Showing papers on "Path integral molecular dynamics published in 1999"

The nature of the hydrated excess proton in water

Abstract: Explanations for the anomalously high mobility of protons in liquid water began with Grotthuss's idea1, 2 of ‘structural diffusion’ nearly two centuries ago Subsequent explanations have refined this concept by invoking thermal hopping3, 4, proton tunnelling5, 6 or solvation effects7 More recently, two main structural models have emerged for the hydrated proton Eigen8, 9 proposed the formation of an H9O4+ complex in which an H3O+ core is strongly hydrogen-bonded to three H2O molecules Zundel10, 11, meanwhile, supported the notion of an H5O2+ complex in which the proton isshared between two H2O molecules Here we use ab initio path integral12,13,14 simulations to address this question These simulations include time-independent equilibrium thermal and quantum fluctuations of all nuclei, and determine interatomic interactions from the electronic structure We find that the hydrated proton forms a fluxional defect in the hydrogen-bonded network, with both H9O4+ and H5O2+ occurring only in thesense of ‘limiting’ or ‘ideal’ structures The defect can become delocalized over several hydrogen bonds owing to quantum fluctuations Solvent polarization induces a small barrier to proton transfer, which is washed out by zero-point motion The proton can consequently be considered part of a ‘low-barrier hydrogen bond’15, 16, in which tunnelling is negligible and the simplest concepts of transition-state theory do not apply The rate of proton diffusion is determined by thermally induced hydrogen-bond breaking in the second solvation shell

Molecular dynamics algorithms for path integrals at constant pressure

Abstract: Extended system path integral molecular dynamics algorithms have been developed that can generate efficiently averages in the quantum mechanical canonical ensemble [M. E. Tuckerman, B. J. Berne, G. J. Martyna, and M. L. Klein, J. Chem. Phys. 99, 2796 (1993)]. Here, the corresponding extended system path integral molecular dynamics algorithms appropriate to the quantum mechanical isothermal–isobaric ensembles with isotropic-only and full system cell fluctuations are presented. The former ensemble is employed to study fluid systems which do not support shear modes while the latter is employed to study solid systems. The algorithms are constructed by deriving appropriate dynamical equations of motions and developing reversible multiple time step algorithms to integrate the equations numerically. Effective parallelization schemes for distributed memory computers are presented. The new numerical methods are tested on model (a particle in a periodic potential) and realistic (liquid and solid para-hydrogen and liquid butane) systems. In addition, the methodology is extended to treat the path integral centroid dynamics scheme, [J. Cao and G. A. Voth, J. Chem. Phys. 99, 10070 (1993)], a novel method which is capable of generating semiclassical approximations to quantum mechanical time correlation functions.

Simulation studies of liquid ammonia by classical ab initio, classical, and path-integral molecular dynamics

Abstract: The structure of liquid ammonia at T=273 K has been studied using classical ab initio molecular dynamics, classical molecular dynamics, and the path-integral molecular dynamics methods. The three different types of calculation are employed to generate new insights into the ability of theoretical methods to model liquid ammonia effectively. Thus, the limitations of using classical nuclei, simple point charge models, small systems, and gradient corrected density functional theory are assessed through a comparison of the results of the different types of calculations to each other and recent experiments in a consistent manner. Briefly, the experimental intermolecular quantum structure is very well reproduced by the classical approximation while the intramolecular classical and quantum structures exhibit large deviations. The intermolecular ab initio partial radial structure factors of liquid ammonia and the associated radial distribution functions are in better agreement with experiment than the empirical models. However, the empirical models also perform reasonably well.

Quantum Molecular Dynamics Simulations of Low-Temperature High Energy Density Matter: Solid p-H2/Li and p-H2/B

Abstract: The metastability of atomic impurities, Li and B, trapped in solid parahydrogen is studied by employing path integral molecular dynamics (PIMD) and centroid molecular dynamics (CMD) simulations at 4 K and zero external pressure. Starting from pure solid hydrogen consisting of 1440 particles, doped systems are prepared by substituting impurity atoms for hydrogen molecules at substitutional defect sites. For various concentrations, thermodynamic quantities are then calculated and the stability of the systems is monitored. For the case of lithium, systems containing 2.5 mol % dopants remain metastable with convergent thermodynamic quantities, but systems with 3.3 mol % or more dopants become unstable. For the case of boron, systems containing as high as 15 mol % dopants remain metastable on the time scale of the simulation, while systems with 25 mol % dopants do not. These results provide evidence of the transition from metastability to global instability and a rough estimate of the maximum doping density of...

Quantum gas in an external field : exact grand canonical expressions and numerical treatment

Abstract: This thesis presents and develops the path integral simulation techniques in application to small quantum systems at finite temperatures. The first goal is to obtain exact thermodynamic expressions for systems of noninteractingThe rest and the major part of the thesis is dedicated to the development and testing of Bead-Fourier path integral molecular dynamics. Although, path integral molecular dynamics as well as path integral Monte Carlo are wellFirst, molecular dynamics under Bead-Fourier scheme was developed and tested on the examples of quantum harmonic oscillator and Hydrogen atom. The main attention was paid to ergodicity problems. Then we addressed the question,Later, the formalism for identical particles was developed.Finally, the question of molecular dynamics efficacy was raised. It was shown, that formalisms for identical and distinguishable particles, both, can be reformulated into a more efficient ones, providing all dynamical variables

Simulation of crystal and liquid potassium via restricted path-integral molecular dynamics

Abstract: The path-integral molecular-dynamics method is employed to study the effect of temperature on a simple metal ~potassium! model system. The simple metal undergoes a phase transformation upon heating. Calculated dynamic properties indicate that the atomic motion changes from a vibrational to a diffusive character identifying the transformation as melting. Calculated structural properties further confirm the transformation. Ionic vibrations in the crystal state and the loss of long-range order during melting modify the electronic structure and in particular localize the electrons inside and at the border of the ion core. @S0163-1829~99!00117-4#