Thomas Bredow

Consistent Gaussian basis sets of triple-zeta valence with polarization quality for solid-state calculations

Consistent basis sets of triple‐zeta valence with polarization quality for main group elements and transition metals from row one to three have been derived for periodic quantum‐chemical solid‐state calculations with the crystalline‐orbital program CRYSTAL. They are based on the def2‐TZVP basis sets developed for molecules by the Ahlrichs group. Orbital exponents and contraction coefficients have been modified and reoptimized, to provide robust and stable self‐consistant field (SCF) convergence for a wide range of different compounds. We compare results on crystal structures, cohesive energies, and solid‐state reaction enthalpies with the modified basis sets, denoted as pob‐TZVP, with selected standard basis sets available from the CRYSTAL basis set database. The average deviation of calculated lattice parameters obtained with a selected density functional, the hybrid method PW1PW, from experimental reference is smaller with pob‐TZVP than with standard basis sets, in particular for metallic systems. The effects of basis set expansion by diffuse and polarization functions were investigated for selected systems.

Theoretical investigation of water adsorption at rutile and anatase surfaces

Abstract The semiempirical MO method SINDO1 was used for the investigation of molecular and dissociative adsorption of water at titanium dioxide surfaces rutile (110) and anatase (001). The surfaces were simulated with model clusters. The influence of long-range interactions was accounted for by increase of the clusters. It was shown that a qualitatively and semiquantitatively correct description of adsorption is possible only with sufficiently large model clusters and under consideration of local relaxations. In agreement with the literature, the dissociative adsorption is energetically favored at both surfaces. For the rutile (110) surface the water adsorption was studied also in the case of oxygen defects. The adsorption energies at defect positions were found to be higher by a factor of two to three than for the ideal surface. This confirms experimental investigations which find increased activity at surfaces with defects. The local relaxation at the defect site plays an important role for the determination of relative stabilities of different adsorption sites. A potential curve was calculated for the dissociation of a water molecule adsorbed at a rutile (110) surface, and the influence of hydrogen bonding and local relaxation on the reaction barrier was determined.

Atomically controlled electrochemical nucleation at superionic solid electrolyte surfaces

Electrochemical equilibrium and the transfer of mass and charge through interfaces at the atomic scale are of fundamental importance for the microscopic understanding of elementary physicochemical processes. Approaching atomic dimensions, phase instabilities and instrumentation limits restrict the resolution. Here we show an ultimate lateral, mass and charge resolution during electrochemical Ag phase formation at the surface of RbAg(4)I(5) superionic conductor thin films. We found that a small amount of electron donors in the solid electrolyte enables scanning tunnelling microscope measurements and atomically resolved imaging. We demonstrate that Ag critical nucleus formation is rate limiting. The Gibbs energy of this process takes discrete values and the number of atoms of the critical nucleus remains constant over a large range of applied potentials. Our approach is crucial to elucidate the mechanism of atomic switches and highlights the possibility of extending this method to a variety of other electrochemical systems.

Organometallic Benzene-Vanadium Wire: A One-Dimensional Half-Metallic Ferromagnet

Using density functional theory we perform theoretical investigations of the electronic properties of a freestanding one-dimensional organometallic vanadium-benzene wire. This system represents the limiting case of multidecker Vn(C6H6)(n+1) clusters which can be synthesized with established methods. We predict that the ground state of the wire is a 100% spin-polarized ferromagnet (half-metal). Its density of states is metallic at the Fermi energy for the minority electrons and shows a semiconductor gap for the majority electrons. We find that the half-metallic behavior is conserved up to 12% longitudinal elongation of the wire. Ab initio electron transport calculations reveal that finite size vanadium-benzene clusters coupled to ferromagnetic Ni or Co electrodes will work as nearly perfect spin filters.

