Spectroscopic Properties of a Biologically Relevant [Fe2(μ-O)2] Diamond Core Motif with a Short Iron-Iron Distance

Diiron cofactors in enzymes perform diverse challenging transformations. The structures of high valent intermediates ( Q in methane monooxygenase and X in ribonucleotide reductase) are debated since Fe−Fe distances of 2.5–3.4 Å were attributed to “open” or “closed” cores with bridging or terminal oxido groups. We report the crystallographic and spectroscopic characterization of a Fe^III₂(μ-O)₂ complex ( 2 ) with tetrahedral (4C) centres and short Fe−Fe distance (2.52 Å), persisting in organic solutions. 2 shows a large Fe K-pre-edge intensity, which is caused by the pronounced asymmetry at the T_D Fe^III centres due to the short Fe−μ−O bonds. A ≈2.5 Å Fe−Fe distance is unlikely for six-coordinate sites in Q or X , but for a Fe₂(μ-O)₂ core containing four-coordinate (or by possible extension five-coordinate) iron centres there may be enough flexibility to accommodate a particularly short Fe−Fe separation with intense pre-edge transition. This finding may broaden the scope of models considered for the structure of high-valent diiron intermediates formed upon O₂ activation in biology.     

Magnetic anisotropy of deposited transition metal clusters

We present results of magnetic torque calculations using the fully relativistic spin-polarized Korringa-Kohn-Rostoker approach applied to small Co and Fe clusters deposited on the Pt(111) surface. From the magnetic torque one can derive amongst others the magnetic anisotropy energy (MAE). It was found that this approach is numerically much more stable and also computationally less demanding than using the magnetic force theorem that allows to calculate the MAE directly. Although structural relaxation effects were not included our results correspond reasonably well to recent experimental data.     

Asphalt

Ohne Zusammenfassung     

Adaptive accuracy control for Car-Parrinello simulations

Summary. The Car-Parrinello (CP) approach to ab initio molecular dynamics serves as an approximation to time-dependent Born-Oppenheimer (BO) calculations. It replaces the explicit minimization of the energy functional by a fictitious Newtonian dynamics and therefore introduces an artificial mass parameter which controls the electronic motion. A recent theoretical investigation shows that the CP-error, i.e., the deviation of the CP–solution from the BO-solution decreases like asymptotically. Since the computational effort increases like , the choice of has to find a compromise between efficiency and accuracy. The asymptotical result is used in this paper to construct an easily implemented algorithm which automatically controls : the parameter is repeatedly adapted during the simulation by choosing as large as possible while pushing an error measure below a user-given tolerance. The performance and reliability of the algorithm is illustrated by a typical example. Received March 10, 1998     

Accuracy and Stability of Computing High-order Derivatives of Analytic Functions by Cauchy Integrals

High-order derivatives of analytic functions are expressible as Cauchy integrals over circular contours, which can very effectively be approximated, e.g., by trapezoidal sums. Whereas analytically each radius r up to the radius of convergence is equal, numerical stability strongly depends on r. We give a comprehensive study of this effect; in particular, we show that there is a unique radius that minimizes the loss of accuracy caused by round-off errors. For large classes of functions, though not for all, this radius actually gives about full accuracy; a remarkable fact that we explain by the theory of Hardy spaces, by the Wiman–Valiron and Levin–Pfluger theory of entire functions, and by the saddle-point method of asymptotic analysis. Many examples and nontrivial applications are discussed in detail.     

Computing isogeny volcanoes of composite degree

Isogeny volcanoes are an interesting structure that have had several recent applications. An isogeny volcano is a connected component of a larger graph called a cordillera. In this paper, we further explore properties of how to compute volcanoes given that we have already computed one of a different degree. This allows us to compute volcanoes of composite degree more efficiently than a direct construction using modular polynomials.     

Contact Angle Measurements Using Sessile Drop and Micro-CT Data from Six Sandstones

Transport in Porous Media - Numerous sessile drop and micro-computed tomography (micro-CT) studies have been conducted to quantify geologic carbon storage formation wettability by measuring static...