J. P. J. Michels

Stability of doubly occupied N2 clathrate hydrates investigated by molecular dynamics simulations

Classical molecular dynamics calculations were performed for a structure II clathrate hydrate with N2 guest molecules in order to investigate the possibility of double occupancy, i.e., two N2 molecules inside one large cage. For all of the pressures, temperatures, and compositions at which the simulations have been performed, the doubly occupied clathrate remained stable. The structure of the host lattice is indistinguishable from that of a singly occupied clathrate hydrate. The volumes and energies are linearly dependent on the filling fraction. The range of values are the same for both the singly as well as doubly occupied clathrates. In the doubly occupied cages, the O–N2 radial distribution function, and therefore the structure in the vicinity of the N2 molecule, is similar to that of the mixed fluid. An extensive investigation of the distances in the cages shows a large similarity between singly and doubly occupied clathrates. All these results indicate that, upon filling the large cages with pairs o...

Molecular dynamics simulation study of the properties of doubly occupied N2 clathrate hydrates

We have performed molecular dynamics calculations to obtain a number of properties of a doubly occupied structure II N2 clathrate hydrate, in particular to study its behavior under higher pressures. The calculated neutron diffraction pattern is in agreement with the experimental result. The effect of the presence of the filling of the small cages and of the large cages (in either single or double occupancy) on the calculated pattern is demonstrated and discussed. The calculated Raman spectra show that the average vibrational frequency of the N2 molecules decreases in going from the singly occupied small cages to the doubly occupied large cages and then to the singly occupied large cages, respectively. The frequency distributions are explained in terms of radial distribution functions. When applying large pressures at low temperatures, a clathrate-amorphous transition occurs for a partially doubly occupied clathrate. The transition occurs at about the same pressure as for single occupations, but the densif...

The shape of ring polymers

Ring and linear polymers, with an without excluded volume, have been studied by Brownian dynamics. The shape of the polymers was investigated by calculating the principal orthogonal components of the radius of gyration. Ring polymers are found to be more spherical than linear ones. (AIP)

Computer simulations of the dynamics of doubly occupied N2 clathrate hydrates

We have studied the dynamical properties of doubly and singly occupied structure II N2 clathrate hydrates by performing a series of classical molecular dynamics calculations. The intermolecular vibrational density of states of the guest molecules show the most explicit changes when going from single to double occupancies. Instead of the narrow peak for single occupancy (at 20 cm−1), a broad range of frequencies occurs between 0 and 200 cm−1 with four modes, which have been assigned to the possible vibrations in the cage. This spectrum provides a unique fingerprint of double occupancy. For the host lattice, the libron band and the optic phonon branch show lower frequencies as compared to single fillings. In contrast, the acoustic phonon branch shifts to lower frequencies for a single filling of the large cases. We find that there is a large vibrational guest–host coupling for both single and double occupancies, although the frequency range is broader in the latter case. There is a significant translational...

COMPUTER SIMULATIONS OF THE LINEWIDTH OF THE RAMAN Q-BRANCH IN FLUID NITROGEN

By means of molecular dynamical simulations, the width of the Raman line in fluid N2 is calculated at room temperature and pressures up to the melting line. The results are compared with experimental results for the linewidth and for the dephasing time. Detailed information is given about the relaxation mechanism of the vibrational frequency. For instance, a marked influence of the vibration‐rotation coupling is seen, in particular at high pressures. Moreover, the time correlation function of the frequency reveals a long time behavior at high pressures. From a comparison of the simulated change in vibrational frequency as a function of pressure with experimental data for the line shift, an estimate is made for the contribution of the so‐called ‘‘attractive part’’ to that shift.

Polymer shapes in three dimensions

The asymmetry and degree of prolate or oblateness of three‐dimensional linear and ring polymers, with and without excluded volume, are investigated by Brownian dynamics. It is found that the simulation results are consistent with theoretical predictions and that the asymmetry is a better measurement parameter of polymer shapes than the degree of prolate or oblateness because it is easier to obtain accurate data about it.