System‐Dependent Dispersion Coefficients for the DFT‐D3 Treatment of Adsorption Processes on Ionic Surfaces

Dispersion-corrected density functional theory calculations (DFT-D3) were performed for the adsorption of CO on MgO and C(2) H(2) on NaCl surfaces. An extension of our non-empirical scheme for the computation of atom-in-molecules dispersion coefficients is proposed. It is based on electrostatically embedded M(4)X(4) (M=Na, Mg) clusters that are used in TDDFT calculations of dynamic dipole polarizabilities. We find that the C(MM)(6) dispersion coefficients for bulk NaCl and MgO are reduced by factors of about 100 and 35 for Na and Mg, respectively, compared to the values of the free atoms. These are used in periodic DFT calculations with the revPBE semi-local density functional. As demonstrated by calculations of adsorption potential energy curves, the new C(6) coefficients lead to much more accurate energies (E(ads)) and molecule-surface distances than with previous DFT-D schemes. For NaCl/C(2) H(2) we obtained at the revPBE-D3(BJ) level a value of E(ads) =-7.4 kcal mol(-1) in good agreement with experimental data (-5.7 to -7.1 kcal mol(-1)). Dispersion-uncorrected DFT yields an unbound surface state. For the MgO/CO system, the computed revPBE-D3(BJ) value of E(ads) =-4.1 kcal mol(-1) is also in reasonable agreement with experimental results (-3.0 kcal mol(-1)) when thermal corrections are taken into account. Our new dispersion correction also improves computed lattice constants of the bulk systems significantly compared to plain DFT or previous DFT-D results. The extended DFT-D3 scheme also provides accurate non-covalent interactions for ionic systems without empirical adjustments and is suggested as a general tool in surface science.

MSINDO parameterization for third‐row transition metals

The recently developed MSINDO version of the semiempirical SCF MO method SINDO1 has been parameterized for third‐row transition metals Sc to Zn. The set of reference data used for the previous parameterization of SINDO1 has been substantially increased by incorporating results of recent experiments and first‐principles calculations. A comparison of calculated heats of formation, geometries, ionization potentials, and dipole moments with literature values for more than 200 gas phase molecules is presented. The accuracy of the modified MSINDO version achieved for heats of formation and bond lengths has been considerably improved compared to SINDO1. Small clusters of transition metals and metal oxides were included in the parameterization to ensure accurate results for studies of larger systems. The application of the method to small transition metal complexes that were not included in the parameterization shows that the optimized parameters are transferable to other compounds.

ATR-FTIR measurements and quantum chemical calculations concerning the adsorption and photoreaction of oxalic acid on TiO2

The adsorption and photoreaction of oxalic acid on the surface of anatase and rutile TiO2 nanoparticles have been studied using a combined experimental and theoretical approach. In the dark, the experimental adsorption reaches an equilibrium state that can be described as a mixture of adsorbed water and oxalic acid molecules, with the latter forming two different surface complexes on anatase and one on rutile particles. When the system is subsequently illuminated with UV(A) light, the surface becomes enriched with absorbed oxalic acid, which replaces photo-desorbed water molecules, and one of the adsorbed oxalic acid structures on anatase is favoured over the other.

Geometrical Correction for the Inter- and Intramolecular Basis Set Superposition Error in Periodic Density Functional Theory Calculations

We extend the previously developed geometrical correction for the inter- and intramolecular basis set superposition error (gCP) to periodic density functional theory (DFT) calculations. We report gCP results compared to those from the standard Boys-Bernardi counterpoise correction scheme and large basis set calculations. The applicability of the method to molecular crystals as the main target is tested for the benchmark set X23. It consists of 23 noncovalently bound crystals as introduced by Johnson et al. (J. Chem. Phys. 2012, 137, 054103) and refined by Tkatchenko et al. (J. Chem. Phys. 2013, 139, 024705). In order to accurately describe long-range electron correlation effects, we use the standard atom-pairwise dispersion correction scheme DFT-D3. We show that a combination of DFT energies with small atom-centered basis sets, the D3 dispersion correction, and the gCP correction can accurately describe van der Waals and hydrogen-bonded crystals. Mean absolute deviations of the X23 sublimation energies can be reduced by more than 70% and 80% for the standard functionals PBE and B3LYP, respectively, to small residual mean absolute deviations of about 2 kcal/mol (corresponding to 13% of the average sublimation energy). As a further test, we compute the interlayer interaction of graphite for varying distances and obtain a good equilibrium distance and interaction energy of 6.75 Å and -43.0 meV/atom at the PBE-D3-gCP/SVP level. We fit the gCP scheme for a recently developed pob-TZVP solid-state basis set and obtain reasonable results for the X23 benchmark set and the potential energy curve for water adsorption on a nickel (110) surface.

