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  • 1
    Electronic Resource
    Electronic Resource
    Tight-binding simulations of argon cation clusters (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 109 (1998), S. 3418-3424 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A simple, semiempirical model was used to study the ground and excited state properties of argon cation clusters at 60 K. The model is a tight-binding Hamiltonian whose parameters are determined from atomic and diatomic properties. Monte Carlo simulations were used to calculate the average properties of these clusters. The photoabsorption spectrum was in good agreement with previous calculations and experiments. The splitting of the photoabsorption spectrum for clusters with greater than 14 atoms was investigated. The two excited states corresponding to the splitting arise from a 3-atom ion core, perturbed by a 4th atom, with solvation from the remaining atoms. The perturbation of the 3-atom ion core by the 4th atom is of the form ψion core±ψ4. The splitting can be decomposed into a contribution solely from the 4 atom wave functions (75% of the splitting) and to additional solvation stabilization of the low energy excited state over the high energy excited state (25%).© 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.476937
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  • 2
    Electronic Resource
    Electronic Resource
    Aperiodic crystal description of diffusion in concentrated polymer-solvent systems (1988)
    s.l. : American Chemical Society
    Macromolecules 21 (1988), S. 239-243 
    Details
    ISSN: 1520-5835
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1021/ma00179a046
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  • 3
    Electronic Resource
    Electronic Resource
    Changes in the Fermi surface at the magnetization reorientation transition in Fe/Cu(100) (1999)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 85 (1999), S. 6211-6213 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We present a study of the evolution of the valence band structure of Fe/Cu(100) as a function of Fe coverage in the 0–12 monolayer range. Angle-resolved photoelectron spectroscopy was used to probe the valence electronic structure and to extract Fermi surface contours as a function of increasing Fe coverage. The first few monolayers show Fe-induced densities of states that are significantly different from those in thicker films. At 5 ML, distinct changes occur in the Fermi surface, where a magnetization reorientation transition has been observed previously. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.370224
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  • 4
    Electronic Resource
    Electronic Resource
    The aperiodic crystal picture and free energy barriers in glasses (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 86 (1987), S. 2943-2948 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The aperiodic crystal picture associates the glass transition with freezing into a nonperiodic structure. Dynamics in the glassy state involves activated jumps between different aperiodic free energy minima. Activation barriers may be estimated through the use of freezing theory and the theory of dense solids. The results resemble, but are distinct from, free volume theory. Reasonable fits to experimental data are obtained.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.452045
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  • 5
    Electronic Resource
    Electronic Resource
    Solvent influence on atomic spectra: The effect of finite size (1985)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 83 (1985), S. 3214-3221 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Time dependent Hartree theory is used to determine the solvent effect on atomic spectra for a given solvent configuration. Configuration averaging is performed as in the mean spherical approximation, resulting in an upper bound to the polarizability. Comparisons are made with previous, more approximate theories, including path integral treatments. It is found that deviations from previous theories can be significant in certain regimes.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.449179
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  • 6
    Electronic Resource
    Electronic Resource
    An adaptive, kink-based approach to path integral calculations (2002)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 116 (2002), S. 1-7 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A kink-based expression for the canonical partition function is developed using Feynman's path integral formulation of quantum mechanics and a discrete basis set. The approach is exact for a complete set of states. The method is tested on the 3×3 Hubbard model and overcomes the sign problem seen in traditional path integral studies of fermion systems. Kinks correspond to transitions between different N-electron states, much in the same manner as occurs in configuration interaction calculations in standard ab initio methods. The different N-electron states are updated, based on which states occur frequently during a Monte Carlo simulation, giving better estimates of the true eigenstates of the Hamiltonian. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1423939
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  • 7
    Electronic Resource
    Electronic Resource
    The exchange potential in path integral studies: Analytical justification (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 1926-1931 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present analytical justification for our previously described exchange pseudopotential. We show how the fermi quantum partition function can be constructed from the Boltzmann (distinguishable particle) wave functions if the states that correspond to like-spin electrons occupying the same quantum state are excluded. A class of weighting functions that satisfy this constraint approximately is discussed. Our previous pseudopotential falls under this class. Essentially, our pseudopotential forces the unwanted states to have high energy and, hence, to make negligible contribution to the partition function. Exchange potentials of the form discussed in this article should be useful for studying systems where the (allowed) correlated Boltzmann wave functions have negligible amplitude for like-spin fermion–fermion distances less than the diameter of the individual particle wave packets. For example, in the case of two spin-up (or spin-down) fermions, if one fermion is located at r, then ||Ψ(r,q)||2 is negligible if q(approximately-equal-to)r. This should be the case for systems where a tight binding model is appropriate or for systems with strong interparticle repulsions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.457098
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  • 8
    Electronic Resource
    Electronic Resource
    Path integral study of the correlated electronic states of Na4–Na6 (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 8211-8219 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Feynman's path integral formulation of quantum mechanics is used to study the correlated electronic states of Na4–Na6. Two types of simulations are performed: in the first, the nuclei are allowed to move at finite temperature in order to find the most stable geometries. In agreement with previous calculations, we find that planar structures are the most stable and that there is significant vibrational amplitude at finite temperatures, indicating that the Born–Oppenheimer surface is relatively flat. In the second type of simulation, the nuclei are held fixed at symmetric and asymmetric geometries and the correlated electron density is found. Our results show that the electrons are localized, rather than delocalized as previous workers have concluded from examination of the single-particle orbitals. We find that the best picture of these clusters is that they contain three-center, two-electron bonds.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.459299
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  • 9
    Electronic Resource
    Electronic Resource
    Development, justification, and use of a projection operator in path integral calculations in continuous space (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 95 (1991), S. 5999-6004 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A projection operator, similar to one previously used by us for problems with a finite set of basis functions, is suggested for use with continuous basis sets. This projection operator requires knowledge of the nodes of the density matrix at all temperatures. We show that a class of nodes, determined from the noninteracting density matrix and present at high temperatures in the interacting system are preserved to first order in the interaction at low temperatures. While we cannot show that the nodes are present at intermediate temperatures, we suspect they do exist and, as a test of this conjecture, we perform a calculation of two electrons confined in a harmonic well, using the projection operator. We find that accurate results are obtained at a range of temperatures, suggesting that our conjecture is indeed correct. We find that the error limits determined using the projection operator are 1–2 times smaller than those obtained with straightforward Monte Carlo integration (corresponding to a reduction in time of 1–4 in obtaining a desired level of accuracy).
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.461591
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  • 10
    Electronic Resource
    Electronic Resource
    Path integral studies of the 2D Hubbard model using a new projection operator (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 1312-1316 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Feynman's path integral formulation of quantum mechanics, supplemented by an approximate projection operator (exact in the case of noninteracting particles), is used to study the 2D Hubbard model. The projection operator is designed to study Hamiltonians defined on a finite basis set, but extensions to continuous basis sets are suggested. The projection operator is shown to reduce the variance by a significant amount relative to straightforward Monte Carlo integration. Approximate calculations are usually within one standard deviation of exact results and virtually always within two to three standard deviations. In addition, the algorithm scales with the number of discretization points P as either P or P2 (depending on the method of implementation), rather than the P3 of the Hubbard–Stratonovich transformation. Accuracy to about 5%–10% in energies and spin–spin correlation functions are found using moderate amounts of computer time.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.459988
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