A Self-Resonant System - Experimental Investigations of Boundary and Operating Conditions

This contribution introduces the setup of a self-resonating system consisting of a freely-sliding mass over a vibrating elastic beam under harmonic excitation. Under specific operating regime, the slider shows tendency to move and adapt its position to where the whole system resonates. The behavior of the slider during that motion is investigated. Experimental observations of the operating regime dependency on some parameters are provided. (© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Isobaric-isothermal monte carlo simulations from first principles: application to liquid water at ambient conditions.

A series of first-principles Monte Carlo simulations in the isobaric-isothermal ensemble were carried out for liquid water at ambient conditions (T=298 K and p=1 atm). The Becke-Lee-Yang-Parr (BLYP) exchange and correlation energy functionals and norm-conserving Goedecker-Teter-Hutter (GTH) pseudopotentials were employed with the CP2 K simulation package to examine systems consisting of 64 water molecules. The fluctuations in the system volume encountered in simulations in the isobaric-isothermal ensemble require a reconsideration of the suitability of the typical charge-density cutoff and the regular grid-generation method previously used for the computation of the electrostatic energy in first-principles simulations in the microcanonical or canonical ensembles. In particular, it is noted that a much higher cutoff is needed and that the most computationally efficient method of creating grids can result in poor simulations. Analysis of the simulation trajectories using a very large charge-density cutoff at 1200 Ry and four different grid-generation methods point to a significantly underestimated liquid density of about 0.8 g cm-3 resulting in a somewhat understructured liquid (with a value of about 2.7 for the height of the first peak in the oxygen-oxygen radial distribution function) for BLYP-GTH water at ambient conditions. In addition, a simulation using a charge-density cutoff at 280 Ry yields a higher density of 0.9 g cm-3, showing the sensitivity of the simulation outcome to this parameter.     

Consistent parametrization of semiempirical MO methods

The parametrization of semiempirical molecular orbital methods is reviewed. The parametrization procedures are classified in three categories. The newly proposed category of consistency of parameters in a row of elements is applied in SINDO 1 to the Si atom. The calculations on test molecules and silicon clusters demonstrate that significant improvements in accuracy can be obtained in this way. © 1992 John Wiley & Sons, Inc.     

The influence of temperature and density functional models in ab initio molecular dynamics simulation of liquid water

The performance of density functional theory methods for the modeling of condensed aqueous systems is hard to predict and validation by ab initio molecular simulation of liquid water is absolutely necessary. In order to assess the reliability of these tests, the effect of temperature on the structure and dynamics of liquid water has been characterized with 16 simulations of 20 ps in the temperature range of 280-380 K. We find a pronounced influence of temperature on the pair correlation functions and on the diffusion constant including nonergodic behavior on the time scale of the simulation in the lower temperature range (which includes ambient temperature). These observations were taken into account in a consistent comparison of a series of density functionals (BLYP, PBE, TPSS, OLYP, HCTH120, HCTH407). All simulations were carried out using an ab initio molecular dynamics approach in which wave functions are represented using Gaussians and the density is expanded in an auxiliary basis of plane waves. Whereas the first three functionals show similar behavior, it is found that the latter three functionals yield more diffusive dynamics and less structure.     

Approximate molecular electrostatic potentials from semiempirical wavefunctions

Approximate molecular electrostatic potentials (MESPs) are calculated with the asymptotic density model (ADM) on the basis of semiempirical wavefunctions generated by the SINDO1 method. The approximate MESP is adjusted to obtain good agreement with the exact MESP from 6–31G^* ab initio calculations for small molecules. This form of the MESP is used for the study of the reactivity of small and medium size silicon clusters with 5 to 45 atoms. Special attention is given to the reactivity of various Si₄₅ structures proposed in the literature. © 1997 by John Wiley & Sons, Inc.     

Simulating fluid-phase equilibria of water from first principles

Efficient Monte Carlo algorithms and a mixed-basis set electronic structure program were used to compute from first principles the vapor-liquid coexistence curve of water. A water representation based on the Becke-Lee-Yang-Parr exchange and correlation functionals yields a saturated liquid density of 900 kg/m3 at 323 K and normal boiling and critical temperatures of 350 and 550 K, respectively. An analysis of the structural and electronic properties of the saturated liquid phase shows an increase of the asymmetry of the local hydrogen-bonded structure despite the persistence of a 4-fold coordination and decreases of the molecular dipole moment and of the spread of the lowest unoccupied molecular orbital with increasing temperature.