A. Lisini

Density functional-time-dependent local density approximation calculations of autoionization resonances in noble gases

Absolute photoionization cross section profiles and asymmetry parameters of Ne, Ar and Kr have been calculated at the time-dependent local density approximation level. We employed a very accurate B-spline finite basis set and the modified Sternheimer approach, which is a first-order perturbative scheme particularly suited to finite basis set calculations. The gradient-dependent van Leeuwen and Baerends (VLB) exchange-correlation potential has been used, since it has the correct Coulombic behaviour at large distances which is a necessary condition for the existence of the Rydberg states. A simple modification of the boundary conditions considerably improves the number of terms of the Rydberg series. The resonance parameters obtained by fitting the calculated Fano profiles with a suitable analytic expression are compared with the experimental parameters.

Density functional calculations of excitation energies and oscillator strengths for C1s → π∗ and O1s → π∗ excitations and ionization potentials in carbonyl containing molecules

Theoretical calculations of C1s → π∗ and O1s → π∗ excitation energies and oscillator strengths together with C1s and O1s ionization potentials in various carbonyl containing molecules have been performed. We employed the DF-LCAO method, with a DZP-STO basis set. Excitation and ionization energies are calculated by the transition state approach, and are more properly discussed as shifts with respect to CO. Comparison of the present results with accurate ab initio and experimental data available for free CO demonstrates the validity of such an approach. The other systems considered range from formylic fluorinated compounds (H2CO, HFCO and F2CO) to transition metal carbonyls (Cr(CO)6, Mo(CO)6, Fe(CO)5, Mn(CO)5Br, Mn(CO)5H and Ni(CO)4) and model molecules (NiCo, PdCO and PtCO). Particular attention is devoted to quantitative considerations about backbonding and to the capability of the excitation intensity to map the π∗ ground state orbitals. Comparison of the present theoretical results with experimental data demonstrates the high reliability of the method employed, which is able to correctly predict small effects (small energy shifts and differential relaxation). Some discrepancies in oscillator strengths are reasonably ascribed to experimental normalization errors.

Molecular orbital description of core excitation spectra in transition metal compounds. An ab initio CI calcultaion on TiCl4 and isoelectronic molecules

Abstract Ab initio calculation of 1s and 2p exicitation spectra are performed in the relaxed 1h-1p CI scheme for a series of isoelectronic and isostructural halides and oxohalide (TiF 4 , TiCl 4 , TiBr 4 , VOF 3 , VOCl 3 , CrO 2 F 2 , CrO 2 Cl 2 , MnO 3 F, MnO 3 Cl). Agreement with experimental spectra available for TiCl 4 is very satisfactory, indicating the reliabilty and accuracy of the present model to describe core excitation spectra even in highly ionic transition metal compounds. The Calculated spectra show significant and well detectable variation upon halogen or metal substitution, and are therefore a sensitive probe to slight electronic structure changes along the series. Significant configuration mixing are observed among different excitation channels in the case of the degenerate 2p core hole, and prevent full account of the spectra in a pure one-electron model.

Theoretical study of the valence photoelectron spectrum of ozone: An analysis of correlation effects and configuration interaction (CI) model spaces

Minimal basis set CI calculations with different configuration spaces including full CI are compared for the ground and ionized states of O3. A double inversion with respect to the Koopmans Theorem (KT) ordering is found, and the inadequacy of approaches based on a single configuration reference is pointed out. The strong correlation effects appear to be well described in terms of fluctuations within a quasidegenerate set comprising the three π orbitals, giving a picture of O3 as a quasi‐open‐shell system. Extended 2h–1p schemes based on the quasidegenerate model give good reproduction of the FCI results. Extension to the whole valence region gives indication of a further shattering of the intensity over the simple 2h–1p results. An assignment for the complete valence spectrum is proposed.

An analysis of configuration-interaction model spaces for the study of the core photoelectron spectra of small molecules

Two-hole, one-particle (2h-1p), 3h-2p and 4h-3p configuration-interaction (CI) calculations of the shake-up in the core photoelectron spectra of H2O, HOF, F2, F2O, N2, CO and O3 have been compared with full configuration interaction (FCI) in a minimal basis set. Explicit inclusion of relaxation is found to be important even at the 3h-2p level, while no definite advantage of the delocalised description is observed beyond the 2h-1p level. The relaxed 3h-2p scheme is found to be adequate for the low-energy part of the spectra; higher excitations become important at higher energy. Both the important correlations and the molecular relaxation appear to be well described within the minimal basis. However, extension of the basis is essential for a quantitative description of the spectrum. The effect of an extended basis has been analysed for the ionisations of CO, using a projection scheme which allows a better definition of the empty orbitals, while keeping the conceptual simplicity of the valence space.

