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An earlier proposed propagator method for the treatment of molecular ionization is tested in first applications. The method referred to as the non-Dyson third-order algebraic-diagrammatic construction [nD-ADC(3)] approximation for the electron propagator represents a computationally promising alternative to the existing Dyson ADC(3) method. The advantage of the nD-ADC(3) scheme is that the (N±1)-electronic parts of the one-particle Green’s function are decoupled from each other and the corresponding equations can be solved separately. For a test of the method the nD-ADC(3) results for the vertical ionization transitions in C2H4, CO, CS, F2, H2CO, H2O, HF, N2, and Ne are compared with available experimental and theoretical data including results of full configuration interaction (FCI) and coupled cluster computations. The mean error of the nD-ADC(3) ionization energies relative to the experimental and FCI results is about 0.2 eV. The nD-ADC(3) method, scaling as n5 with the number of orbitals, requires the solution of a relatively simple Hermitian eigenvalue problem. The method renders access to ground-state properties such as dipole moments. Moreover, also one-electron properties of (N±1) electron states can now be studied as a consequence of a specific intermediate-state representation (ISR) formulation of the nD-ADC approach. Corresponding second-order ISR equations are presented. [ABSTRACT FROM AUTHOR]

The general intermediate state representation (ISR) for single-electron ionization is adapted to the case of K-shell (or core-level) ionization in molecules. The development is based on the so-called core–valence separation (CVS) approximation leading to a considerable simplification of the ISR secular equations. Using the CVS approximation the core-level ISR can be formulated entirely in terms of the intermediate states of the valence electron excitation problem, which allows one to construct consistent nth-order approximation schemes for the (single-hole) ionization energies by a specific extension of the (n-2)-nd order ISR approximation for electronic excitation. In particular, the CVS-ISR concept is used to derive a consistent fourth-order approximation for core-level ionization based on the existing second-order algebraic-diagrammatic construction [ADC(2)] approximation to electron excitation. The computational scheme combines the diagonalization of a Hermitian secular matrix with finite perturbation expansions for the secular matrix elements. The explicit configuration space is spanned by one-hole (1h), two-hole-one-particle (2h-1p), and (3h-2p) ionic states with exactly one hole in the core-level shell of interest, while the configurations considered implicitly via perturbation theory extend to the class of 5h-4p states. A characteristic of the method is that the dominant valence electron relaxation effect is accounted for at the post-Hartree–Fock (HF) level. This calls for the relatively high order of perturbation-theoretical consistency, but avoids, on the other hand, the necessity of a localized (symmetry breaking) one-particle representation in the case of molecules with equivalent 1s orbitals. The method is size consistent and thus suitable for applications to large systems. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The necessity to determine the static self-energy part Σ(∞) in many-body Green’s function studies of atomic and molecular ionization introduces difficulties affecting both the accuracy and the efficiency of the method. We show how this bottleneck can be overcome under an approximation obtained by truncating the Dyson expansion for the one-particle Green’s function G(ω). Here the essential computational step consists of an inversion or a Lanczos diagonalization of constant Hermitian secular matrices associated with the dynamical self-energy part M(ω). Both methods are very practical and efficient as is demonstrated in an exemplary application to the CO molecule. The same approximation and numerical techniques apply also to the problem of extracting ground state information from G(ω), i.e., correlation energy and expectation values of single-particle operators. [ABSTRACT FROM AUTHOR]

Presents a third-order propagator method for ionization potentials and electron attachment computation of atoms and molecules. Application of the algebraic diagrammatic construction (ADC) procedure to the electron propagator parts; Comparison of the scheme with the Dyson-type approach; Relation of the non-Dyson ADC method with coupled cluster methods.

Many-body Green’s-function calculations which employ large configurational expansions and which are accurate to third and fourth order of perturbation theory are reported for the valence ionic states of HCl. Large polarized basis sets including Rydberg functions s-, p-, and d-type symmetries have been used. Third-order calculations are sufficient in the outer valence region, but in the inner valence region where the complete breakdown of the one-particle picture of ionization is observed a fourth-order theory is necessary in conjunction with large basis sets. The synchrotron and x-ray excited spectra can be assigned nearly up to the double-ionization threshold in a very satisfactory way. No indication of strong outer valence satellite lines is found. [ABSTRACT FROM AUTHOR]

A recently developed complete fourth-order Green’s function method referred to as ADC(4) was used to calculate ionization energies and relative intensities of 1s hole states in N2 and CO. Very accurate 1s electron binding energies and a good qualitative description of the shakeup spectra at low and medium energies have been obtained. For the N1s and C1s results comparison is made with previous large-scale configuration interaction treatments. New results are presented for the O1s shakeup spectrum of CO. Here striking differences with respect to the C1s spectrum are found which are explained in terms of charge–transfer effects induced by electronic excitation. We also present results for shakeup states of symmetries other than 2Σ+ which are not observed in the usual photoemission experiment. [ABSTRACT FROM AUTHOR]

Because of the six equivalent carbon atoms in benzene, there is only one type of single core vacancy, but four different types of double core vacancies. The binding energies of single and double core vacancies are calculated and analyzed. Particular attention is paid thereby to reveal the relation between the single and double hole states. The π and σ electrons are found to contribute differently to the different types of double vacancies. It is generally stressed that double core ionization probes the bonding properties much more sensitively than single core ionization. [ABSTRACT FROM AUTHOR]

The outer valence double ionization transitions in the benzene molecule have been computed using Green’s functions and the results are discussed in connection with the Auger spectrum of this molecule. It is found that already at low energy the double ionization transitions are characterized by strong correlation effects and the appearance of a very large number of satellite states. The 226 computed dicationic states are analyzed in terms of their energy distribution weighted by their two-hole components. It is shown that the relative energies of the maxima in this distribution agree with the experimental Auger peaks to within 0.3 eV. These results emphasize the extreme usefulness of the method in the investigation of double ionization spectra of large molecules, which are practically beyond the reach of conventional ab initio approaches. [ABSTRACT FROM AUTHOR]

Abstract: The valence shell photoelectron spectra of uracil, 1-methyluracil and 6-methyluracil have been studied experimentally and theoretically. Synchrotron radiation has been used to record spectra at photon energies of 40 and 80eV. Photoelectron angular distributions have been determined and these provide an experimental means of distinguishing between σ- and π-type orbitals. Vertical ionization energies and spectral intensities have been evaluated using the many-body Green’s function approach, thereby enabling theoretical photoelectron spectra to be derived. The calculated spectra display a satisfactory agreement with the experimental data and this has allowed most of the photoelectron bands to be assigned. Two of the outer-valence vertical ionization energies are similar to one another and the vibrational progressions associated with these transitions overlap strongly. Vibronic interaction between these states, induced through the excitation of out-of-plane vibrational modes, may lead to nonadiabatic effects. Preliminary theoretical investigation of this interaction has been performed. [Copyright &y& Elsevier]