Volker Staemmler

Bio: Volker Staemmler is an academic researcher from Ruhr University Bochum. The author has contributed to research in topics: Ab initio & Ab initio quantum chemistry methods. The author has an hindex of 39, co-authored 161 publications receiving 5862 citations.

Exchangelike Effects for Closed-Shell Adsorbates: Interface Dipole and Work Function

Abstract: Based on detailed theoretical analyses, we present, for the first time, direct evidence that the significant interface dipole commonly observed for atoms and molecules physisorbed on metal surfaces originates from exchangelike effects. In the case of Xe, previously proposed contributions from chemical interactions do not play a significant role.

PNO–CI (pair natural orbital configuration interaction) and CEPA–PNO (coupled electron pair approximation with pair natural orbitals) calculations of molecular systems. I. Outline of the method for closed‐shell states

Abstract: The methods of configuration interaction with double substitutions to pair natural orbitals (PNO−CI) and of the coupled electron pair approximation (CEPA) proposed by W. Meyer are improved by combination with a new scheme of the calculation of the pair natural orbitals (PNO) and an efficient iterative scheme for the diagonalization of the CI matrix. The relevant matrix elements for the closed shell case are tabulated, the quantities that are pertinent for an analysis of the correlation energy are defined, and the organization of the computer programs is described.

PNO–CI (pair natural orbital configuration interaction) and CEPA–PNO (coupled electron pair approximation with pair natural orbitals) calculations of molecular systems. II. The molecules BeH2, BH, BH3, CH4, CH−3, NH3 (planar and pyramidal), H2O, OH+3, HF and the Ne atom

Abstract: PNO–CI and CEPA–PNO calculations are performed for the molecules MgH2, AlH, AlH3, SiH4, PH3, H2S, HCl, and the Ar atom. Two types of Gaussian basis sets are used; both sets contain one p‐set on H. The ’’small’’ set includes one d‐set on the heavy atom, the ’’standard’’ basis two d‐sets and one f‐set. Both for MgH2 and Ar, a ’’large’’ and a ’’very large’’ basis are used as well, which contain additional polarization functions. The energy improvement due to the different polarization functions is analyzed. Hartree–Fock limits for the molecular energies are estimated. The computed valence shell correlation energies are analyzed in terms of quantities defined in part I, in particular in terms of the IEPA (independent electron pair) correlation energies eμ IEPA and the error ΔE IEPA of the IEPA scheme. Both the valence shell interorbital pair correlation energies and the IEPA error are smaller in absolute value than those of the corresponding first row hydrides, provided that one uses the localized representation. MgH2, AlH3, and SiH4 are ’’good IEPA molecules’’, i.e., ‖ΔE IEPA‖ is unusually small for them. For Ar, calculations that include the L and M shell, and the K, L, and M shell correlation energy are reported. The best known variational energy of the Ar atom is obtained as −527.2592 a.u., the (nonvariational) CEPA energy being −527.2916 a.u. For the unknown molecules MgH2 and AlH3, binding energies, equilibrium geometries, and symmetric‐stretching force constants are predicted. MgH2 has a binding energy of 104 kcal/mol referred to Mg+2H; it is hence expected to be unstable with respect to Mg+H2. The predicted binding energy of AlH3 is ? 205 kcal/mol referred to Al+3H. The inversion barrier of PH3 which amounts to ? 38 kcal/mol on SCF level is reduced by electron correlation to ? 35 kcal/mol.

Molecular adsorption on oxide surfaces: Electronic structure and orientation of NO on NiO(100)/Ni(100) and on NiO(100) as determined from electron spectroscopies and ab initio cluster calculations

