Calculation of the photoabsorption of HF in the Vacuum UV

The photoabsorption cross section of the HF molecule up to 12 eV arising from the X-->A transition is computed by a large one-electron basis set of atomic natural orbitals and highly correlated wavefunctions. The quality of the results is checked by the accordance of the transition probabilities evaluated both in the dipole length and velocity gauges. Cross sections within an accuracy of 7% are obtained by multireference extended CI with selected CAS-SCF reference states. Comparison with two experimental results, in large disagreement with each other, does not permit an assessment of which experiment is more reliable

Performance of polynomial Gaussian functions in describing the molecular electronic continuum

The continuum molecular orbitals arising from L-2 calculations using special polynomial Gaussian functions are accurately tested to probe their capability in producing accurate photoemission cross sections. The molecular potential is represented by the local X alpha approximation, which permits a comparison with numerically integrated (exact) orbitals. Several strong anisotropic molecules are taken into account to check the flexibility of the L-2 bases in the presence of strong attractive potentials. This work shows that for adequate choices of Gaussian functions, phase shifts may be estimated with reasonable accuracy while the errors in the continuum transition probabilities are within 2-3% in the more unfavourable cases. Therefore, the proposed basis sets appear to be suitable for the description of the electronic continuum in molecular photoionization calculations

Theoretical study of the dissociation of HF+ in the B 2Sigma+ state by quantum mechanical methods

The dissociative pathway of the HF+ molecular ion, initially in its B (2)SIGMA+ electronic state, is studied by a full quantum-mechanical technique. The vibronic time-dependent wave function is expanded on the twelve lowest (2)SIGMA+ electronic states computed by a limited configuration-interaction calculation. The nuclear functions are projected on a monodimensional grid and the time-evolution operator is represented by its (1,1) Pade approximant. The calculations are performed both in the adiabatic and in a semidiabatic representation of the electronic states. The decay pattern shows strong vibronic interactions often involving several states at a time. The results are compared with those obtained in a previous calculation, where the nuclear motion was treated by a classical trajectories approach

Calculation of the differential photoionization cross section of HF

Cross sections, photoelectron angular distribution parameters and branching ratios for the ionization of the valence 1 pi, 3 sigma and inner-valence 2 sigma shells of hydrogen fluoride have been calculated in the Random Phase Approximation (RPA). The computational procedure is based on the K-matrix technique and projection on basis sets of L(2) functions to solve the RPA equations for the three coupled ionization channels in a way that recovers the electronic continuum degeneracy

Parameterization and validation of intramolecular force fields derived from DFT calculations

The energy and its first and second geometrical derivatives obtained by DFT calculations for a number of conformations of a single molecule are used to parametrize intramolecular force fields, suitable for computer simulations. A systematic procedure is proposed to adequately treat either fully atomistic or more simplified force fields, as within the united atom approach or other coarse grained models. The proposed method is tested and validated by performing molecular dynamics simulations on several different molecules, comparing the results with literature force fields and relevant experimental data. Particular emphasis is given to the united atom approach for flexible molecules characterized by "soft" torsional potentials which are known to retain a high degree of chemical specificity

Solvent effect on the optical properties of [(NH3)(5)Ru-pyrazine](+m) (m=2,3) complexes by ab initio calculations

We have carried out a study, by ab initio methods, of the solvent effect on [(NH3)(5)Ru-pyrazine](m+) (m=2 and 3) complexes, which are of interest as basic units of metallic chains with potential application in molecular electronics and photonics. We have performed multireference configuration interaction (CI) calculations in which solvent effects are included by the Polarizable Continuum Model. A new method for the estimate of the size of the cavities in this approach, which has proven to be suitable for the specific case under study, is also proposed. Our calculations account for the red shift of the metal-to-ligand charge transfer band, observed experimentally for the Ru(LI) compound as the solvent donor number increases, and furnish an explanation with a solid theoretical foundation. For the Ru(III) compound we find that the ground state configuration is different in vacuum and in electron donor solvents. (C) 1998 American Institute of Physics. [S0021-9606(98)31243-X]

Calculation of the differential photoionization cross-section of PH3

Cross-sections, photoelectron angular distribution parameters and branching ratios for the ionization of the outer-valence 5a(1), 2e and inner-valence 4a(1) shells of phosphine have been calculated up to a photon energy of 90 eV. Ab-initio calculations have been performed, by the K-matrix technique and projection on basis sets of L-2 functions, in the random phase approximation for the three coupled ionization channels. The theoretical results, compared with the available experimental data and with recent semi-empirical calculations, show a quantitative agreement with the experiment. (C) 1999 Elsevier Science B.V. All rights reserved