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Recently, we have proposed an efficient scheme for Monte Carlo simulations, the multiple "time step" Monte Carlo (MTS-MC) [J. Chem. Phys. 117, 8203 (2002)] based on the separation of the potential interactions into two additive parts. In this paper, the structural and thermodynamic properties of the simple point charge water model combined with the Ewald sum are compared for the MTS-MC real-/reciprocal-space split of the Ewald summation and the common Metropolis Monte Carlo method. We report a number of observables as a function of CPU time calculated using MC and MTS-MC. The correlation functions indicate that speedups on the order of 4.5-7.5 can be obtained for systems of 108-500 waters for n=10 splitting parameter., ((c) 2004 American Institute of Physics.)

We propose a sampling scheme to reduce the CPU time for Monte Carlo simulations of atomic systems. Our method is based on the separation of the potential energy into parts that are expected to vary at different rates as a function of coordinates. We perform n moves that are accepted or rejected according to the rapidly varying part of the potential, and the resulting configuration is accepted or rejected according to the slowly varying part. We test our method on a Lennard-Jones system. We show that use of our method leads to significant savings in CPU time. We also show that for moderate system sizes the scaling of CPU time with system size can be improved (for n = 40 the scaling is predominantly linear up to 1000 particles). [ABSTRACT FROM AUTHOR]

The dipole absorption spectrum of an electron in fluid helium is calculated by the maximum entropy method (MEM) numerical inversion of quantum Monte Carlo data obtained from a path integral Monte Carlo (PIMC) simulation at 309 K at the reduced densities ρ*=0.1, 0.3, 0.5, 0.7, and 0.9. Our results agree with the RISM-polaron theory results of Nichols and Chandler [A. L. Nichols III and D. Chandler, J. Chem. Phys. 87, 6671 (1987)] and the grid wave function calculation of Coker and Berne [D. F. Coker and B. J. Berne, J. Chem. Phys. 89, 2128 (1988)]. The method generated the expected long high frequency tail and the low density zero-frequency intensity caused by high conductivity. The method has also been tested by comparing the MEM absorption spectrum to the analytical spectrum of an electron confined in a spherical cavity of fluctuating radius, a model for a solvated electron in a localized state. © 1994 American Institute of Physics. [ABSTRACT FROM AUTHOR]

An algorithm based on allowing the form of the intermolecular potential to fluctuate is introduced into molecular dynamics and Monte Carlo simulations. It is shown that this method accelerates the approach to equilibrium in frustrated systems. It provides a very useful method for determining all of the local minima in complex systems. Examples are mixing in dense binary liquids and folding of long polymer chains. When this method is combined with umbrella sampling techniques it accelerates the simulations of a wide class of chemically important systems. [ABSTRACT FROM AUTHOR]

A new short time propagator for a particle interacting with a hard sphere or with a hard sphere cavity is derived. This new propagator is shown to be a much better approximation than the image approximations used in the literature and should prove useful for path integral or Green function Monte Carlo simulations. The simple example of the radial distribution function of a hard sphere gas is calculated using the new propagator as well as the older image approximation propagator and is shown to converge much more rapidly to the ab initio result. [ABSTRACT FROM AUTHOR]

We study the nature of the excess electronic states in fluid helium by calculating the excess electron energies and wave functions for a set of configurations of solvent atoms taken from path integral Monte Carlo calculations on the solvated electron system. The eigenvalues and eigenfunctions for the different configurations of the solvent are used to calculate the inhomogeneously broadened density of states and absorption line shape of the excess electron over a range of fluid densities. The predictions of a simple theory of the excess electronic states in fluid helium due to Springett et al. [B. E. Springett, M. H. Cohen, and J. Jortner, Phys. Rev. 159, 183 (1967)] are found to agree quite well with our computer simulation results. This simple theory, however, predicts an inhomogeneously broadened electronic absorption linewidth which is much narrower than that obtained from simulation. It is found that the RISM-polaron theory of Nichols and Chandler [A. L. Nichols III and D. Chandler, J. Chem. Phys. 87, 6671 (1987)] gives an absorption line shape which is in better agreement with our simulations. We observe a transition from lower energy states in which the electron is bound to density fluctuations in the fluid to the situation where the excess electron scatters through the fluid in continuum states at higher energies. [ABSTRACT FROM AUTHOR]