X-ray photoelectron spectroscopy and ab initio CI study of the core and valence ionisation of formamide

Abstract High resolution X-ray photoelectron spectra have been recorded of the core and valence levels of the formamide molecule using monochromatised Al Kα radiation for the excitation of the spectra. Ab initio CI calculations of the binding energies and intensities have been carried out in order to interpret the experimental results and to provide additional information on the electronic structure. The core level shake-up spectra are found to be similar to those observed earlier of the CO molecule, acetaldehyde and acetone. In particular, the substitution of a hydrogen atom by methyl or amino groups tends to give rise to an additional shake-up line corresponding to the additional valence orbital of π symmetry. The valence photoelectron spectrum reveals new structures compared to previous studies. This has enabled an assignment in terms of both single hole states and many-electron states and the study confirms that the orbital ordering in the outermost region should be given as 10a′ (n)2a″ (π)2a″ (π)9a′ (σ) in order of increasing binding energy. Ionisation from the 7a′ (σ) orbital at about 18 eV leads to two structures in the spectrum with almost equal intensity. The two strong photoelectron bands in the inner valence region are found to be composed of four- or five-component states, which implies a more or less complete breakdown of the orbital model in this region.

Calculation of dynamical correlation effects by quasidegenerate perturbation theory. An application to photoionization spectra

Abstract The Rayleigh-Schrodinger quasidegenerate perturbation theory is applied for the treatment of the dynamical and weak non-dynamical correlation effects within the CI scheme. The strongest QD correlation effects are taken into account at the zeroth order by including the most interacting configurations in the model space, while the weaker correlations are included perturbatively. The intruder states problem is avoided with a prediagonalization of a larger space and taking as a the model space the subspace spanned by the states of interest. The scheme is applied to the calculation of the photoionization spectra of Be, O 3 and HCl, which have been chosen as representative test cases. Results are in good agreement with those of the full Hamiltonian and the experimental data available are obtained already at the second order of perturbation, showing the reliability of the method proposed.

On the validity of the Xα method in the computation of ionization potentials. A comparison of the results for small molecules

Abstract Xα calculations of the ionization potentials were performed on a series of small molecules by the MS-Xα (tangent and overlapping spheres, and LCAO-Xα methods. The results are compared with the experimental values and the results of previous accurate calculations. It is inferred that the Xα formalism is rather effective for the calculation of ionization potentials and possibly superior to the Koopmans theorem. Indeed in all considered cases the ordering inversions supplied by Koopmans theorem are removed in the Xα predictions. The LCAO-Xα method seems to be the most accurate of the Xα methods examined.

Ab initio CI investigation of valence photoelectron spectra of conjugated hydrocarbons

Abstract The valence photoelectron spectra of unsaturated hydrocarbons of increasing complexity, ethylene and all-trans butadiene, hexatriene, octatetraene and pentadecaene, have been calculated at the ab initio CI level. The analysis of the influence of the many-body effects on the spectral features and their evolution along the series can be the basis upon which to anticipate the spectral behaviour of longer chain molecules with alternating double bonds. The capability of the 2h-lp CI and 2h-lv CI schemes to give a qualitative description both of the outer and the inner valence structures has been proved, pointing out the usefulness of the simpler scheme, i.e. 2h-lv CI, for investigating the whole valence spectrum of all molecules considered.

Calculation of the valence shell correlation effects on ionization potentials. A test on N2, C2N2 and H2NN

Abstract Two schemes for the definition of the valence space from an extended basis are proposed and tested for the calculation of ionic states in N2, C2N2 and H2NN. Good results are obtained with a projection scheme, leading to a 2h-1v model reproducing the essential features of the full basis 2h-1p results. Notably excellent agreement with experiment is obtained for the main ionization potentials in N2 and C2N2. Unsatisfactory results are instead given by a simple extension of the minimal basis set SCF space, indicating the importance of employing accurate occupied orbitals as a starting point.