M. Sprik

Computer simulation of muonium in water

The structure of water molecules surrounding an isolated hydrogen atom (H) or muonium (Mu) is investigated using computer simulation. The water solvent is treated using classical mechanics and a simple effective two body potential that is designed to yield a reasonable solvent structure. Feynman’s path integral approach is used to treat the single quantum impurity (H or Mu). Both the H and Mu atoms are found to be clathrated with average coordination numbers of 18 and 23 water molecules, respectively. The value for the H atom differs little from the results of a purely classical calculation. It is suggested that the lack of isotope effect in certain diffusion controlled reactions of H and Mu is due to the enclathration of u2009both species.

Study of electron solvation in liquid ammonia using quantum path integral Monte Carlo calculations

The solvation of an electron in liquid ammonia has been studied using quantum path integral Monte Carlo calculations. In agreement with previous experimental and theoretical deductions the charge distribution of the electron is compact. Various distribution functions characterizing the structure around the solvated electron are presented and the surrounding solvent structure is compared to that around a classical atomic anion. A qualitative discussion is given of the absorption spectrum based upon the form of the complex time dependence of the electron mean squared displacement correlation function.

Simulation of an excess electron in a hard sphere fluid

A Monte Carlo simulation of an electron in a hard sphere fluid is described. Five thermodynamic states along the isotherm λ=6σ have been examined (λ is the thermal wavelength of the electron, σ is the diameter of a sphere, d=σ/2 is taken as the distance of closest approach between a sphere and the electron). At fluid densities below 0.2σ−3, the electron fluctuates in extended configurations. At higher densities, we find that the fluid of random scatterers localizes the electronic configurations into compact yet fluctuating structures occupying voids in the fluid. Between the densities 0.1 to 0.2σ−3 we observe two relatively stable yet distinct electronic structures, one compact and the other extended. This observation of apparent metastable states seems to imply nonlinearities in the fluctuations of the electron suggestive of phase transition behavior. The statistics of the fluctuations in the compact structures are in perfect agreement with earlier results obtained for an electron in rigid disordered array of scatterers. The results of the simulation for both extended and localized states are compared with those obtained from the integral equation theory of Chandler et al. The Monte Carlo calculations were made possible by the use of the staging algorithm. This renormalization procedure allows for the efficient sampling of electronic and fluid fluctuations that extend over many length scales. The competition between the variety of length scales is intrinsic to the physics of the solvated electron.A Monte Carlo simulation of an electron in a hard sphere fluid is described. Five thermodynamic states along the isotherm λ=6σ have been examined (λ is the thermal wavelength of the electron, σ is the diameter of a sphere, d=σ/2 is taken as the distance of closest approach between a sphere and the electron). At fluid densities below 0.2σ−3, the electron fluctuates in extended configurations. At higher densities, we find that the fluid of random scatterers localizes the electronic configurations into compact yet fluctuating structures occupying voids in the fluid. Between the densities 0.1 to 0.2σ−3 we observe two relatively stable yet distinct electronic structures, one compact and the other extended. This observation of apparent metastable states seems to imply nonlinearities in the fluctuations of the electron suggestive of phase transition behavior. The statistics of the fluctuations in the compact structures are in perfect agreement with earlier results obtained for an electron in rigid disordered arr...

Study of electron solvation in polar solvents using path integral calculations

A path-integral Monte Carlo algorithm for the simulation of electrons solvated in polar liquids is briefly outlined, and results are presented for the electron solvated in liquid water.

Solvation and ionisation of alkali metals in liquid ammonia: a path integral Monte Carlo study

Quantum path integral Monte Carlo calculations have been used to study the properties of the alkali atoms Li, Na and Cs immersed in liquid ammonia. The solvent has been treated using a pairwise additive intermolecular potential fitted to experimental data. The alkali-atom solvent potential consists of two parts; an ion-core-solvent interaction fitted to quantum chemical calculations and a valence-electron-solvent pseudopotential taken from the solid-state literature. Two distinct forms of pseudopotential have been employed, one having an attractive, and the other a repulsive core. In the latter case, the equilibrium structure of Na and Cs is found to be an ionised state consisting of a fully solvated ion core, plus a well-separated, compact solvated valence electron. In the case of Li, the equilibrium structure for both models appears to be a dipolar or excitonic atom. The relative merits of the two models are discussed and, where possible, contact is made with data on metal ammonia solutions.

The orientational relaxation of methane molecules in the solid phase II at low temperatures

A model for the dynamics of the coupling between the orientations of the ordered CH4 molecules in phase II of solid methane at low temperatures is proposed. The model is equivalent to the dynamics of disordered solid hydrogen. The effective interaction strength is determined by the overlap of the librational ground states in the molecular field potential and vanishes in the classical limit. An approximate expression for the effective interaction strength is derived, showing an exponential dependence on the uncertainty of orientation in the librational ground states. This parameter is estimated from the experimental values of the tunnel energies. The second moments of the spectral densities of several anisotropic operators are evaluated in the infinite temperature limit. The resulting gaussian approximations for the spectra are applied in a derivation of the spin lattice relaxation time. The calculated values of the spin lattice relaxation time are compared to experiment.

A high pressure NMR study of solid methane II. Coupled and uncoupled relaxation of the zeeman and tunnel system in ordered four spin 12 systems

Abstract A theory of the spin-lattice relaxation and the relaxation of the tunnel systems (spin species interconversion) of the tetrahedral four spin 1 2 systems is presented. Resonances between the tunnel system and the Zeeman system are treated. Similarities and differences with the relaxation of triangular three spin 1 2 systems are emphasized. In particular the absence in tetrahedral four spin 1 2 systems of coupling of the magnetization to rotational polarization is explained by means of symmetry arguments. The theory is applied to the relaxation of the magnetization of silane (SiH 4 ).

Simulation of the cubic to orthorhombic phase transition in potassium cyanide

Constant pressure (NPH) ensemble molecular dynamics calculations have been used to study the cubic to orthorhombic phase transition that occurs upon cooling potassium cyanide at low pressures. A rigid ion model consisting of interionic electrostatic terms plus nonbonded atom–atom interactions has been found to yield an almost quantitative account of the transition. In particular, the extreme softening of the shear elastic constant C44 and the anomalous dispersion of the transverse acoustic phonons propagating along the crystal [100] direction in the cubic phase are well reproduced.

The thermodynamics of discontinuous orientational phase transitions in quantum crystals

Generalized Clausius-Clapeyron relations are derived for discontinuous orientational phase transitions in the quantum-mechanical regime. Some qualitative features of the isotope effect on the transition temperature are explained. The quantum correction to the density dependence of the h.c.p.-f.c.c. transition temperature in solid hydrogen, the stability of the partially ordered phase of solid CH4 and the anomalous isotope effect in ammonium bromide are treated in thermodynamic terms.

The isotope effect in the phase diagram of solid methane: I. Proton NMR experiment

Abstract The pressure-temperature phase diagram of solid CH3D and CHD3 up to 3 kbar is determined by means of the observation of the anomaly in the proton spin-lattice relaxation time. The proton second moment of CH4, CH3D and CHD3 is determined by applying the zero time resolution technique. The experimental value of the isothermal compressibility, resulting from these data, is used to transform the pressure-temperature transition curves of CH4, CH3D and CHD3 to temperature-density relations.