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  • 1
    Electronic Resource
    Electronic Resource
    Molecular dynamic simulation of the structure of water in the vicinity of a solvated ion (1981)
    s.l. : American Chemical Society
    The @journal of physical chemistry 〈Washington, DC〉 85 (1981), S. 1498-1505 
    Details
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1021/j150611a010
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  • 2
    Electronic Resource
    Electronic Resource
    Reversible multiple time scale molecular dynamics (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 97 (1992), S. 1990-2001 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The Trotter factorization of the Liouville propagator is used to generate new reversible molecular dynamics integrators. This strategy is applied to derive reversible reference system propagator algorithms (RESPA) that greatly accelerate simulations of systems with a separation of time scales or with long range forces. The new algorithms have all of the advantages of previous RESPA integrators but are reversible, and more stable than those methods. These methods are applied to a set of paradigmatic systems and are shown to be superior to earlier methods. It is shown how the new RESPA methods are related to predictor–corrector integrators. Finally, we show how these methods can be used to accelerate the integration of the equations of motion of systems with Nosé thermostats.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.463137
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  • 3
    Electronic Resource
    Electronic Resource
    Density dependence of excess electronic ground-state energies in simple atomic fluids (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 97 (1992), S. 2002-2021 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The ground-state energies of an excess electron E0 as a function of solvent density are computed using model electron–atom pseudopotentials in fluid helium, argon, and xenon. E0 is a lower bound to the experimentally measurable threshold to photoconductivity, V0. The nonuniqueness of the pseudopotential description of electron–molecule interactions is demonstrated. We find that when many-body polarization effects are included, our calculated E0 results are in close agreement with experimental V0 values indicating that the conduction-band energy lies close to the ground-state energy across a broad range of densities in these polarizable fluids. If the many-body nature of the polarization interaction is ignored the ground-state energies deviate significantly from the V0 results highlighting the importance of accurate treatment of many-body polarization interactions. It is shown that a mean-field theory of polarization gives substantial agreement with full many-body calculations. This allows us to introduce a mean-field, density-dependent pair potential which greatly simplifies such many-body calculations. In the more polarizable systems, it is found that the spatial extent of the ground-state wave function as a function of solvent density is correlated with the density dependence of both V0 and the electron mobility, and it becomes uniformly spread throughout our simulation cell as the electron mobility goes through its maximum value at intermediate solvent densities.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.463138
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  • 4
    Electronic Resource
    Electronic Resource
    Many-body dispersion forces of polarizable clusters and liquids (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 97 (1992), S. 8628-8636 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A system of atoms with embedded Drude dispersion oscillators interacting through dipole–dipole forces is simulated. Using path integrals it is shown that after the coordinates of the dispersion oscillators are integrated out, the atoms interact through many-body dispersion forces to all orders of the dipole–dipole interaction. Simulations are carried out on clusters to see if the presence of many-body forces leads to ground state geometries different from those predicted from two-body potentials. In addition, the polarizability tensor of clusters is determined as a function of cluster size. Simulations are also carried out for fluids to see how many-body forces affect the pair correlation function. Lastly, the long-range interaction between van der Waals clusters is compared with the predictions of a summation over site–site two-body interactions. It is found that many-body forces have only a minor effect on the low energy geometries of van der Waals clusters, a somewhat surprising result given that many-body forces do give an important contribution to surface free energies of clusters and liquids. The vibrational frequencies of the breathing mode decrease by approximately 10%.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.463381
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  • 5
    Electronic Resource
    Electronic Resource
    Monte Carlo methods for accelerating barrier crossing: Anti-force-bias and variable step algorithms (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 1980-1985 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: For computer simulations of systems in which particles must cross large potential energy barriers, slow convergence is a problem. Basically there are two very disparate time scales: one characterizing motion in the potential wells and one characterizing the rare jumps from one stable well to another. Multiple time scale problems like this sorely test computer resources, and stand in the way of progress on simulations of chain folding, glass transitions, nucleation phenomena, activated barrier crossing, and quantum tunneling processes. Here several new methods are developed and tested on classical and quantum barrier crossings in double well problems. These new methods, called the anti-force-bias and variable step methods, lead to much faster convergence than standard methods. Convergence is tested by studying the deviation in the mean of the cumulative spatial distribution function from the exact distribution function.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458029
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  • 6
    Electronic Resource
    Electronic Resource
    A new quantum propagator for hard sphere and cavity systems (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 97 (1992), S. 2382-2385 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: 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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.463076
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  • 7
    Electronic Resource
    Electronic Resource
    Excess electronic states in fluid helium (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 89 (1988), S. 2128-2137 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: 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 elecronic 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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.455057
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  • 8
    Electronic Resource
    Electronic Resource
    Low-temperature variational approximation for the Feynman quantum propagator and its application to the simulation of quantum systems (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 7531-7539 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The low-temperature propagator of Mak and Andersen [J. Chem. Phys. 92, 2953 (1990)] allows for much more rapid convergence of Feynman path integral computer simulations of quantum systems. The effectiveness of this propagator is very sensitive to the choice of an effective frequency and the choice of this made by Mak and Andersen, although good, is not optimum. In this paper a harmonic reference system is used together with a variation principle to compute this effective frequency. Simulations show that when this is used in the low temperature propagator, the results converge much more rapidly than for other choices of the frequency. Moreover, an energy estimator is derived, which allows this effective potential to be used for the determination of the energy of the quantum system. In addition, using a cumulant expansion of the centroid density in the free particle reference system, an effective potential along with a corresponding energy estimator is derived and compared to the above.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458189
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  • 9
    Electronic Resource
    Electronic Resource
    Dynamic friction on rigid and flexible bonds (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 5084-5095 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A wide variety of problems involving molecular motion in liquids can be formulated in terms of the generalized Langevin equation (GLE). The friction coefficient on a molecular bond or on some more complicated reaction coordinate is then required. An often used approximation is to set the dynamic friction constant equal to the autocorrelation function of the fluctuating force exerted on the frozen bond by the remaining unfrozen coordinates. The true friction involves projection operators and should differ from this approximation. In this paper we derive various identities and show that the rigid bond approximation is the high frequency limit of the true dynamic friction coefficient. We compute the "true'' dynamic friction and the friction approximated on the basis of the rigid or frozen bond and show that the asymptotic limit is very accurate even for frequencies not much larger than the peak frequency of the solvent spectral density. Two different dynamical systems are studied using MD simulations with our newly devised NAPA integrator for systems with disparate time scales. In one the molecule is not allowed to rotate and in the other it is allowed to rotate. Interestingly, even for very long rotational reorientation times, small but significant differences in the long time decay of the bond dynamic friction are observed for rotational and nonrotational molecules—differences, however, that do not produce large differences in the static friction constants.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458647
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  • 10
    Electronic Resource
    Electronic Resource
    Method for accelerating chain folding and mixing (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 99 (1993), S. 6071-6077 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: 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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.465904
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