Georg Schreckenbach

QUASI: A general purpose implementation of the QM/MM approach and its application to problems in catalysis

Abstract We describe the work of the European project QUASI (Quantum Simulation in Industry, project EP25047) which has sought to develop a flexible QM/MM scheme and to apply it to a range of industrial problems. A number of QM/MM approaches were implemented within the computational chemistry scripting system, ChemShell, which provides the framework for deploying a variety of independent program packages. This software was applied in several large-scale QM/MM studies which addressed the catalytic decomposition of N 2 O by Cu-containing zeolites, the methanol synthesis reaction catalysed by Cu clusters supported on ZnO surfaces, and the modelling of enzyme structure and reactivity.

Calculation of NMR shielding tensors based on density functional theory and a scalar relativistic Pauli-type Hamiltonian. The application to transition metal complexes

This article deals with the calculation of the shielding tensor of nuclear magnetic resonance (NMR) spectroscopy based on a scalar relativistic two-component Pauli-type Hamiltonian. A complete formulation of the method within the framework of the gauge including atomic orbitals (GIAO) is given. Further, an implementation, based on density functional theory (DFT) is presented. The new method is applied to the 17O shielding in transition-metal oxo complexes [MO4]n- (M = Cr, Mo, W; Mn, Tc, Rh; Ru, Os) and to the metal chemical shift in transition-metal carbonyls M(CO)6 (M = Cr, Mo, W). This represents the first calculation of heavy-element shifts that is based on a relativistic first-principle quantum mechanical method. The inclusion of relativity is crucial for a proper description of ligand and metal shieldings in 5d complexes, but it is also important in 4d complexes. Limitations of the method, among them the neglect of the spin-orbit coupling, are discussed in detail.

DENSITY FUNCTIONAL CALCULATIONS OF NMR CHEMICAL SHIFTS AND ESR G-TENSORS

Abstract. An overview is given on recent advances of density functional theory (DFT) as applied to the calculation of nuclear magnetic resonance (NMR) chemical shifts and electron spin resonance (ESR) g-tensors. This is a new research area that has seen tremendous progress and success recently; we try to present some of these developments. DFT accounts for correlation effects efficiently. Therefore, it is the only first-principle method that can handle NMR calculations on large systems like transition-metal complexes. Relativistic effects become important for heavier element compounds; here we show how they can be accounted for. The ESR g-tensor is related conceptually to the NMR shielding, and results of g-tensor calculations are presented. DFT has been very successful in its application to magnetic properties, for metal complexes in particular. However, there are still certain shortcomings and limitations, e.g., in the exchange-correlation functional, that are discussed as well.

Density Functional Calculations on Actinide Compounds: Survey of Recent Progress and Application to [UO2X4]2 (X = F, Cl, OH) and AnF6 (An = U, Np, Pu)

The subject of this article, the application of density functional theory (DFT) to molecular systems containing actinide elements, is discussed in two parts. In the first part, a survey is given of DFT applications on actinide‐containing molecules. Various methodological developments are reviewed, including, among others, new relativistic effective core potentials (ECP), and newly developed stable relativistic DFT methods. Actual DFT calculations of actinide molecular systems are discussed, covering the time from about 1991 to the present. In the second part, two different DFT‐based relativistic methods are applied to some actinide molecules. These are ECPs and the quasirelativistic (QR) method. Systems studied include actinide hexafluorides AnF6 (An=U, Np, Pu) and uranyl (VI) anions [UO2X4]2− (X=OH, F, Cl). Calculated geometries and vibrational frequencies are discussed and compared with experiment. The two relativistic methods have been combined with the BLYP and B3LYP density functionals. The ECP‐B3LYP and QR‐BLYP approaches gave the best bond lengths and frequencies. The existence of stable structures with a bent uranyl bond (“cis‐uranyl”) is predicted for all three [UO2X4]2− ions. ECP‐B3LYP predicts the following order for the stability of the “cis” conformers of [UO2X4]2− (relative to the respective global energy minimum): OH>F>Cl with the “cis”‐[UO2Cl4]2− being least stable. The article concludes with a discussion of future directions for the application of DFT to the f‐block chemistry. © 1999 John Wiley & Sons, Inc. J Comput Chem 20: 70–90, 1999

Strongly coupled binuclear uranium–oxo complexes from uranyl oxo rearrangement and reductive silylation

The most common motif in uranium chemistry is the d(0)f(0) uranyl ion [UO(2)](2+) in which the oxo groups are rigorously linear and inert. Alternative geometries, such as the cis-uranyl, have been identified theoretically and implicated in oxo-atom transfer reactions that are relevant to environmental speciation and nuclear waste remediation. Single electron reduction is now known to impart greater oxo-group reactivity, but with retention of the linear OUO motif, and reactions of the oxo groups to form new covalent bonds remain rare. Here, we describe the synthesis, structure, reactivity and magnetic properties of a binuclear uranium-oxo complex. Formed through a combination of reduction and oxo-silylation and migration from a trans to a cis position, the new butterfly-shaped Si-OUO(2)UO-Si molecule shows remarkably strong U(V)-U(V) coupling and chemical inertness, suggesting that this rearranged uranium oxo motif might exist for other actinide species in the environment, and have relevance to the aggregation of actinide oxide clusters.

