Adsorption energies of benzene on close packed transition metal surfaces using the random phase approximation

The authors acknowledge financial support from the Scottish Funding Council (through EaStCHEM and SRD-Grant HR07003) and from EPSRC (PhD studentship for JAGT, EP/M506631/1). Funding by the Austrian Science Fund (FWF): F41 (SFB ViCoM) is grateful acknowledged.The adsorption energy of benzene on various metal substrates is predicted using the random phase approximation (RPA) for the correlation energy. Agreement with available experimental data is systematically better than 10% for both coinage and reactive metals. The results are also compared with more approximate methods, including vdW-density functional theory (DFT), as well as dispersion corrected DFT functionals. Although dispersion corrected DFT can yield accurate results, for instance, on coinage metals, the adsorption energies are clearly overestimated on more reactive transition metals. Furthermore, coverage dependent adsorption energies are well described by the RPA. This shows that for the description of aromatic molecules on metal surfaces further improvements in density functionals are necessary, or more involved many body methods such as the RPA are required.PostprintPeer reviewe

Coupling Epitaxy, Chemical Bonding, and Work Function at the Local Scale in Transition Metal-Supported Graphene

Resonance tunneling spectroscopy and density functional theory calculations are employed to explore local variations in the electronic surface potential of a single graphene layer grown on Rh(111). A work function modulation of 220 meV is experimentally measured, indicating that the chemical bonding strength varies significantly across the supercell of the Moire pattern formed when graphene is bonded to Rh(111). In combination with high-resolution images, which provide precise knowledge of the local atomic registry at the carbon metal interface, we identify experimentally, and confirm theoretically, the atomic configuration of maximum chemical bonding to the substrate. Our observations are at odds with reported trends for other transition metal substrates. We explain why this is the case by considering the various factors that contribute to the bonding at the graphene/metal interface.PostprintPeer reviewe

Name A new algorithm for the evaluation of the Incomplete Gamma Function on vector computers

Several methods of evaluating the Incomplete Gamma Function are compared according to accuracy and computation time. An improved algorithm is presented which allows significantly faster computation of two-electron integrals on most vector computers, especially in the case of GLO functions. Parameters for F-0(t) are tabulated.</p

Parallelization and vectorization of quantum mechanical methods—I. Integral program for polymers and molecules

The interest in theoretical investigations of macromolecular structures in chemistry with quantum mechanical methods is steadily growing. Many problems can only be attacked when the complexity of the chemical composition is taken into account. Supercomputers are the condition to perform the numerical investigations. The most promising developments in computer science and technology are massively parallel multiprocessor systems, not only with respect to cost-efficiency ratio in comparison with supercomputers. The variety of possible processor units that can be used to build up flexible architectures ranges from transputers, workstations and even large-scale computers. The consequences are for the users to adjust their programs and even to develop new algorithms appropriate for the specific parallel computer system. The calculation of the energy band structure and electronic wave function is the basic step in the theoretical study of physical and chemical properties of polymers. To obtain theoretical results comparable with experimental evidences the ab initio Hartree-Fock Crystal Orbital method has to be applied. We have developed new integral programs (one- and two-electron integrals) for multiprocessor systems with special emphasis on vectorizable structures and algorithms. Speed-up, efficiency and parallelization overhead are analysed for parallel computers with distributed memory and message-passing communication. The results prove that for large chemical systems the parallelization will be very efficient.</p

Itinerant magnetism of fcc-iron and disordered iron-nickel-Invar alloys

We apply a tight-binding method for the self-consistent calculation of the itinerant magnetic properties of transition metal alloys to face-centered cubic iron and disordered iron-nickel alloys with particular emphasis on the Invar region around and above 65% Fe. We find an increase of the magnetization with decreasing density for every material. For fcc iron at high densities we obtain ferrimagnetic and spin-glass like states. The magnetization curve of Fe/Ni alloys is very well reproduced in the whole concentration region. The drastic change of the magnetization curve in the Invar region is found to follow from the concentrations- and environment-dependence of the Fe moments.</p

The impact of the Resolution of the Identity approximate integral method on modern <em>ab initio</em> algorithm development

The computation of the two-electron four-center integrals over gaussian basis functions is a significant component of the overall work of many ab initio methods used today, Improvements in the computational efficiency of the base algorithms have provided significant impact, Somewhat overlooked are methods that provide approximations to these integrals and their implementation in application software. A partial review of approximate integral techniques focused on the resolution of the identity (RI) four-center, two-electron integral approximation is given. The past and current uses of the RI algorithms are presented along with possibilities for further exploitation of the technology.</p

The potential energy landscape of noradrenaline. An electronic structure study

The relaxed potential energy pro. les for interconversion between the conformers in the two most stable noradrenaline families (AG1 and GG1, each containing four members) have been calculated at the density functional B3LYP/6-31+G* level of theory. Rigid rotation ( in which the molecule is kept frozen during variation of the torsional angle) is found to introduce large errors in the relative energies and geometries of the stationary points; it appears to be best to compute relaxed potential energy surfaces wherever possible, even if this can only be accomplished by using a lower level of theory. The barriers for interconversion between the noradrenaline conformers ( from least to most stable structure, with inclusion of scaled zero-point vibrational effects) range from 7 to 15 kJ mol(-1). The barrier for conversion between the two most stable noradrenaline conformers, AG1a and GG1a, is among the highest of those computed: 15.4 kJ mol(-1) at the B3LYP/6-31+G* level and 21.7 kJ mol(-1) at the MP2/ 6-31+G* level. Temperature effects further increase the barrier between AG1a and GG1a. The high transition barriers indicate that conformer conversion is unlikely to occur during collisional cooling of noradrenaline in a supersonic expansion.</p

V-51 NMR parameters of VOCl3: static and dynamic density functional study from the gas phase to the bulk

V-51 NMR parameters have been calculated for VOCl3, the reference compound in V-51 NMR spectroscopy, in order to capture environmental effects in both the neat liquid and the solid state. Using a combination of periodic geometry optimizations and Car-Parrinello molecular dynamics simulations with embedded cluster NMR calculations, we are able to test the ability of current computational approaches to reproduce V-51 NMR properties (isotropic shifts, anisotropic shifts and quadrupole coupling constants) in the gas, liquid and solid states, for direct comparison with liquid and solid-state experimental data. The results suggest that environmental effects in the condensed phases can be well captured by an embedded cluster approach and that the remaining discrepancy with experiment may be due to the approximate density functionals in current use. The predicted gas-to-liquid shift on the isotropic shielding constant is small, validating the common practice to use a single VOCl3, molecule as reference in V-51 NMR computations.</p