A neural network/trajectory approach is presented for the development of accurate potential-energy hypersurfaces that can be utilized to conduct ab initio molecular dynamics (AIMD) and Monte Carlo studies of gas-phase chemical reactions, nanometric cutting, and nanotribology, and of a variety of mechanical properties of importance in potential microelectromechanical systems applications. The method is sufficiently robust that it can be applied to a wide range of polyatomic systems. The overall method integrates ab initio electronic structure calculations with importance sampling techniques that permit the critical regions of configuration space to be determined. The computed ab initio energies and gradients are then accurately interpolated using neural networks (NN) rather than arbitrary parametrized analytical functional forms, moving interpolation or least-squares methods. The sampling method involves a tight integration of molecular dynamics calculations with neural networks that employ early stopping and regularization procedures to improve network performance and test for convergence. The procedure can be initiated using an empirical potential surface or direct dynamics. The accuracy and interpolation power of the method has been tested for two cases, the global potential surface for vinyl bromide undergoing unimolecular decomposition via four different reaction channels and nanometric cutting of silicon. The results show that the sampling methods permit the important regions of configuration space to be easily and rapidly identified, that convergence of the NN fit to the ab initio electronic structure database can be easily monitored, and that the interpolation accuracy of the NN fits is excellent, even for systems involving five atoms or more. The method permits a substantial computational speed and accuracy advantage over existing methods, is robust, and relatively easy to implement.

Chemical vapors originating from the explosive charge within landmines and unexploded ordnance (UXO) form a chemical "signature" unique to these devices. The fact that canines can detect this signature was a primary motivation for the Defense Advanced Research Projects Agency's (DARPA) Dog's Nose Program. One goal of this program was to develop electronic chemical sensors that mimic the canine's ability to detect landmines. The sensor described here, developed under this program, utilizes novel fluorescent polymers to detect landmine signature vapors in air at ultratrace concentration levels (parts-per-trillion or less). Thin films of the polymers are highly emissive but undergo a dramatic reduction in emission intensity when molecules of target analytes bind to the polymer. Binding of a single explosive molecule can quench the fluorescence from hundreds of polymer repeat units, resulting in an amplification of the quenching response. The polymer structure contains receptor sites designed to interact specifically with nitroaromatic explosives, enhancing the selectivity of the polymers for target analytes. A man-portable sensor prototype, similar in size and configuration to metal detectors currently used for mine detection, has demonstrated performance comparable to that of canines during field tests monitored by DARPA at Fort Leonard Wood, MO.

Analysis of the spectrum suggests that the formation of pF involves electron transfer from one bacteriochlorophyll molecule to another within the reaction center, or possibly from bacteriochlorophyll to the bacteriopheophytin of the complex. The initial absorbance changes after flash excitation also include a bleaching of an absorption band at 800 nm. The bleaching decays with T AZ 30 psec. The bleaching appears not to be a secondary effect, but rather to reveal another early step in the primary photochemical reaction.The primary photochemical reaction of bacterial photosynthesis is the transfer of an electron from a bacteriochlorophyll complex, P, to an acceptor, X, whose identity is uncertain (1-3). The electron transfer reaction occurs with a quantum yield of essentially 100% (4), and it occurs with great speed, even at temperatures below 40K. It can be studied in preparations of isolated "reaction centers," which contain three different polypeptides, four equivalents of bacteriochlorophyll, two of bacteriopheophytin, and one each of ubiquinone and nonheme iron (1-3).In a search for clues to the mechanism of the electron transfer reaction, Parson et al. (5) State pF is unlikely to be the lowest excited singlet state of the bacteriochlorophyll complex, P*, because measurements of the fluorescence yield indicate that the lifetime of P* is only 20 to 40 psec when X is in the reduced form (and less when X is not reduced) (6, 7). The proposal that the primary reaction involves an intermediate state such as pF, rather than proceeding directly from P*, would explain a longstanding anomaly in the quantitative relationship between the quantum yields of photochemistry and fluorescence (3-5). The possibility has remained, however, that pF is a side-product which forms only if the normal reaction is blocked. A decision on the role of state pF in photosynthesis, therefore, requires information on whether pF is formed under conditions that permit the electron transfer reaction to occur. We report here on an investigation of this point, using picosecond kinetic techniques. The results appear to establish pF as an intermediate in the electron transfer reaction. While this work was in progress, Kaufmann et al. (8) reached the same conclusion, using somewhat different techniques of psec spectroscopy. MATERIALS AND METHODSReaction centers were obtained from cells of Rhodopseudomonas sphaeroides strain R-26 by the method of Clayton and Wang (9). The detergent lauryldimethylamine oxide was replaced by Triton X-100 by dialysis (5, 10).The apparatus used in the psec experiments was essentially that of Magde and Windsor (11). We shall summarize the approach only briefly here for clarity; ref. 11 provides additional details. A single pulse from a mode-locked Nd+3/glass laser was frequency-doubled to provide a 530 nm excitation flash lasting about 8 psec. This illuminated only a small band across the center of the sample. A measuring flash of white light, which was generated by self phase modulation of residual 1060 nm light from th...

A previously reported method for conducting molecular dynamics simulations of gas-phase chemical dynamics on ab initio potential-energy surfaces using modified novelty sampling and feedforward neural networks is applied to the investigation of the unimolecular dissociation of vinyl bromide. The neural network is fitted to a database comprising the MP4(SDQ) energies computed for 71 969 nuclear configurations using an extended basis set. Dissociation rate coefficients and branching ratios at an internal excitation energy of 6.44 eV for all six open reaction channels are reported. The distribution of vibrational energy in HBr formed in three-center dissociation is computed and found to be in excellent accord with experimental measurements. Computational requirements for the electronic structure calculations, neural network training, and trajectory calculations are given. The weight and bias matrices required for implementation of the neural network potential are made available through the Supplementary Material.

Nanosecond and picosecond kinetic techniques have been used to study electron transfer from the first excited singlet state (Bph*) and the first excited triplet state (Bph') of bacteriopheophytin to p-benzoquinone. Quenching of the first excited singlet state by 40 m M p-benzoquinone results in a decrease in the lifetime of Bph* but does not lead directly to the formation of the n-cation radical (Bpht). In the presence of 8 M methyl iodide and 40 m M p-benzoquinone together, the singlet lifetime is reduced further; however, the quantum yield of Bph' is enhanced due to the increased rate of intersystem crossing between Bph* and BphT. Electron transfer from BphT to p-benzoquinone leads to the formation and detection of Bphf. The results are discussed in terms of the spin-selectivity of the reverse electron transfer process within the intermediate charge transfer complexes.