Precision shooting: Sampling long transition pathways

The kinetics of collective rearrangements in solution, such as protein folding and nanocrystal phase transitions, often involve free energy barriers that are both long and rough. Applying methods of transition path sampling to harvest simulated trajectories that exemplify such processes is typically made difficult by a very low acceptance rate for newly generated trajectories. We address this problem by introducing a new generation algorithm based on the linear short-time behavior of small disturbances in phase space. Using this ``precision shooting'' technique, arbitrarily small disturbances can be propagated in time, and any desired acceptance ratio of shooting moves can be obtained. We demonstrate the method for a simple but computationally problematic isomerization process in a dense liquid of soft spheres. We also discuss its applicability to barrier crossing events involving metastable intermediate states.Comment: 9 pages, 12 figures, submitted to J. Chem. Phy

Two-stage coarsening mechanism in a kinetically constrained model of an attractive colloid

We study an attractive version of the East model using the real-space renormalization group (RG) introduced by Stella et al. The former is a kinetically constrained model with an Ising-like interaction between excitations, and shows striking agreement with the phenomonology of attractive colloidal systems. We find that the RG predicts two nonuniversal dynamic exponents, which suggests that in the out-of-equilibrium regime the model coarsens via a two-stage mechanism. We explain this mechanism physically, and verify this prediction numerically. In addition, we find that the characteristic relaxation time of the model is a non-monotonic function of attraction strength, again in agreement with numerical results.Comment: 10 page

Force-induced unfolding of a homopolymer on fractal lattice: exact results vs. mean field predictions

We study the force-induced unfolding of a homopolymer on the three dimensional Sierpinski gasket. The polymer is subject to a contact energy between nearest neighbour sites not consecutive along the chain and to a stretching force. The hierarchical nature of the lattice we consider allows for an exact treatment which yields the phase diagram and the critical behaviour. We show that for this model mean field predictions are not correct, in particular in the exact phase diagram there is {\em not} a low temperature reentrance and we find that the force induced unfolding transition below the theta temperature is second order.Comment: 15 pages, 5 eps figure

Equilibrium free energies from fast-switching trajectories with large time steps

Jarzynski's identity for the free energy difference between two equilibrium states can be viewed as a special case of a more general procedure based on phase space mappings. Solving a system's equation of motion by approximate means generates a mapping that is perfectly valid for this purpose, regardless of how closely the solution mimics true time evolution. We exploit this fact, using crudely dynamical trajectories to compute free energy differences that are in principle exact. Numerical simulations show that Newton's equation can be discretized to low order over very large time steps (limited only by the computer's ability to represent resulting values of dynamical variables) without sacrificing thermodynamic accuracy. For computing the reversible work required to move a particle through a dense liquid, these calculations are more efficient than conventional fast switching simulations by more than an order of magnitude. We also explore consequences of the phase space mapping perspective for systems at equilibrium, deriving an exact expression for the statistics of energy fluctuations in simulated conservative systems

The case for new rural centralities in agricultural enclaves: three scenarios for Muggiano

Agricultural enclaves represent for cities both a challenge, they have to deal with connectivity and development issues, and an opportunity, they can be considered as resources for achieving major policy objectives such as better environmental protection or increased local food production. Among European metropolises, Milan’s case is quite original since it presents unique institutional arrangements and chiefly its agricultural park (Parco Agricolo Sud Milano – PASM). The territory of Muggiano is part of the PASM, at the fringe of the municipal territory, cut out from both the rest of the city and the rest of the park, it is undergoing a slow decline both in its agricultural and urban functions. Yet, field exploration revealed a more complex territory than expected, carrying a strong latent potential and presenting different challenges than expected. In a prospective effort, we elaborated 3 scenarios for the development of Muggiano: from a fully agricultural production one to another extreme one, focusing on residential uses. Our exploration led us to concluding that the “rural centrality” scenario, investing fully on agriculture but not only for its productive function, and spatially balancing the development along a “backbone”, was the best option Muggiano could go for

The case for new rural centralities in agricultural enclaves: three scenarios for Muggiano

Agricultural enclaves represent for cities both a challenge, they have to deal with connectivity and development issues, and an opportunity, they can be considered as resources for achieving major policy objectives such as better environmental protection or increased local food production. Among European metropolises, Milan\u2019s case is quite original since it presents unique institutional arrangements and chiefly its agricultural park (Parco Agricolo Sud Milano \u2013 PASM). The territory of Muggiano is part of the PASM, at the fringe of the municipal territory, cut out from both the rest of the city and the rest of the park, it is undergoing a slow decline both in its agricultural and urban functions. Yet, field exploration revealed a more complex territory than expected, carrying a strong latent potential and presenting different challenges than expected. In a prospective effort, we elaborated 3 scenarios for the development of Muggiano: from a fully agricultural production one to another extreme one, focusing on residential uses. Our exploration led us to concluding that the \u201crural centrality\u201d scenario, investing fully on agriculture but not only for its productive function, and spatially balancing the development along a \u201cbackbone\u201d, was the best option Muggiano could go for

Transferable Pair Potentials for CdS and ZnS Crystals

A set of interatomic pair potentials is developed for CdS and ZnS crystals. We show that a simple energy function, which has been used to describe the properties of CdSe [J. Chem. Phys. 116, 258 (2002)], can be parametrized to accurately describe the lattice and elastic constants, and phonon dispersion relations of bulk CdS and ZnS in the wurtzite and rocksalt crystal structures. The predicted coexistence pressure of the wurtzite and rocksalt structures, as well as the equation of state are in good agreement with experimental observations. These new pair potentials enable the study of a wide range of processes in bulk and nanocrystalline II-VI semiconductor materials

Volcanism on Io: Insights from Global Geologic Mapping

We are preparing a new global geo-logic map of Jupiter s volcanic moon, Io. Here we report the type of data that are now available from our global mapping efforts, and how these data can be used to investigate questions regarding the volcano-tectonic evolution of Io. We are using the new map to investigate several specific questions about the geologic evolution of Io that previously could not be well addressed, including (for example) a comparison of the areas vs. the heights of Ionian mountains to assess their stability and evolution (Fig. 1). The area-height relationships of Io s visible mountains show the low abundance and low relief of volcanic mountains (tholi) relative to tectonic mountains, consistent with formation from low-viscosity lavas less likely to build steep edifices. Mottled mountains are generally less high than lineated mountains, consistent with a degradational formation

Volcanism on Io: Results from Global Geologic Mapping

We have completed a new 1:15,000,000 global geologic map of Jupiter's volcanic moon, Io, based on a set of 1 km/pixel combined Galileo- Voyager mosaics produced by the U.S. Geological Survey. The map was produced over the last three years using ArcGIS(TM) software, and has undergone peer-review. Here we report some of the key results from our global mapping efforts, and how these results relate to questions regarding the volcano-tectonic evolution of Io