The behavior of an excess electron in helium (at T=309 K) and xenon (at T=309 K and T=248 K) is studied over a range of fluid densities (ρ*=ρσ3=0.1–0.9). A path integral Monte Carlo technique is used to model the ‘‘quantum’’ electron which interacts through pseudopotentials with the ‘‘classical’’ solvent particles. In helium, the electron becomes confined in a cavity in the solvent and behaves like a particle in a spherical box. We observe contrasting behavior in the more polarizable xenon solvent where the electron exists in a ‘‘quasifree’’ state. A variety of equilibrium properties of the electron and the solvent are presented to characterize the structure of the different systems. The anomolous density dependence of the experimental electron mobility along the coexistence curve in xenon can be understood qualitatively in terms of the equilibrium structures we observe at the different solvent densities. [ABSTRACT FROM AUTHOR]

The structure of an excess electron in water at room temperature is investigated using the Feynman path integral technique. The interaction potential between the electron and water is modeled by an effective potential, made up of three terms: a static potential, a repulsive potential, and a polarization potential. The polarization part is treated in two different ways: approximated as pairwise additive, and exactly with the many body polarization effects treated self-consistently. It is shown that the excess electron forms a cavity with the radius of the electron being 2.24 Å for pairwise additive polarization and 2.11 Å for the self-consistent treatment of the polarization. There is no sharp geometrical coordination number of water molecules around the electron. The water molecules within a distance of about 3.5 Å from the center of the electronic change distribution point their OH bonds towards the electron, and form only three hydrogen bonds. It is also found that the pair correlation function of the solvent molecules close to the electron are considerably different from the corresponding quantities calculated in the bulk. The electron, therefore, is shown to modify the local density to a large extent. There are important structural differences between the many body polarization model and the two body polarization model. It is concluded that for a quantitative description of the structure of the hydrated electron, the self-consistent treatment of the long range (many body) polarization effects are important. [ABSTRACT FROM AUTHOR]

Simulation of e-–(H20)n for n=1,2,3 done at 5 K, using path integral Monte Carlo methods shows that a water dimer as well as a water trimer in the linear configuration can bind an electron in a diffuse surface state. The binding energy of the electron dimer and the electron trimer is estimated to be between 3–6 and 4–9 meV, respectively. The results indicate that the electron does not alter the structure of the water dimer but does induce observable changes in the water trimer. It is also shown that an electron does not bind to a monomer. These results are discussed in connection with recent molecular beam experiments. [ABSTRACT FROM AUTHOR]

Presents an erratum from the article published in the 1993 issue of the 'Journal of Chemical Physics,' which describes the theory and simulation of polar and nonpolar polarizable fluids.

We present a way of combining real-time path integral Monte Carlo simulations with a maximum entropy numerical analytic continuation scheme in a new approach for calculating time correlation functions for finite temperature many body quantum systems. The real-time dynamics is expressed in the form of the symmetrized time correlation function, which is suitable for Monte Carlo methods, and several simulation techniques are presented for evaluating this function accurately up to moderate values of time. The symmetrized time correlation function is then analytically continued in combination with imaginary time data to obtain the real-time correlation function. We test this approach on several exactly solvable problems, including two one-dimensional systems, as well two cases of vibrational relaxation of a system coupled to a dissipative environment. The computed time correlation functions are in good agreement with exact results over several multiples of the thermal time βh, and exhibit a significant improvement over analytic continuation of imaginary time correlation functions. Moreover, we show how the method can be systematically improved. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]