Diagnosing broken ergodicity using an energy fluctuation metric

The Mountain and Thirumalai energy fluctuation metric, Omega(t), has been used to study the effective ergodicity of 60- and 256-atom binary Lennard-Jones mixtures in order to determine the reliability of the calculated diffusion constants at different energies. A plot of Omega(t) against 1time allows the identification of two distinct regimes: ergodic supercooled liquids, where Omega(t) approaches zero, and nonergodic glassy states, where Omega(t) asymptotically approaches a nonzero value on the molecular dynamics time scale. This approach seems to be more appropriate than attempting to define a threshold value for Omega(t)/Omega(0). The behavior of systems between these two limits, which are nonergodic on the time scale considered but may be approaching ergodicity, was examined for a range of simulation times. The calculated diffusion constants change as effective ergodicity is approached, moving closer to the Vogel-Tammann-Fulcher fit defined by higher-energy systems that are already considered to be effectively ergodic. Using the form of the decay of the metric as a measure of ergodicity, we have been able to reproduce the trend in fragility obtained by Sastry for a 256-atom system [Nature (London) 409, 164 (2001)], correcting some of our earlier results [J. Chem. Phys. 120, 8314 (2004)].     

Finding pathways between distant local minima

We report a new algorithm for constructing pathways between local minima that involve a large number of intervening transition states on the potential energy surface. A significant improvement in efficiency has been achieved by changing the strategy for choosing successive pairs of local minima that serve as endpoints for the next search. We employ Dijkstra's algorithm [E. W. Dijkstra, Numer. Math. 1, 269 (1959)] to identify the "shortest" path corresponding to missing connections within an evolving database of local minima and the transition states that connect them. The metric employed to determine the shortest missing connection is a function of the minimized Euclidean distance. We present applications to the formation of buckminsterfullerene and to the folding of various biomolecules: the B1 domain of protein G, tryptophan zippers, and the villin headpiece subdomain. The corresponding pathways contain up to 163 transition states and will be used in future discrete path sampling calculations.     

Global minima of (C60)nCa, (C60)nF and (C60)nI clusters

Likely candidates for the lowest potential energy minima of (C₆₀)_nCa²⁺, (C₆₀)_nF⁻ and (C₆₀)_nI⁻ clusters are located using basin-hopping global optimisation. In each case, the potential energy surface is constructed using the Girifalco form for the C₆₀ intermolecular interaction, an averaged Lennard–Jones C₆₀–ion interaction, and a polarisation potential, which depends on the first few non-vanishing C₆₀ multipole polarisabilities. We find that the ions generally occupy the interstitial sites of a (C₆₀)_n cluster, the coordination shell being tetrahedral for Ca²⁺ and F⁻. The I⁻ ion has an octahedral coordination shell in the global minimum for (C₆₀)₆I⁻, however for 12  n  8 the preferred coordination geometry is trigonal prismatic.     

Symmetry and density functionals

The energies of different states belonging to an orbitally degenerate term are not consistent when evaluated with density functionals currently in use. This problem arises because the functionals do not transform according to the totally symmetric irreducible representation of the appropriate point group. A physically correct form can be obtained by projection.     

Counterion-Trapped-Molecules: From High Polarity and Enriched IR Spectra to Induced Isomerization

We report extensive computational studies of some novel intermolecular systems and their properties. Recombination of alkali-halide counterions separated by a noncovalently trapped hydrocarbon molecule is prevented by significant potential energy barriers, resulting in unusual metastable insertion complexes. These systems are extremely polar, while the inserted molecule is strongly counter-polarized, leading to significant cooperative nonadditivity effects. The compression and electric field produced by the counterions favours isomerization of the trapped molecule via a significant reduction of the barriers to bond rearrangement, in a field-induced mechanochemical process. The predicted IR intensity spectra clearly reflect (1) formation of the insertion complex, rather than simple attachment of alkali halide, and (2) isomerization of the trapped molecule, thus allowing experimental access to these events.     

A doubly nudged elastic band method for finding transition states

A modification of the nudged elastic band (NEB) method is presented that enables stable optimizations to be run using both the limited-memory Broyden-Fletcher-Goldfarb-Shanno (L-BFGS) quasi-Newton and slow-response quenched velocity Verlet minimizers. The performance of this new "doubly nudged" DNEB method is analyzed in conjunction with both minimizers and compared with previous NEB formulations. We find that the fastest DNEB approach (DNEB/L-BFGS) can be quicker by up to 2 orders of magnitude. Applications to permutational rearrangements of the seven-atom Lennard-Jones cluster (LJ7) and highly cooperative rearrangements of LJ38 and LJ75 are presented. We also outline an updated algorithm for constructing complicated multi-step pathways using successive DNEB runs.     

基于Matlab/Simulink的电压型超导储能功率调节控制仿真

本文首先对电压型超导储能（Voltage-Superconducting-Magnet-Energy-Storage,简称VSMES）功率调节系统（Power-Conditioning-System,简称PCS）的原理进行介绍,并运用Matlab/Simulink仿真分析软件,针对三相电压型超导储能功率调节系统的控制过...     

Self-injection length in La0.7Ca0.3MnO3-YBa2Cu3O7-δ ferromagnet-superconductor multilayer thin films

We have carried out extensive studies on the self-injection problem in barrierless heterojunctions between La_0.7Ca_0.3MnO₃ (LCMO) and YBa₂Cu₃O_7-δ (YBCO) thin films. The heterojunctions were formed in situ by sequentially growing LCMO and YBCO films on 〈100〉 LaAlO₃ (LAO) substrate using a pulsed laser deposition (PLD) system. YBCO micro-bridges with 64 μm width were patterned both on the LAO (control) and LCMO side of the substrate. Critical current, I _c, was measured at 77 K on both the control side as well as the LCMO side for different YBCO film thickness. It was observed that while the control side showed a J _c of ∼ 2 × 10⁶ A/cm², the LCMO side showed about half the value for the same thickness (1800 ?). The difference in J _c indicates that a certain thickness of YBCO has become ‘effectively’ normal due to self-injection. From the measurement of J _c at two different thicknesses (1800 ? and 1500 ?) of YBCO films both on the LAO as well as the LCMO side, the value of self-injection length (at 77 K) was estimated to be ∼ 900 ?. To the authors’ best knowledge, this is the first time that self-injection length has been quantified. A control experiment carried out with LaNiO₃ deposited by PLD on YBCO did not show any evidence of self-injection.