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  • 1
    Electronic Resource
    Electronic Resource
    A Monte Carlo simulation of liquid 1,2-dimethoxyethane (1990)
    s.l. : American Chemical Society
    The @journal of physical chemistry 〈Washington, DC〉 94 (1990), S. 4299-4302 
    Details
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1021/j100373a077
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  • 2
    Electronic Resource
    Electronic Resource
    Spatial-partitioning-based acceleration for variational Monte Carlo (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 6180-6189 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The problem of inherently differing time scales of core and valence electrons in Monte Carlo (MC) simulations is circumvented in a straightforward and intuitive manner. By appropriately subdividing into equivalent subspaces the high-dimensional (many-electron) space in which Monte Carlo integration is done, it is possible to choose completely independent and appropriate sampling times for each "electron." This approach trivially satisfies detailed balance. The partitioning of space is applicable to both variational and Green's function MC. Such a partitioning, however, only provides a significant computational advantage in variational MC. Using this approach we were able to have inner electrons move with reasonably large steps and yet avoid excessive rejection, while outer electrons were moved great distances in few steps. The net result is a large decrease in the sampling autocorrelation time, and a corresponding increase in convergence rate. Results of several standard algorithms are compared with the present acceleration algorithm for the atoms Be and Ne, and the molecule Li2. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.479922
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  • 3
    Electronic Resource
    Electronic Resource
    A spline approach to trial wave functions for variational and diffusion Monte Carlo (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 6230-6237 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We describe how to combine the variational Monte Carlo method with a spline description of the wave function to obtain a powerful and flexible method to optimize electronic and nuclear wave functions. A property of this method is that the optimization is performed "locally": During the optimization, the attention is focused on a region of the wave function at a certain time, with little or no perturbation in far away regions. This allows a fine tuning of the wave function even in cases where there is no experience on how to choose a good functional form and a good basis set. After the optimization, the splines were fitted using more familiar analytical global functions. The flexibility of the method is shown by calculating the electronic wave function for some two and three electron systems, and the nuclear wave function for the helium trimer. For 4He3, using a two-body helium–helium potential, we obtained the best variational function to date, which allows us to estimate the exact energy with a very small variance by a diffusion Monte Carlo simulation. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.479927
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  • 4
    Electronic Resource
    Electronic Resource
    Quantum Monte Carlo investigation of small 4He clusters with a 3He impurity (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 717-722 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Small helium (4He) clusters containing the lighter isotope 3He are studied by means of quantum Monte Carlo methods. Accurate ground state energies and structural properties are obtained using accurate trial wave functions and the Tang–Tonnies–Yiu (TTY) helium–helium pair potential. The dimer 4He–3He is not bound; as well as the trimer 4He3He2. The smallest cluster containing 3He is 4He2 3He with a nonrigid structure having a marked linear contribution. Interestingly, this weakly bound system, with an energy one order of magnitude less than the 4He3 trimer, is able to bind another 3He atom, forming the tetramer 4He2 3He2, which shows the odd feature of having five out of six unbound pairs. In general, the substitution of a single 4He atom in a pure cluster with a 3He atom leads to an energetic destabilization, as the pair 4He–3He is not bound. The isotopic impurity is found to perturb only weakly the distributions of the remaining 4He atoms, which retain the high floppiness already found in the pure clusters. As the number of atoms increases the isotopic impurity has the marked tendency to stay on the surface of the cluster. This behavior is consistent with the formation of the so-called "Andreev states" of a single 3He in liquid 4He helium and droplets, where the impurity tends to form single-particle states on the surface of the pure 4He. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.480604
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  • 5
    Electronic Resource
    Electronic Resource
    Positron chemistry by quantum Monte Carlo. II. Ground-state of positron-polar molecule complexes (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 109 (1998), S. 1716-1720 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The stability of the ground-state of positron-polar molecule complexes [M,e+] has been explored for M=LiH,HF,H2O,BeO,LiF using variational and diffusion Monte Carlo techniques. Our simulations show that the ground-state of the complexes [LiH,e+]2,1Σ+, [BeO,e+]2,1Σ+, and [LiF,e+]2,1Σ+ is stable against the dissociation either in the two fragments M and e+ or in the other two fragments M+ and Ps=[e+,e−], while the ground-state of [H2O,e+]2,1A1, and of [HF,e+]2,1Σ+ has an energy equal to the dissociation threshold, M and e+. We also compare the predicted vertical positron affinity (PA) with high quality vertical electron affinity (EA) and discuss the relevant difference between the two values. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.476745
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  • 6
    Electronic Resource
    Electronic Resource
    Positron and positronium chemistry by quantum Monte Carlo. III. Ground state of [OH,Ps], [CH,Ps], and [NH2,Ps] complexes (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 109 (1998), S. 5931-5934 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The stability of the [OH,Ps], [CH,Ps] and [NH2,Ps] complexes, where Ps = (e+,e−), have been studied using Monte Carlo techniques and explicitly correlated trial wave functions. For these systems we have computed the ground state energy values using both variational Monte Carlo and fixed node diffusion Monte Carlo methods. Diffusion Monte Carlo results allow us to predict [OH,Ps] and [CH,Ps] systems to be stable against positron and Positronium (Ps) loss. As far as [NH2,Ps] is concerned, its small value of the Binding Energy (BE) does not allow us to conclude about its stability: this is discussed qualitatively including zero point motion of the atoms in the complex. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.477216
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  • 7
    Electronic Resource
    Electronic Resource
    Positronium chemistry by quantum Monte Carlo. I. Positronium-first row atom complexes (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 108 (1998), S. 4756-4760 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The stability of the ground and excited states of Positronium–atom complexes [A, Ps], Ps =[e+,e−], has been explored for A=Li, B, C, O, F using variational and diffusion Monte Carlo techniques. From the numerical results of our simulations it turns out that the ground state of the complexes [Li, Ps]2S, [C, Ps]3S, [O, Ps]1P, and [F, Ps]2S is stable against the break up in the two neutral fragments A and Ps, while the ground state of [B, Ps]2P has an energy above the same dissociation threshold. As to the excited states, the only possible candidate, [F, Ps]2P, has a total energy statistically equal to the lower dissociation threshold, i.e. it does not seem to be stable against the dissociation. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.475887
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  • 8
    Electronic Resource
    Electronic Resource
    Robust wave function optimization procedures in quantum Monte Carlo methods (2002)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 116 (2002), S. 5345-5350 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The energy variance optimization algorithm over a fixed ensemble of configurations in variational Monte Carlo often encounters problems of convergence. Being formally identical to a problem of fitting data, we re-examine it from a statistical maximum-likelihood point of view. We show that the assumption of an underlying Gaussian distribution of the local energy, implicit in the standard variance minimization scheme, is not theoretically nor practically justified, and frequently generates convergence problems. We propose alternative procedures for optimization of trial wave functions in quantum Monte Carlo and successfully test them by optimizing a trial wave function for the helium trimer. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1455618
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