric polarizabilities of some polyatomic molecules are evaluated, within the HF pertubative scheme, by using LCAO SCF MOs wave functions.

Molecular photoionization cross sections and asymmetry parameters by L2 basis functions calculations: H2O

A K‐matrix technique using a basis set of square‐integrable functions is applied to the calculation of differential photoionization cross sections in molecules. Continuum orbitals are variationally determined in the static‐exchange approximation of the ion field. Integrated photoionization cross sections and asymmetry parameters β of the three main valence ionization processes in H2O are calculated, in the independent channel approximation, for the photon energy in the range of 14–50 eV and compared with the available experimental data.

Photoionization cross section calculations of HCl by the Stieltjes technique: Effect of the channel coupling

Partial and total valence- and inner-shell photoionization cross sections of HCl are calculated in the random phase approximation employing an extended basis set and the Stieltjes imaging procedure. Results of independent-channel as well as multi-channel calculations in the static-exchange approximation are reported for comparison. The generally good agreement with recent experimental measurements indicates that the random phase approximation provides an efficient description of photoionization channel coupling. The calculated vertical spectra in the ‘discrete’ region of valence- and inner-shell are also reported.

Magnetic properties of polyatomic molecules. III. Magnetic shielding constants of heavy nuclei in H217O, 14NH3, 13CH4 and 13CH3F☆

Abstract The nuclear magnetic shielding tensor components for 17 O in H 2 17 O. 14 N in 14 NH 3 and 13 C in both 13 CH 4 and 13 CH 3 F have been evaluated by coupled HF perturbation theory. All quantities needed to calculate the tensor components for any gauge are also reported. The gauge-invariance of the computed quantities is not uniformly good for the various examples investigated.

H2S photoabsorption and photoionization cross sections by Stieltjes imaging

Abstract Photoabsorption and photoionization cross sections of H2S have been calculated by using extended basis sets of integrable functions and the Stieltjes imaging procedure. The calculations were performed in static-exchange approximation (SEA) and random-phase approximation (RPA) and the excitation spectrum was obtained either in the single-channel or in the multichannel approach. The gauge dependence of the SEA results tends to inhibit a reliable assessment of the channel interaction effects. The situation is noticeably improved in the RPA where the gauge dependence of the results, even in the single-channel approximation, is considerably lessened.

Electronic properties of the SF6 molecule: Ab-initio calculation for the ground state☆

Abstract We report an ab-initio computation for the ground-state wavefunction of sulphur hexafluoride where 55 Slater-type orbitals were used for a basis-set expansion with 3d-functions included on the central atom. Several molecular properties are examined and, among other results, a rather satisfactory agreement is found between the computed and measured binding energies of all the electrons.