THE IMPORTANCE OF THE ANISOTROPIC ENERGY TERM FOR THE STRUCTURE OF THE SOLID PHASES OF NITROGEN

We have performed Monte Carlo simulations on the solid phases of nitrogen and improved the nitrogen–nitrogen intermolecular site–site potential derived by Etters et al. Previous simulations with the Etters potential reproduced the properties of dense fluid nitrogen and the structures of the α, β, and δ phase, but failed to describe the low temperature, high pressure γ and e phases. We have examined the influence of the Coulomb term on the stability of the e phase with various values for the quadrupole and hexadecapole moments. When the quadrupole is taken about 15% larger than the experimental value, the e phase could be stabilized, although the volume and box angle did not correspond to the experimental values. The e phase could also be stabilized by introducing an anisotropic term, which influences the shape of the molecule. The two anisotropy parameters have been determined with respect to the e‐phase structure, and resulted also in the stabilization of the γ phase. The anisotropic term changes the sha...

High pressure study on the Raman spectra of fluid nitrogen and nitrogen in helium

A study on the Raman shift and width of nitrogen and nitrogen in helium has been performed as a function of pressure and temperature by means of experiments,molecular dynamics (MD) simulations and hard fluid (HF) theory. The experiments have been performed using Raman spectroscopy in a diamond anvil cell at pressures up to 10 GPa and temperatures between 250 and 400 K. Both the experimental shift and width results of pure nitrogen link up very well with accurate measurements at lower pressures and with less accurate measurements at higher pressures. For the first time the Raman shift and width have been determined as a function of temperature at an isobar, such that a sensitive test of theoretical models can be made. The MD calculations on the linewidth along an isobar show very good agreement with experiment. The influence on the linewidth of the bondlength dependence of the site–site interaction parameters (often called the attractive contribution) appears to be small, which indicates that this has a small anisotropy. For pure N2 the MD and the HF calculations of the repulsive contribution to the Raman shift are about the same. This shows that both ways of calculation are consistent. The experimental Raman shift of nitrogen diluted in helium appears to be much larger than that of pure nitrogen. In contrast, the linewidth is much smaller than that of pure nitrogen. HF calculations were also performed for the Raman shift of N2, infinitely diluted in He. The results for the bondlength independent (repulsive) contribution give clearly smaller values than those of the experiment, which means that the effect of the change of the potential parameters at excitation must be positive. This implies that that part of the intermolecular potential, which is due to the overlap of the molecular charge distributions has a dependence on the bondlength, that results in a positive contribution to the Raman shift. It will be shown that for N2 the good agreement with experiment of earlier HF calculations with an attractive contribution, based on a purely dispersive model, is due to a cancellation of errors. For nondiluted mixtures of He–N2 under noncritical conditions the plot of experimental FWHM values as a function of the volume fraction shows a broad maximum, which is indicative for inhomogeneous broadening. This behavior is described with the help of the Knapp–Fischer model.

The influence of the density of the solvent on the static and dynamic properties of star polymers: a molecular dynamics study

The static and dynamic properties of a star polymer in a solvent at several densities have been studied by computer simulations using molecular dynamics. In addition to the repulsive Lennard‐Jones potential which interacts between all particles we have used a harmonic potential to link 19 particles into a star polymer. It was found that both the static properties (e.g., the end‐to‐end distance and the radius of gyration) as well as the dynamic properties (e.g., the autocorrelation functions and the center‐of‐mass diffusion coefficient) are significantly influenced by the density of the solvent.

Scaling in two‐dimensional linear and ring polymers

Bead size effects on the excluded volume of two‐dimensional linear and ring polymers are investigated with Brownian dynamics. It is found that the mean dimensions of the chains follow a scaling relation with scaling variable X=N(σ/l)d/φ, where N is the number of units on the chain, σ is the size of the unit, l is the link length, d is the dimension, and φ is the crossover exponent. The scaling law is 〈R2〉/〈R2〉0 or 〈S2〉/〈S2〉0∼X2ν−1 for X→∞. Here ν is the critical exponent for the mean dimensions of an isolated polymer chain and the subscript 0 denotes the nonexcluded volume case.