Implementation of Empirical Dispersion Corrections to Density Functional Theory for Periodic Systems

A recently developed empirical dispersion correction (Grimme et al., J. Chem. Phys. 2010, 132, 154104) to standard density functional theory (DFT‐D3) is implemented in the plane‐wave program package VASP. The DFT‐D3 implementation is compared with an implementation of the earlier DFT‐D2 version (Grimme, J. Comput. Chem. 2004, 25, 1463; Grimme, J. Comput. Chem. 2006, 27, 1787). Summation of empirical pair potential terms is performed over all atom pairs in the reference cell and over atoms in shells of neighboring cells until convergence of the dispersion energy is obtained. For DFT‐D3, the definition of coordination numbers has to be modified with respect to the molecular version to ensure convergence. The effect of three‐center terms as implemented in the original molecular DFT‐D3 version is investigated. The empirical parameters are taken from the original DFT‐D3 version where they had been optimized for a reference set of small molecules. As the coordination numbers of atoms in bulk and surfaces are much larger than in the reference compounds, this effect has to be discussed. The results of test calculations for bulk properties of metals, metal oxides, benzene, and graphite indicate that the original parameters are also suitable for solid‐state systems. In particular, the interlayer distance in bulk graphite and lattice constants of molecular crystals is considerably improved over standard functionals. With the molecular standard parameters (Grimme et al., J. Chem. Phys. 2010, 132, 154104; Grimme, J. Comput. Chem. 2006, 27, 1787) a slight overbinding is observed for ionic oxides where dispersion should not contribute to the bond. For simple adsorbate systems, such as Xe atoms and benzene on Ag(111), the DFT‐D implementations reproduce experimental results with a similar accuracy as more sophisticated approaches based on perturbation theory (Rohlfing and Bredow, Phys. Rev. Lett. 2008, 101, 266106).

Adsorption of oxalate on anatase (100) and rutile (110) surfaces in aqueous systems: experimental results vs. theoretical predictions.

A combined experimental and theoretical study of the adsorption of oxalic acid from the aqueous phase at the surface of anatase nanoparticles has been performed. The interfaces were investigated by ATR-FTIR measurements and quantum-chemical calculations using the semiempirical method MSINDO. The vibration spectra of the most stable surface complexes have been calculated and used for the interpretation of experimental results. The theoretical studies have been done using the anatase (100) surface to model the adsorption of oxalic acid and water. The effect of interaction of water and oxalic acid on the adsorption mechanism and the vibration spectra was taken into account in the theoretical models. Inclusion of solvation effects was found crucial to determine the type of denticity and structure of adsorbed complexes. By comparison of experimental data and theoretical calculations the most likely surface species and the effects of hydration in their relative stabilities were determined. The present results are compared to previous studies preformed also by combination of experimental and theoretical calculations of analogous systems using nanoparticulate rutile [C. B. Mendive et al., Phys. Chem. Chem. Phys., 2008, 10, 1960, ref. 1]. Differences between surface complexes on anatase and rutile lie mainly on the denticity type. Whilst in the case of rutile the most stable species consist of two bidentate surface complexes followed in third place by a monodentated form, anatase allows the formation of four species in which the stability order is reversed with respect to the denticity type. In the case of anatase, the main contributors to the surface speciation are two monodentate species differing in the position of the H atom within the molecule (being more stable when it is placed in the O-C-O moiety not bound to the surface); and two bidentate species, one deprotonated and one monoprotonated, in which the C-C bond was parallel or perpendicular to the TiO2 surface, respectively.