Self-assembly on multiple length scales: a Monte Carlo algorithm with data augmentation

We present a Monte Carlo algorithm that allows simulations where portions of the system of variable size are moved. The algorithm requires the definition of an augmented space that contains information on the bonding between components of the system and is updated as the simulation proceeds. With this method it is possible to incorporate, within the same simulation, processes involving motion of smaller and larger portions of a given system. The algorithm is presented in general terms and illustrated for a simple one-dimensional lattice model.     

PM-10 and heavy metals in particulate matter of the province of Lecce (Apulia, Southern Italy)

This paper shows the results of a preliminary study of air monitoring in the province of Lecce (Apulia, Southern Italy). In particular, the attention has been focused on the determination of the PM-10 level and of the concentration of nine metals (Cd, Cr, Cu, Fe, Mn, Ni, Pb, V and Zn) present on the filters which were collected in two towns (Lecce and Campi Salentina) from 2002 until 2003. The metals have been chosen on the basis of their toxicity and of their possible use as chemical tracers. The results have proved that PM-10 values and metals concentrations did not show substantial difference between the two towns and that PM-10 level and lead concentration are below the limit established by Italian law in force. The experiments have demonstrated a high correlation between iron and manganese in both sampling sites; this could be attributed to pollution of metallurgical origin. Multivariate statistical analysis, carried out by HCA and PCA methods, has been used in order to differentiate samples in relation to sampling sites, sampling period and meteorological conditions.     

Diffusion and viscosity of a calamitic liquid crystal model studied by computer simulation

We report a molecular dynamics simulation study on an ensemble of rod-like particles, each composed of nine soft spheres held rigidly along a line. We have calculated translational mean square displacements and velocity autocorrelation functions in the fluid phases exhibited by the model, i.e., smectic A, nematic and isotropic. These quantities have then been used to compute diffusion coefficients. In addition, we have calculated viscosities in the nematic and isotropic phases. Despite its crude nature, the model is capable of providing a faithful reproduction of many features of the transport behavior observed in real liquid-crystalline materials. The simulation results have been compared with the predictions of the modified affine transformation theory, finding only a fair agreement.     

Viscoelasticity and metastability limit in supercooled liquids

A supercooled liquid is said to have a kinetic spinodal if a temperature Tsp exists below which the liquid relaxation time exceeds the crystal nucleation time. We revisit classical nucleation theory taking into account the viscoelastic response of the liquid to the formation of crystal nuclei and find that the kinetic spinodal is strongly influenced by elastic effects. We introduce a dimensionless parameter lambda, which is essentially the ratio between the infinite frequency shear modulus and the enthalpy of fusion of the crystal. In systems where lambda is larger than a critical value lambda(c) the metastability limit is totally suppressed, independently of the surface tension. On the other hand, if lambda相似文献   

Inducing chaos by resonant perturbations: theory and experiment

We propose a scheme to induce chaos in nonlinear oscillators that either are by themselves incapable of exhibiting chaos or are far away from parameter regions of chaotic behaviors. Our idea is to make use of small, judiciously chosen perturbations in the form of weak periodic signals with time-varying frequency and phase, and to drive the system into a hierarchy of nonlinear resonant states and eventually into chaos. We demonstrate this method by using numerical examples and a laboratory experiment with a Duffing type of electronic circuit driven by a phase-locked loop. The phase-locked loop can track the instantaneous frequency and phase of the Duffing circuit and deliver resonant perturbations to generate robust chaos.     

A computational study of the excited states of bilirubin IX

We have determined the lowest excited states of bilirubin IX by TD-DFT calculations. The lowest pair of excited states, S(1) and S(2), turn out to be of charge-transfer (CT) nature. Although DFT based methods tend to underestimate the energy of CT states, the small oscillator strengths we have computed indicate that such states may actually exist in this spectral region, but would have escaped spectroscopic detection. The next pair of excited states, S(3) and S(4), account for the most prominent spectral feature of bilirubin. They can be accurately described by the exciton coupling model, as we show by a thorough analysis of wavefunctions and properties. This finding therefore supports the interpretation of bilirubin photoisomerization behaviour, based on the exciton coupling model.     

Assessing the accuracy of metadynamics

Metadynamics is a powerful technique that has been successfully exploited to explore the multidimensional free energy surface of complex polyatomic systems and predict transition mechanisms in very different fields, ranging from chemistry and solid-state physics to biophysics. We here derive an explicit expression for the accuracy of the methodology and provide a way to choose the parameters of the method in order to optimize its performance.