methods are utilized as computational tools in many areas of chemical physics. In this paper, we present the theoretical basis for a dynamical Monte Carlo method in terms of the theory of Poisson processes. We show that if: (1) a "dynamical hierarchy" of transition probabilities is created which also satisfy the detailed-balance criterion; (2) time increments upon successful events are calculated appropriately; and (3) the effective independence of various events comprising the system can be achieved, then Monte Carlo methods may be utilized to simulate the Poisson process and both static and dynamic properties of model Hamiltonian systems may be obtained and interpreted consistently.

After forever changing the drug discovery process in the pharmaceutical industry, combinatorial chemistry methodologies are increasingly being applied to the discovery and optimization of more efficient catalysts and materials (see picture). With the advent of new combinatorial synthesis and screening technologies, coupled with integrated data management systems, the application of these technologies to materials science and catalyst research holds tremendous potential and brings high expectations to this new and exciting field.

letters to nature 944 NATURE | VOL 389 | 30 OCTOBER 1997 pyrochlore, a large degree of spin-polarization is primarily due to the extremely small number of carriers (estimated at 0.0001-0.0005 carriers per formula unit from Hall effect measurements 6 ). Thus even moderate exchange splitting of Tl 6s-derived bands can fully polarize the conducting electrons.What, then, can account for the different temperature dependences of the transport process at the grain boundary between the two systems? In the perovskite manganites, the surface magnetism will in general be quite different from that of the bulk, owing to the lower effective magnetic coupling, the tendency of spin canting (an important feature in the perovskite manganites 21 ), as well as the disorder due to the surface termination of the crystal structure. Also important is the influence of the Jahn-Teller lattice distortions at the surface. One possible consequence of these effects would be a lower surface T C and surface spin polarization with respect to the bulk. The hopping electron is strongly coupled to these surface spins by the large on-site J H , and thus the TMR is susceptible to fast degradation with increasing temperature via the magnetically altered surface spins. In the pyrochlore, however, there is no strong J H coupling the conducting electrons to the Mn spins, and our results imply less sensitivity to the effects of the surface.Another distinction can be visualized by considering the temperature dependence of MR* in absolute units (r 0 MR* shown in the lower panel of Fig. 3. In the perovskite, the absolute magnitude of MR* diminishes rapidly with increasing temperature, much below T C . In contrast, the same data for the pyrochlore is virtually constant for all temperatures up to T C . This difference may indicate that for Tl 2 Mn 2 O 7 , the potential barrier ∆ B is less than the electron energy E ↑ (referring to Fig. 4). In this case the grain boundary transport would not be considered tunnelling, but rather a scattering process across a disordered region acting as a series resistance. This implies that the low-field MR can be made even larger by raising the relative resistance of the grain boundaries with respect to the bulk resistivity, much as a thin-film tunnel junction increases this ratio with respect to bulk polycrystalline samples in perovskite manganites. Ⅺ

The energy released in low-energy chemisorption or physisorption of molecules on metal surfaces is usually expected to be dissipated by surface vibrations (phonons). Theoretical descriptions of competing electronic excitations are incomplete, and experimental observation of excited charge carriers has been difficult except at energies high enough to eject electrons from the surface. We observed reaction-induced electron excitations during gas interactions with polycrystalline silver for a variety of species with adsorption energies between 0.2 and 3.5 electron volts. The probability of exciting a detectable electron increases with increasing adsorption energy, and the measured time dependence of the electron current can be understood in terms of the strength and mechanism of adsorption.

A comparative study of experimental and theoretical combinatorial and high-throughput screening methods for the development of novel materials is presented. Both methods were applied to the development of new anode fuel cell alloy catalysts with improved CO tolerance. Combinatorial experimental electrocatalysis was performed on a 64-element electrode array. Sputter-deposited ternary thin-film electrocatalysts of composition PtRuM (M = Co,Ni,W) were screened in parallel for their methanol oxidation activity, and their individual geometric and specific chronoamperometric current density were monitored and evaluated against standard PtRu catalysts. Density functional theory calculations of a variety of model ternary PtRuM alloy catalysts yielded detailed adsorption energies and activation barriers. Feeding these thermodynamic and kinetic data into a simple micro kinetic model for the CO electro oxidation reaction, the relative activities of a number of PtRuM ternary alloys were calculated. The experimental and theoretical computational results reveal very similar trends in electrocatalytic activity as a function of alloy composition; they also point at similar ternary PtRuM alloys as candidates for improved anode catalysts for low-temperature fuel cells.