Abstract: We have investigated the adsorption of NO on a thin NiO(100) film of several layers thickness grown on top of a Ni(100) surface in comparison with data of an in vacuo cleaved NiO(100) single crystal. The layer exhibits a high defect density. We demonstrate via application of several surface-sensitive electron-spectroscopic techniques [i.e., x-ray photoelectron spectroscopy (XPS), angle-resolved ultraviolet photoelectron spectroscopy (ARUPS), near-edge x-ray-absorption fine structure (NEXAFS), and high-resolution electron-energy-loss spectroscopy (HREELS)] that this layer has similar occupied (ARUPS) and unoccupied (NEXAFS) states as a bulk NiO(100) sample. In spite of its limited thickness, the band structure of the film exhibits dispersions perpendicular to the surface compatible with bulk NiO(100). It is shown that the electronic structure of the oxygen sublattice can be described in a band-structure picture while for the Ni sublattice electron localization effects lead to a breakdown of the band-structure picture. NO on NiO desorbs at 220 K. This indicates weak chemisorption. The NO coverage is close to 0.2 relative to the number of Ni surface atoms as determined by XPS.HREELS reveals that there is only one species on the surface documented by the observation of only one bond-stretching frequency. NEXAFS data on the system and a comparison with previous data on the system NO/Ni(100) indicate that the molecular axis of adsorbed NO is tilted by an angle of approximately 45\ifmmode^\circ\else\textdegree\fi{} relative to the surface normal. The N 1s XP spectra of the weakly chemisorbed species show giant satellites similar to the previously observed cases for weak chemisorption on metal surfaces. This is the first observation of an intense satellite structure for an adsorbate on an insulator surface, which shows that there must be sufficient screening channels even on an insulating surface. A theoretical assignment of the peaks is discussed. We compare the spectroscopic properties of the NO species on the thin-film oxide surface, which is likely to contain a certain number of defects, with NO adsorbed on a basically defect-free bulk oxide surface by thermal-desorption (TDS) and XP spectra. TDS and XP spectra of the bulk system are basically identical as compared with the oxide film, indicating that the majority of species adsorbed on the film is not adsorbed on defects but rather on regular NiO sites. Results of ab initio oxide cluster calculations are used to explain the bonding geometry of NO on regular NiO sites.

A multi-configuration reference CEPA method based on pair natural orbitals

Abstract: A multi-reference CI scheme is proposed which is aiming at a considerable reduction of the generally very large number of configurations of CI expansions in multi-configuration reference cases This reduction is achieved by combining the idea of internal contraction, the concept of pair natural orbitals (PNO's) and CEPA (coupled electron pair) type approximations for the contributions of higher than double excitations This latter estimate leads to size consistent results and also permits to employ reference wavefunctions that contain only the dominantly occupied configurations of the considered system Applications to two test cases, the lowest states (3 P,1 D and1 S) of the carbon atom and the symmetry forbiddenC 2v insertion reaction of Be and H2, show that our method is able to truncate CI expansions to lengths of no more than 103–104 without losing more than 1–2% of the correlation energy The calculated excitation energies and energy barriers agree with the full CI results in the respective basis within about 1 kcal/mol Thus the MC-CEPA-PNO method presents a very efficient way to obtain “chemical accuracy” in CI-calculations for molecular systems

Zinc oxide nanostructures: growth, properties and applications

Abstract: Zinc oxide is a unique material that exhibits semiconducting and piezoelectric dual properties. Using a solid–vapour phase thermal sublimation technique, nanocombs, nanorings, nanohelixes/nanosprings, nanobelts, nanowires and nanocages of ZnO have been synthesized under specific growth conditions. These unique nanostructures unambiguously demonstrate that ZnO probably has the richest family of nanostructures among all materials, both in structures and in properties. The nanostructures could have novel applications in optoelectronics, sensors, transducers and biomedical sciences. This article reviews the various nanostructures of ZnO grown by the solid–vapour phase technique and their corresponding growth mechanisms. The application of ZnO nanobelts as nanosensors, nanocantilevers, field effect transistors and nanoresonators is demonstrated.

Magnetic metal-organic frameworks.

Abstract: The purpose of this critical review is to give a representative and comprehensive overview of the arising developments in the field of magnetic metal–organic frameworks, in particular those containing cobalt(II). We examine the diversity of magnetic exchange interactions between nearest-neighbour moment carriers, covering from dimers to oligomers and discuss their implications in infinite chains, layers and networks, having a variety of topologies. We progress to the different forms of short-range magnetic ordering, giving rise to single-molecule-magnets and single-chain-magnets, to long-range ordering of two- and three-dimensional networks (323 references).

Natural localized molecular orbitals

Abstract: The method of natural localized molecular orbitals (NLMOs) is presented as a novel and efficient technique for obtaining LMOs for SCF and CI wave functions. It is an extension of the previously developed natural atomic orbital (NAO) and natural bond orbital (NBO) methods, and uses only the information contained in the one‐particle density matrix. Results are presented for methane and cytosine to indicate that NLMOs closely resemble LMOs obtained by the Boys and Edmiston–Ruedenberg methods, with the exception that the NLMO procedure automatically preserves the MO σ–π separation in planar molecules. The computation time is modest, generally only a small fraction of the SCF computation time. In addition, the derivation of NLMOs from NBOs gives direct insight into the nature of the LMO ‘‘delocalization tails,’’ thus enhancing the role of LMOs as a bridge between chemical intuition and molecular wave functions.