Development of a parallel molecular dynamics code on SIMD computers: Algorithm for use of pair list criterion

In recent years several implementations of molecular dynamics (MD) codes have been reported on multiple instruction multiple data (MIMD) machines. However, very few implementations of MD codes on single instruction multiple data (SIMD) machines have been reported. The difficulty in using pair lists of nonbonded interactions is the major problem with MD codes for SIMD machines, such that, generally, the full connectivity computation has been used. We present an algorithm, the global cut-off algorithm (GCA), which permits the use of pair lists on SIMD machines. GCA is based on a probabilistic approach and requires the cut-off condition to be simultaneously verified on all nodes of the machine. The MD code used was taken from the GROMOS package; only the routines involved in the pair lists and in the computation of nonbonded interactions were rewritten for a parallel architecture. The remaining calculations were performed on the host computer. The algorithm has been tested on Quadrics computers for configurations of 32, 128, and 512 processors and for systems of 4000, 8000, 15,000, and 30,000 particles. Quadrics was developed by Istituto Nazionale di Fisica Nucleare (INFN) and marketed by Alenia Spazio. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 685–694, 1998     

Metal-to-Metal Distance Modulated Au(I)/Ru(II) Cyclophanyl Complexes: Cooperative Effects in Photoredox Catalysis

The modular synthesis of Au(I)/Ru(II) decorated mono- and heterobimetallic complexes with π-conjugated [2.2]paracyclophane is described. [2.2]Paracyclophane serves as a rigid spacer which holds the metal centers in precise spatial orientations and allows metal-to-metal distance modulation. A broad set of architectural arrangements of pseudo -geminal, -ortho, -meta, and -para substitution patterns were employed. Metal-to-metal distance modulation of Au(I)/Ru(II) heterobimetallic complexes and the innate transannular π-communication of the cyclophanyl scaffold provides a promising platform for the investigations of structure-activity relationship and cooperative effects. The Au(I)/Ru(II) heterobimetallic cyclophanyl complexes are stable, easily accessible, and exhibit promising catalytic activity in the visible-light promoted arylative Meyer-Schuster rearrangement.     

Time-Resolved Spectroscopy and Electronic Structure of Mono-and Dinuclear Pyridyl-Triazole/DPEPhos-Based Cu(I) Complexes

Chemical and spectroscopic characterization of the mononuclear photosensitizers [(DPEPhos)Cu(I)(MPyrT)]^0/+ ( CuL , CuLH ) and their dinuclear analogues ( Cu₂L’ , Cu₂L'H₂ ), backed by (TD)DFT and high-level GW-Bethe-Salpeter equation calculations, exemplifies the complex influence of charge, nuclearity and structural flexibility on UV-induced photophysical pathways. Ultrafast transient absorption and step-scan FTIR spectroscopy reveal flattening distortion in the triplet state of CuLH as controlled by charge, which also appears to have a large impact on the symmetry of the long-lived triplet states in Cu₂L’ and Cu₂L'H₂ . Time-resolved luminescence spectroscopy (solid state), supported by transient photodissociation spectroscopy (gas phase), confirm a lifetime of some tens of μs for the respective triplet states, as well as the energetics of thermally activated delayed luminescence, both being essential parameters for application of these materials based on earth-abundant copper in photocatalysis and luminescent devices.     

Quenching and blinking of fluorescence of a single dye molecule bound to gold nanoparticles

A fluorescein derivative (SAMSA) bound to gold nanoparticles of different diameters is investigated by time-resolved fluorescence at the single molecule level in a wide dynamic range, from nanosecond to second time scale. The significant decrease of both SAMSA excited state lifetime and fluorescence quantum yield observed upon binding to gold nanoparticles can be essentially traced back to an increase of the nonradiative deactivation rate, probably due to energy transfer, that depends on the nanoparticle size. A slow single molecule fluorescence blinking, in the ms time scale, has a marked dependence on the excitation intensity both under single and under two photon excitation. The blinking dynamics is limited by a low probability nonlinear excitation to a high energy state from which a transition to a dark state occurs. The results point out a strong coupling between the vibro-electronic configuration of the dye and the plasmonic features of the metal nanoparticles that provide dye radiationless deactivation channels on a wide dynamic range.     

Poly(m-phenylene disulfide): Antimony pentafluoride doping and infrared spectroscopy

Poly(m-phenylene disulfide) treated with SbF₅ gives an unstable blue-black material. Heating of doped samples at 155°C leads to some stabilization to ambient conditions; doped and heated pellets exhibit a conductivity of 10^?9 and 10^?5 S/cm for vapor and liquid doping, respectively. To investigate backbone modifications, polymer powders have been treated with large amounts of liquid SbF₅ and an analysis of the IR spectra of the extracted samples has been tried in comparison with that of pristine polymer. Evidence for SbF ion has been found in the IR spectra of the complexes.     

Temporal fluctuations in the SERRS spectra of single iron–protoporphyrin IX molecule

Surface enhanced resonance Raman spectra of Fe–protoporphyrin IX, adsorbed on silver colloidal nanoparticles immobilized onto a polymer-coated glass slide have been investigated at very low concentrations. The spectra exhibit drastic temporal fluctuations on a time scale of seconds in both line frequency and intensity; such a trend suggesting that the single molecule limit is approached. Sequences of spectra have been analyzed in terms of an underlying continuum and of Raman peaks superimposed on this continuum. A statistical analysis of the spectrum intensity has allowed us to put into evidence that main contribution to the intensity fluctuations arises from the continuum. In addition, a high correlation between the total integrated intensity and the intensity detected at different Raman peaks has been revealed. Furthermore, the ratio between the intensity detected in correspondence of different FePP vibrational modes shows a temporal variability likely reflecting the intrinsic dynamics of the molecule. All these findings have been ascribed to a desorption–adsorption mechanism of the molecules at the silver surface.     

Direct evidence for associated charm production in 340 GeV π−p interactions

In a 2.5 events/μb exposure of a small 20 cm diameter, rapid cycling hydrogen bubble chamber equipped with high resolution optics, clear evidence is observed for the first time of the associated production of charmed particles, identified by their short decays. A significant excess of 12 events which we interpret as charm pairs corresponding to a total cross section of ～ 40 μb is seen above the expected strange particle background. In addition, 8 charged 3 prong events consistent with D^± decay are observed with an expected background of ～ 2 events     

Comparison of theory and experiment in electron-beam-controlled CO2 laser oscillators

A four-temperature model describing the electron-beam-controlled CO₂ pulsed laser has been developed and used to study the time evolution of a laser pulse and the influence thereon of experimental parameters. This paper explains the experimental results of two different electron-beam CO₂ pulsed laser systems which have been built in the laboratories at Frascati.     

Semileptonic (Λ b → Λcev) decay in a field theoretic quark model

The semileptonic decay width of heavy baryons such as (Λ _b → Λcev) has been estimated in the framework of a nonrelativistic field theoretic quark model where four component quark field operators along with a harmonic oscillator wave function are used to describe translationally invariant hadronic states. The present estimation does not make an explicit use of heavy quark symmetry and has a reasonable agreement with the experimentally measured decay width, polarisation ratio and form factors with the harmonic oscillator radii and quark momentum distribution inside the hadron as free parameters.