Site / 2008  
AG Paulus Archive
AG Paulus Website

The electronic structure of linear Beryllium chains has been theoretically studied by using ab initio methods. It turns out that, for internuclear distances close to the equilibrium structure, two partially filled edge orbitals (‘end states’) localize at the chain ends. This gives rise to two lowlying states, a singlet ground state, 1Σg, and a quasidegenerate triplet, 3Σu. The energy splitting goes rapidly to zero as the number of atoms in the chain is increased. Preliminary investigations indicate that this could be a general behavior shared by the metals belonging to the Groups 2 and 12.
The precursor of the MetalInsulator transition is studied at abinitio level in equallyspaced Lin and Ben linear chains. In particular, largesize Full ConfigurationInteraction (FCI) calculations (up to one billion determinants for Li_{8} and half a billion for Be_{5} ) have been performed. At FCI level, it is possible to take into account the different nature of the electronic wavefunction at the different internuclear distances. Several indicators of the MetalInsulator transition (minimum of the Koopmans energy gap, maximum of the localization and polarizability tensors) are considered and discussed. In is shown that the three considered properties indicate a MetalInsulator transition at distances that are in remarkable agreement.
Calculations based upon the incremental approach, i.e. an expansion of the correlation energy in terms of onebody, twobody, and higherorder contributions from localized orbital groups, have been performed for metallic beryllium. We apply an embedding scheme which has been successfully applied recently to groundstate properties of magnesium and group 12 elements. This scheme forces localization in metalliclike model systems and allows for a gradual delocalization within the incremental approach. Quantumchemical methods of the coupledcluster and multireference configuration interaction type are used for evaluating individual increments. Results are given for the cohesive energy and lattice constants of beryllium, and it is shown that further development of the approach is needed for this difficult case.
Two types of oxygendeficient perovskites RBaCo_{2}O_{5.5}(R=Ho,Gd) related to the “122” type structure (a p × 2a p × 2a p ) have been studied on the basis of ab initio cluster calculations. We consider the peculiar behavior of the trivalent ions Co^{3+}(3d^{6}) in either octahedral or pyramidal oxygen coordinations, which is related to a structural firstorder phase transition in both compounds. Relative energy positions of low spin (LS, S = 0), intermediate spin (IS, S = 1) and high spin (HS, S = 2) electron configurations are calculated for the lowand hightemperature lattice structures of RBaCo_{2}O_{5.5}. A combined analysis of the calculated results and experimental structural data leads to a simple model that captures the most prominent features of the phase transition common to both compounds.
The effect of electroncorrelation on the groundstate properties of CeN and LaN is studied by ab initio quantumchemical methods. The approach which is used combines two separate steps: (1) the groundstate HartreeFock calculations for the crystal; (2) application of the method of increments to the studied system, which allows an expansion of bulk properties using the information from quantumchemical calculations performed for finite clusters. As can be expected, for CeN correlation plays a significant role: with HartreeFock method only 49% of the experimental cohesive energy has been recovered, whereas after correlation corrections (coupledcluster approach) the groundstate properties were found to be in good agreement with the experimental data found in literature. Thus, we obtained about 90% of the expected cohesive energy; the computed lattice constants and bulk moduli also agree well with the experimental values. For comparison, the equivalent treatment has been performed for LaN, where no f orbital is occupied. There the HF contribution to the groundstate properties is larger and hence the correlation effects weaker.
Spin and angleresolved photoelectron spectroscopy was applied for studies of electronic and magnetic structures of Eu/Gd and Ce/Fe. Ferromagnetic coupling of 4f moments of Eu and Gd was found in the 1 ML Eu/Gd(0 0 0 1) system with high net Eu magnetization at low temperatures reflected by a spin polarization of 15% of the Eu 4f state. In case of the 1 ML Ce/Fe(1 1 0) system the antiparallel orientation of the Ce 4f spins with respect to the magnetization direction of the Fe substrate was observed. Very different shapes of the spinup and spindown Ce 4f spectral weights can be explained within periodic Anderson model by spindependent hybridization between Ce localized 4f and itinerant valence band states.
Zinc crystallizes in the hcp structure, but with an anomalously large c/a ratio, indicating a strong distortion away from ideal packing. Coupled cluster calculations within the framework of the method of increments, improved by an embedding scheme for metals, were performed to explore the potential energy surface of zinc with respect to the hexagonal lattice parameters. The inclusion of the filled d shell in the correlation treatment proved to be essential. From the exceptional shape of the potential energy surface the existence of a zinc modification with a nearly ideal c/a ratio can be deduced.
We have combined an embeddedcluster model with an extension of the method of increments to treat the adsorption of molecules on a surface. In this way we are able to investigate the physisorption of CO on CeO_{2}(110) at the MP2, MP4(SDTQ), and CCSD(T) levels with only moderate computational costs. We find that, at the CCSD(T) level, 25% of the adsorption energy originates from electron correlation. The interactions of the CO molecule with its five nearest cerium and oxygen neighbors in the surface layer make the largest contributions to the electron correlation. Approximately 97% of the adsorptioninduced electron correlation energy part of the adsorption energy is recovered by the method of increments (in our chosen expansion), at the MP2 level. 
↑ Top
View
Edit
Attributes
History
Attach
Print
This page was last modified on September 24, 2021, at 01:48 PM 