Fernando M. S. Silva Fernandes

Ab initio study of the conformational equilibrium of ethylene glycol

SummaryThe conformational equilibrium of ethylene glycol (CH2OHCH2OH) has been examined by performing geometry optimizations at the 6-31G*, MP2/6-31G* and 6-31G** levels. Final energies have been calculated at the MP3 level with the optimized geometries. The two most stable conformers are atGg′ andgGg′ but it is verified that the inclusion of electronic correlations reduces their energy difference of 0.6 kcal/mol at the HF level to less than 0.2 kcal/mol. The possible coexistence of the two most stable conformers is in agreement with some previous studies of Frei et al. For thetXg′ conformer a detailed analysis of the intramolecular potential as a function of rotation around the C-C bond is also reported.

Phase diagrams of alkali halides using two interaction models : A molecular dynamics and free energy study

Phase diagrams for potassium and sodium chlorides are determined by molecular dynamics and free energy calculations. Two rigid-ion interaction models, namely, the Born-Mayer-Huggins (BMH) and Michielsen-Woerlee-Graaf (MWG) effective pair potentials, have been used. The critical and triple point properties are discussed and compared with available experimental and simulation data. The MWG model reproduces the experimental liquid-gas equilibria better than the BMH model, being the accordance very good in the lowest temperature region of the coexistent liquids, particularly for NaCl. However, both models underestimate the critical temperatures of KCl and NaCl. Relatively to the solid-gas equilibria, the models do not reproduce well the experimental data. As for the solid-liquid coexistences either the BMH or the MWG models appear unrealistic.

Monte Carlo simulation of the adsorption of phenol on gold electrodes: a simple model

Canonical Monte Carlo simulations, at 298 K, of the adsorption of phenol in a dilute aqueous solution on gold electrodes, at the potential of zero charge, are presented. The results suggest that the process occurs in two distinct and successive steps. Adsorption starts with the phenol oxygen atom pointing towards the gold surface and the aromatic ring in a quasi-perpendicular orientation relative to the surface. This is followed by the reorientation of the aromatic ring to a parallel configuration. The effect of the solvent is analyzed through the calculation of the potential of mean force acting on phenol. Despite the simplicity of the model and the interaction potentials used in the simulations, the results are in good agreement with experimental observations, giving insight into the microscopic details of the adsorption process of phenol at low concentrations.

Hypervolumes in microcanonical Monte Carlo

A Monte Carlo method to perform microcanonical simulations by sampling the configurational and momenta spaces is presented. The technique was inspired by the method of hypervolumes for calculating the entropy in a microcanonical ensemble. Although this method is strictly proven in the thermodynamic limit, the hypervolume Monte Carlo (HVMC) algorithm, presented in this article, works well with a relatively small number of particles. In contrast to other algorithms for microcanonical Monte Carlo simulations, the HVMC method does not involve previous integrations over the momenta space or demons. Therefore, it can be used with any form of Hamiltonian. Thermal and structural properties for the Lennard-Jones system obtained by NVE molecular dynamics are compared with results from the HVMC method. The agreement is excellent. Additionally, the method provides the speed distribution functions of the system which are, also, in excellent agreement with the results from molecular dynamics. A discussion of the HVMC method in the context of the statistical mechanical theory of the microcanonical ensemble is presented.

The starting state in simulations of the fluid-solid coexistence by Gibbs-Duhem integration

A direct method to determine the starting state in simulations of the fluid–solid coexistence by Gibbs–Duhem integration is presented. It is based on the limiting behavior of the Gibbs Ensemble vapor–liquid calculations at the lowest temperature range. The approach estimates the location of the triple point and, from there up, the fluid–solid coexistence properties, at higher temperatures, are calculated by Gibbs–Duhem simulations. The technique is illustrated by tracing the phase diagram of the Lennard–Jones system. The usefulness of the method is discussed in view of our recent study on the phase behavior of C60 [Int. J. Quantum Chem. 84 (2001) 375].  2001 Elsevier Science B.V. All rights reserved.

A time saving algorithm for the Monte Carlo method of Metropolis

A time saving algorithm for the Monte Carlo method of Metropolis is presented. The technique is tested with different potential models and number of particles. The coupling of the method with neighbor lists, linked lists, Ewald sum and reaction field techniques is also analyzed. It is shown that the proposed algorithm is particularly suitable for computationally heavy intermolecular potentials.

Hypervolume Monte Carlo method at constant pressure

Abstract The Hypervolume Monte Carlo (HVMC) method recently proposed by Fernandes and Ramalho (Comput. Phys. Commun. 90 (1995) 73) to perform microcanonical simulations, sampling the configurational and momenta spaces, has been extended to the isoenthalpic-isobaric ensemble. Thermal and structural properties as well as the speed distribution functions of the Lennard-Jones system, obtained from Andersens isoenthalpic-isobaric molecular dynamics method, are compared with the results from the proposed HVMC method at constant pressure. The agreement is excellent. The statistical-mechanical roots of the isoenthalpic-isobaric HVMC method are also presented.

Phase transitions in ionic clusters

Molecular dynamics simulations of unconstrained KCl clusters with 8, 64, 216 and 512 ions were carried out using the Born‐Mayer‐Huggins potential. All the clusters exhibit first‐order‐like melting and freezing transitions with hysteresis regions. The transitions are detected by sharp variations of the radius of gyration, configurational energy and by the analysis of the radial density functions, mean square displacements and power spectra. The ‘‘melting temperature’’ increases with the number of ions and approaches the melting temperature of the bulk. The radius of gyration turns out to be a good indicator of the transitions and of the sizes of the clusters. Its variation suggests that, for the present length of the simulation runs and temperature range, no evaporation took place. Glass‐like transitions were also detected. Snapshots for solid, liquid and glass‐like states show the structure of the clusters and some aspects of the nucleation.

Gibbs ensemble Monte Carlo

The Gibbs ensemble Monte Carlo algorithm is discussed in the context of gas-liquid coexistence. The details of the algorithm and a Java based program are presented.

The hypervolume Monte Carlo method at constant pressure applied to liquid methyl chloride

Abstract The hypervolume Monte Carlo (HVMC) in the isoenthalpic–isobaric ensemble (NpH), recently proposed by Fernandes and Freitas [1] has been applied to a molecular system. Thermal and structural properties as well as speed distribution functions of the liquid methyl chloride, obtained in a series of thermodynamics states, have been compared with NpT molecular dynamics (MD) [2] , [3] and some experimental results [4] . The agreement between the calculated and experimental densities is fairly good and the comparison with molecular dynamics results is excellent.