THE CALCULATION OF NMR SHIELDING TENSORS BASED ON DENSITY FUNCTIONAL THEORY AND THE FROZEN-CORE APPROXIMATION

The application of the frozen-core approximation to the calculation of the shielding tensor of nuclear magnetic resonance (NMR) spectroscopy is discussed and an implementation is presented. A complete formulation of the shielding calculation within the frozen-core approximation is given, both in general terms and for the special case of density functional theory (DFT) and “gauge including atomic orbitals” (GIAOs). The practical implementation is validated by a detailed discussion of the consequences of the approximation. The general conclusion is drawn that the frozen-core approximation is a useful tool for shielding calculations—if the valence space is increased to contain at least the ns, np, (n − 1)p, (n − 1)d (fourth period and higher) shells, where n is the number of the given period in the periodic table of elements. The new method is applied to 77Se shieldings and chemical shifts for a small number of compounds. The agreement between theory and experiment is good for relative shifts, whereas calculated absolute shieldings are generally too small by about 300–400 ppm. This difference is attributed to the relativistic contraction of the core density at the selenium atom that had been explicitly incorporated into the experimental absolute shielding scale.

Large Enhancement and Tunable Band Gap in Silicene by Small Organic Molecule Adsorption

Adsorption of eight organic molecules (acetone, acetonitrile, ammonia, benzene, methane, methanol, ethanol, and toluene) onto silicene has been investigated using van der Waals density functional theory calculations (DFT-D). The calculated values of the adsorption energies vary from −0.11 to −0.95 eV. Quantitatively, these values are higher than the corresponding adsorption energies of the molecules adsorbed on graphene. In addition, electronic structure calculations have been performed. The obtained values of the band gap range from 0.006 to 0.35 eV for acetonitrile to acetone, respectively. Furthermore, the effective mass of the electron is estimated and found to be comparatively small, which is expected to result in high electron mobility. In addition, we study the effect of Li atoms doped in pristine and acetone adsorbed silicene. In particular, we focus on the variation of the adsorption energy with respect to the number of Li atoms in the systems. Our results suggest new approaches for the use of si...

The 57Fe nuclear magnetic resonance shielding in ferrocene revisited. A density-functional study of orbital energies, shielding mechanisms, and the influence of the exchange-correlation functional

The 57Fe nuclear magnetic resonance (NMR) shielding and chemical shift in ferrocene, Fe(C5H5)2, are studied using density functional theory (DFT) and gauge-including atomic orbitals (GIAO). Electronic factors contributing to the chemical shift are discussed in detail. It is shown that the chemical shift is entirely determined by paramagnetic contributions which in turn are dominated by metal based occupied-virtual d→d couplings. In particular, the HOMO-1(a1′) and the HOMO (e2′) couple with the LUMO (e1″). It is argued that the 57Fe nucleus in ferrocene is less shielded than in the reference compound (iron pentacarbonyl) due to a smaller HOMO-LUMO gap, resulting in stronger interactions between occupied and virtual orbitals. The influence of the XC functional on the calculated molecular orbital (MO) energies of frontier orbitals is discussed. Different generalized gradient approximations (GGA) give similar results whereas hybrid functionals that incorporate part of the Hartree–Fock exchange stabilize occup...

The calculation of 77Se chemical shifts using gauge including atomic orbitals and density functional theory

Calculations of 77Se nuclear magnetic resonance chemical shifts and shieldings are reported for a number of selenium containing compounds. The calculated shifts span a range of about 2800 ppm, and therefore cover almost the complete range of known 77Se chemical shifts. The calculations are based on density functional theory (DFT); gauge including atomic orbitals (GIAO’s) are used. The DFT‐GIAO method is tested against other theoretical approaches. It is able to predict chemical shifts as well as individual tensor components of the shielding tensor with about the same quality as sophisticated ab initio methods. DFT is even capable of handling highly correlated systems like Se2+4. Other methods fail in this particular case.

Current developments in silicene and germanene

Exploration of the unusual properties of the two-dimensional materials silicene and germanene is a very active research field in recent years. This article therefore reviews the latest developments, focusing both on the fundamental materials properties and on possible applications.