| 1. |
- Hone, Tyler D, et al.
(författare)
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Comparison of Approximate Quantum Simulation Methods Applied to Normal Liquid Helium at 4 K
- 2007
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Ingår i: The journal of physical chemistry. B. - : American Chemical Society (ACS). - 1520-6106 .- 1520-5207. ; 112:2
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Tidskriftsartikel (refereegranskat)abstract
- The Feynman-Kleinert linearized path integral molecular dynamics (FK-LPI), ring polymer molecular dynamics (RPMD), and centroid molecular dynamics (CMD) methods are applied to the simulation of normal liquid helium. Comparisons of the simulation results at the T = 4 K and = 0.01873 Å-3 state point are presented. The calculated quantum correlation functions for the three methods show significant differences, both in the short time and in the intermediate regions of the spectrum. Our simulation results are also compared to the recent results of other approximate quantum simulation methods. We find that FK-LPI qualitatively agrees with other approximate quantum simulation results while CMD and RPMD predict a qualitatively different impulsive rebound in the velocity autocorrelation function. Frequency space analysis reveals that RPMD exhibits a broad high-frequency tail similar to that from quantum mode coupling theory and numerical analytic continuation approaches, while FK-LPI provides a somewhat more rapid decay at high frequency than any of these three methods. CMD manifests a high-frequency component that is greatly reduced compared with the other methods.
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| 2. |
- Poulsen, Jens Aage, 1972, et al.
(författare)
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Determination of the Van Hove spectrum of liquid He(4): An application of the Feymnan-Kleinert linearized path integral methodology
- 2004
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Ingår i: Journal of Physical Chemistry A. - : American Chemical Society (ACS). - 1089-5639 .- 1520-5215. ; 108:41, s. 8743-8751
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Tidskriftsartikel (refereegranskat)abstract
- The spectrum of the Van Hove correlation function (CF) is calculated for liquid He(4) at 27 K and a density of 0.25 g/cm(3) by utilizing a recently proposed approximation to quantum CFs derived by combining an effective-frequency Boltzmann-Wigner transform with a linearized path integral expression for CFs. For this anharmonic system and highly nonlinear CF, we obtain excellent agreement with available accurate results. In particular, the first, second, and third moments of the spectrum are in very good agreement with both experiment and earlier theoretical results. Also, the calculated kinetic energy is in excellent agreement with accurate path integral Monte Carlo results.
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| 3. |
- Poulsen, Jens Aage, 1972, et al.
(författare)
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Feynman-Kleinert Linearized Path Integral (FK-LPI) Algorithms for Quantum Molecular Dynamics, with Application to Water and He(4)
- 2006
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Ingår i: Journal of Chemical Theory and Computation. ; 2:6, s. 1482-1491
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Tidskriftsartikel (refereegranskat)abstract
- The Feynman-Kleinert Linearized Path Integral (FK-LPI) representation of quantum correlation functions is extended in applications and algorithms. Diffusion including quantum effects for a flexible simple point charge model of liquid water is explored, including new tests of internal consistency. An ab initio quantum correction factor (QCF) is also obtained to correct the far-infrared spectrum of water. After correction, a spectrum based on a classical simulation is in good agreement with the experiment. The FK-LPI QCF is shown to be superior to the so-called harmonic QCF. New computational algorithms are introduced so that the quantum Boltzmann Wigner phase-space density, the central object in the implementation, can be obtained for arbitrary potentials. One scheme requires only that the standard classical force routine be replaced when turning from one molecular problem to another. The new algorithms are applied to the calculation of the Van Hove spectrum of liquid He(4) at 27 K. The spectrum moments are in very good agreement with the experiment. These observations indicate that the FK-LPI approach can be broadly effective for molecular problems involving the dynamics of light nuclei.
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| 4. |
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| 5. |
- Poulsen, Jens Aage, 1972, et al.
(författare)
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Quantum density fluctuations in liquid neon from linearized path-integral calculations
- 2007
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Ingår i: Physical Review B. ; 75:22
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Tidskriftsartikel (refereegranskat)abstract
- The Feynman-Kleinert linearized path-integral [J. A. Poulsen , J. Chem. Phys. 119, 12179 (2003)] representation of quantum correlation functions is applied to compute the spectrum of density fluctuations for liquid neon at T=27.6 K, p=1.4 bar, and Q vector 1.55 Aa(-1). The calculated spectrum as well as the kinetic energy of the liquid are in excellent agreement with the experiment of Cunsolo [Phys. Rev. B 67, 024507 (2003)].
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| 6. |
- Poulsen, Jens Aage, 1972, et al.
(författare)
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Quantum diffusion in liquid para-hydrogen: An application of the Feymnan-Kleinert linearized path integral approximation
- 2004
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Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-6106 .- 1520-5207. ; 108:51, s. 19799-19808
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Tidskriftsartikel (refereegranskat)abstract
- Quantum effects on diffusion in liquid para-hydrogen at temperatures of T = 17 and 25 K and saturated vapor pressure is studied by calculating the diffusion coefficient from the standard Green-Kubo formula, using both the ordinary velocity correlation function (CF) and its Kubo-transformed counterpart. All CFs are calculated with a recently proposed linearized path integral expression for general CFs, using an approximate Wigner transformed Boltzmann operator based on Feynman-Kleinert variational path integral theory. Also, the ability of the approximate Wigner transform to predict the radial distribution function and kinetic energy of the liquid is investigated. The conclusions are as follows: (i) The predicted structure of liquid parahydrogen is in excellent agreement with accurate path integral Monte Carlo calculations at both temperatures. (ii) The calculated liquid kinetic energy is in very good agreement with the accurate value at T = 25 K but deviates somewhat from the accurate value at T = 17 K. (iii) The diffusion coefficients based on the Kubo-transformed CF are in very good agreement with experiment, at both temperatures, whereas results from the ordinary velocity CF are not accurate at T = 17 K. The reason for the better performance of the Kubo CF approach is attributed to the latter's robustness toward errors in the approximate Boltzmann operator Wigner transform. The kinetic energy derived from the Kubo-transformed CFs is in excellent agreement with accurate values at both temperatures.
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| 7. |
- Poulsen, Jens Aage, 1972, et al.
(författare)
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Static and dynamic quantum effects in molecular liquids: a linearized path integral description of water.
- 2005
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Ingår i: Proceedings of the National Academy of Sciences of the United States of America. - : Proceedings of the National Academy of Sciences. - 0027-8424. ; 102:19, s. 6709-14
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Tidskriftsartikel (refereegranskat)abstract
- Structure, transport properties, and IR spectra including quantum effects are calculated for a flexible simple point charge model of liquid water. A recently introduced combination of a variational local harmonic description of the liquid potential surface and the classical Wigner approximation for the dynamics is used. The potential energy and interatomic radial distribution functions are in good agreement with accurate results from the literature and are significantly closer to experiment than predictions found from classical theory. The oxygen and hydrogen velocity correlation functions are also calculated, and the corresponding molecular diffusion coefficient is in good accord with existing theoretical estimates including quantum effects. Of most interest, an ab initio quantum correction factor is obtained to correct the far IR spectrum of water. When corrected, a spectrum based on a classical simulation yields results that agree well with experiment. Combined with internal tests of consistency, these observations indicate that this quite flexible approach will be effective for a variety of molecular problems involving the dynamics of light nuclei.
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| 8. |
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