The DIRAC code for relativistic molecular calculations.

DIRAC is a freely distributed general-purpose program system for one-, two-, and four-component relativistic molecular calculations at the level of Hartree-Fock, Kohn-Sham (including range-separated theory), multiconfigurational self-consistent-field, multireference configuration interaction, electron propagator, and various flavors of coupled cluster theory. At the self-consistent-field level, a highly original scheme, based on quaternion algebra, is implemented for the treatment of both spatial and time reversal symmetry. DIRAC features a very general module for the calculation of molecular properties that to a large extent may be defined by the user and further analyzed through a powerful visualization module. It allows for the inclusion of environmental effects through three different classes of increasingly sophisticated embedding approaches: the implicit solvation polarizable continuum model, the explicit polarizable embedding model, and the frozen density embedding model.

Effect of spin-orbit coupling on decay widths of electronic decay processes.

Auger-Meitner processes are electronic decay processes of energetically low-lying vacancies. In these processes, the vacancy is filled by an electron of an energetically higher lying orbital, while another electron is simultaneously emitted to the continuum. In low-lying orbitals, relativistic effects can not, even for light elements, be neglected. At the same time, lifetime calculations are computationally expensive. In this context, we investigate which effect spin-orbit coupling has on Auger-Meitner decay widths and aim for a rule of thumb for the relative decay widths of initial states split by spin-orbit coupling. We base this rule of thumb on Auger-Meitner decay widths of Sr4p-1 and Ra6p-1 obtained by relativistic FanoADC-Stieltjes calculations and validate it against Auger-Meitner decay widths from the literature.

The Second-Order-Polarization-Propagator-Approximation (SOPPA) in a four-component spinor basis.

A theoretical framework for understanding molecular structures is crucial for the development of new technologies such as catalysts or solar cells. Apart from electronic excitation energies, however, only spectroscopic properties of molecules consisting of lighter elements can be computationally described at a high level of theory today since heavy elements require a relativistic framework, and thus far, most methods have only been derived in a non-relativistic framework. Important new technologies such as those mentioned above require molecules that contain heavier elements, and hence, there is a great need for the development of relativistic computational methods at a higher level of accuracy. Here, the Second-Order-Polarization-Propagator-Approximation (SOPPA), which has proven to be very successful in the non-relativistic case, is adapted to a relativistic framework. The equations for SOPPA are presented in their most general form, i.e., in a non-canonical spin-orbital basis, which can be reduced to the canonical case, and the expressions needed for a relativistic four-component SOPPA are obtained. The equations are one-index transformed, giving more compact expressions that correspond to those already available for the four-component RPA. The equations are ready for implementation in a four-component quantum chemistry program, which will allow both linear response properties and excitation energies to be calculated relativistically at the SOPPA level.

Relativistic decay widths of autoionization processes: the relativistic FanoADC-Stieltjes method.

Electronic decay processes of ionized systems are, for example, the Auger decay or the Interatomic/ Intermolecular Coulombic Decay. In both processes, an energetically low lying vacancy is filled by an electron of an energetically higher lying orbital and a secondary electron is instantaneously emitted to the continuum. Whether or not such a process occurs depends both on the energetic accessibility and the corresponding lifetime compared to the lifetime of competing decay mechanisms. We present a realization of the non-relativistically established FanoADC-Stieltjes method for the description of autoionization decay widths including relativistic effects. This procedure, being based on the Algebraic Diagrammatic Construction (ADC), was adapted to the relativistic framework and implemented into the relativistic quantum chemistry program package Dirac. It is, in contrast to other existing relativistic atomic codes, not limited to the description of autoionization lifetimes in spherically symmetric systems, but is instead also applicable to molecules and clusters. We employ this method to the Auger processes following the Kr3d(-1), Xe4d(-1), and Rn5d(-1) ionization. Based on the results, we show a pronounced influence of mainly scalar-relativistic effects on the decay widths of autoionization processes.

Interatomic decay of inner-valence ionized states in ArXe clusters: relativistic approach.

In this work we investigate interatomic electronic decay processes taking place in mixed argon-xenon clusters upon the inner-valence ionization of an argon center. We demonstrate that both interatomic Coulombic decay and electron-transfer mediated decay (ETMD) are important in larger rare gas clusters as opposed to dimers. Calculated secondary electron spectra are shown to depend strongly on the spin-orbit coupling in the final states of the decay as well as the presence of polarizable environment. It follows from our calculations that ETMD is a pure interface process taking place between the argon-xenon layers. The interplay of all these effects is investigated in order to arrive at a suitable physical model for the decay of inner-valence vacancies taking place in mixed ArXe clusters.

Possible electronic decay channels in the ionization spectra of small clusters composed of Ar and Xe: A four-component relativistic treatment.

Electronic decay of the inner-valence Ar 3s(-1) vacancy is energetically forbidden in an isolated argon atom and in all rare gas dimers where argon is present. However, if an argon atom has at least two suitable rare gas atoms in its neighborhood, the Ar 3s(-1) vacancy may decay electronically via an electron transfer mediated decay (ETMD) mechanism. An ArXe(2) cluster is considered in the present paper as an example of such systems. The single and double ionization spectra of different ArXe(2) isomers as well as of homonuclear Ar(2) and Xe(2) and heteronuclear ArXe clusters have been calculated by means of propagator methods to reveal possible electronic decay channels. A four-component version of the one-particle propagator utilizing the Dirac-Coulomb Hamiltonian was employed to obtain the single ionization potentials of the clusters studied. Hereby electron correlation, scalar relativistic effects, and spin-orbit couplings are described in a consistent manner. A two-particle propagator in its one-component form, in conjunction with effective core potentials to account consistently for correlation and scalar relativistic effects, was used to calculate the double ionization potentials. ETMD is shown to be the only possible electronic decay process of the Ar 3s(-1) vacancy in the ArXe(2) cluster. In clusters with more Xe atoms, alternative electronic decay mechanisms may appear.