Jaroslav Koutecký

Effective core potential-configuration interaction study of electronic structure and geometry of small neutral and cationic Agn clusters : predictions and interpretation of measured properties

The ground state geometries of small neutral Agn(n=2–9) and cationic Agn+ (n=2–9) clusters have been determined in the framework of the Hartree–Fock and complete active space self‐consistent field (CASSCF) procedure employing a relativistic effective core potential (RECP) accounting for core–valence correlation effects (CVC). Similarities and differences between topologies of neutral and charged structures have been found. Large scale configuration interaction (CI) calculations for 5s electrons only have been carried out for determining stabilities and ionization potentials (IP). A comparison between predicted and measured observables allows tentative geometrical assignments. Structural and electronic properties of small Ia and Ib clusters are compared.

Silicon and germanium clusters. A theoretical study of their electronic structures and properties

Silicon and germanium clusters containing three to seven atoms have been studied with the pseudopotential MO‐LCAO method followed by configuration interaction procedure. Si and Ge clusters have very similar electronic structures and consequently analogous physico‐chemical properties but differ substantially from small carbon clusters. Linear structures are clearly less favorable than more compact structures. On the other hand, some planar geometries possess considerable stability. The Si and Ge clusters which are sections of the diamond‐type crystal lattice are less stable than clusters which can be considered as segments of closed‐packed lattices or as steps in pentagonal crystal growth. The reason is that the majority of atoms in small clusters are surface atoms which cannot assume the tetrahedral coordination characteristic of Si and Ge bulk atoms. The appearance of typical bulk properties is expected only for very large Si and Ge clusters with small surface atoms/bulk atoms ratio.

Evolution of the electronic structure of lithium clusters between four and eight atoms

Absorption spectra of lithium clusters containing four to eight atoms have been measured using depletion spectroscopy. Few intense transitions are observed, always located in two predominant spectral regions, ∼480 and 680 nm. The spectra are interpreted using ab initio configuration interaction (CI) calculations, leading to a complete characterization of the excited states and a straightforward determination of the ground state geometrical structure. Intense transitions are explained by interference effects in the transition amplitude and symmetry considerations. Comparisons with semiclassical models, in which an effective mass correction is introduced, are also presented.

Theoretical interpretation of the photoelectron detachment spectra of Na−2–5 and of the absorption spectra of Na3, Na4, and Na8 clusters

The configuration‐interaction (CI) study of excited states of alkali metal clusters accounts for spectroscopical patterns obtained from (i) the photoelectron detachment spectra of their anions and from (ii) the photodepletion spectra of the neutral species, reproduces observed excitation energies, intensities for allowed transitions, and permits an assignment of cluster structures. For Na−2–4 the linear anionic geometries are responsible for the photoelectron detachment spectra. In the case of Na−5, both planar and linear anionic isomers seem to contribute to the recorded spectrum. The calculation of optically allowed states for Na3(C2v) and Na4(D2h) structures and oscillator strengths yield rich spectra which have been fully assigned to the observed ones. In the case of Na8, the Td and the related D2d forms give rise to an intense transition located at ∼495 nm and the weak fine structure shifted to the red in full agreement with the measured spectrum. A molecular versus collective excitation interpretati...

Effective core potential‐configuration interaction study of electronic structure and geometry of small anionic Agn clusters: Predictions and interpretation of photodetachment spectra

The ground state geometries of small anionic Ag−n(n=2–9) clusters were determined in the framework of the Hartree–Fock procedure employing a relativistic effective core potential (RECP) accounting for core–valence correlation (CVC) effects. Large scale configuration interaction (CI) calculations for 5s electrons only were carried out in order to determine the ground state energies of anionic and neutral species as well as of excited states of the latter in the geometries of the former. The calculated vertical detachment and excitation energies account for the observed photodetachment spectroscopic patterns and permit an assignment of the cluster geometries. Structural and electronic properties of small Ia and Ib anionic clusters are compared.

Electronic and geometric structure of LI4 and Na4 clusters

The properties of the Li4 and Na 4 clusters are determined utilizing ab initio CI methods. The rhombic and bent square geometries of these alkalai metal clusters are predicted to be the most stable arrangements for the singlet and triplet states respectively. More symmetrical planar square and tetrahedron Li4 and Na4 clusters exhibit pronounced biradical features and are therefore highly unstable. The conjecture is made that the biradical character of some cluster arrangements might be connected with the catalytic activity of geometric irregularities in the surface of catalysator.

Abinitio MRD CI investigation of the optical spectra of C4 and C5 clusters

The ground and excited electronic states of linear and rhombic C4 and linear C5 clusters have been studied with ab initio single reference and multireference configuration interaction calculations. The spectrum of linear C4 is characterized by the existence of low‐lying Π states at 1–1.5 eV above the 3Σ−g ground state. In rhombic C4, which has very similar ground state energy as the linear form, the first allowed transition is found at 2.4 eV. The optical spectrum of linear C5 exhibits some similarities with that of linear C3: in both molecules the ground state is 1Σ+g and the lowest allowed transition, 1Πu←1Σ+g, is about 3 eV above the ground state.

Comparative study of tetramers built from Ia, IIa, IIIa, and IVa atoms

Abstract A simple analysis of the correlated (MRD-CI) wavefunctions of Ia, IIa, IIIa, and IVa tetramers explains the different stabilities of their geometries. The Jahn-Teller and pseudo-Jahn-Teller effects can destabilize very compact, highly symmetrical cluster shapes. Moreover, the s-p hybridization and the electron correlation are very important phenomena which can have decisive influence on the cluster stability.

Calculated properties of alkali metal clusters with fivefold symmetry

Ab initio calculations indicate a high stability for the pentagonal pyramid Li6 and the pentagonal bipyramid Li7 clusters. The pivotal role of the Jahn–Teller or pseudo‐Jahn–Teller effect in determining the optimal geometrical shape of small neutral and anionic Li clusters is demonstrated. The calculated electronic properties of the pentagonal bipyramids Li7, Na7, and K7 are in satisfactory agreement with recent ESR measurements.

Quantum molecular interpretation of the absorption spectra of Na5, Na6, and Na7 clusters

The configuration‐interaction study of the excited states of the most‐stable structures of Na5, Na6, and Na7 clusters employing ab initio effective‐core potential corrected by the core polarization potential predicts spectroscopic patterns which are in good agreement with the measured depletion spectra. A comparison of the transition energies and the oscillator strengths with the experimental data makes possible tentative structural assignments. Planar Na5 and Na6 structures and the three‐dimensional pentagonal bipyramid for Na7 appear responsible for the recorded spectra. The full agreement between theory and experiment is present for Na6 and Na7. The measured cross sections and calculated oscillator strengths for Na5 compare better for lower transition energies than for higher ones. A many‐electron description of the excited states of Na5, Na6, and Na7 yields a complete quantum molecular interpretation of the absorption spectra. From a comparison of the experimental and theoretical results the conclusio...