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Anne B. Mccoy - One of the best experts on this subject based on the ideXlab platform.
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Diffusion Monte Carlo Studies on the Detection of Structural Changes in the Water Hexamer upon Isotopic Substitution.
The journal of physical chemistry. A, 2020Co-Authors: Victor G. M. Lee, Nicholas J. Vetterli, Mark A. Boyer, Anne B. MccoyAbstract:The ground state structure of water hexamer and several deuterated variants are studied using Diffusion Monte Carlo (DMC). We demonstrate that a recently developed guided DMC approach allows us to ...
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An Efficient Approach for Studies of Water Clusters Using Diffusion Monte Carlo
The journal of physical chemistry. A, 2019Co-Authors: Victor G. M. Lee, Anne B. MccoyAbstract:An efficient and accurate approach for using Diffusion Monte Carlo (DMC) to study molecular clusters is described and applied to an investigation of (H2O)n with n = 1, 2, 3. In this approach, impor...
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Evaluation of Matrix Elements Using Diffusion Monte Carlo Wave Functions.
The journal of physical chemistry. A, 2019Co-Authors: Victor G. M. Lee, Lindsey R. Madison, Anne B. MccoyAbstract:Approaches for using Diffusion Monte Carlo (DMC) to evaluate matrix elements involving two vibrational wave functions are explored. In the first part of this study, overlaps between a wave function...
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Diffusion Monte Carlo in internal coordinates.
The journal of physical chemistry. A, 2013Co-Authors: Andrew S. Petit, Anne B. MccoyAbstract:An internal coordinate extension of Diffusion Monte Carlo (DMC) is described as a first step toward a generalized reduced-dimensional DMC approach. The method places no constraints on the choice of internal coordinates other than the requirement that they all be independent. Using H(3)(+) and its isotopologues as model systems, the methodology is shown to be capable of successfully describing the ground state properties of molecules that undergo large amplitude, zero-point vibrational motions. Combining the approach developed here with the fixed-node approximation allows vibrationally excited states to be treated. Analysis of the ground state probability distribution is shown to provide important insights into the set of internal coordinates that are less strongly coupled and therefore more suitable for use as the nodal coordinates for the fixed-node DMC calculations. In particular, the curvilinear normal mode coordinates are found to provide reasonable nodal surfaces for the fundamentals of H(2)D(+) and D(2)H(+) despite both molecules being highly fluxional.
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Using fixed-node Diffusion Monte Carlo to investigate the effects of rotation-vibration coupling in highly fluxional asymmetric top molecules: application to H2D+.
The Journal of chemical physics, 2013Co-Authors: Andrew S. Petit, Bethany A. Wellen, Anne B. MccoyAbstract:A fixed-node Diffusion Monte Carlo approach for obtaining the energies and wave functions of the rotationally excited states of asymmetric top molecules that undergo large amplitude, zero-point vibrational motions is reported. The nodal surfaces required to introduce rotational excitation into the Diffusion Monte Carlo calculations are obtained from the roots of the asymmetric top rigid rotor wave functions calculated using the system's zero-point, vibrationally averaged rotational constants. Using H2D+ as a model system, the overall accuracy of the methodology is tested by comparing to the results of converged variational calculations. The ability of the fixed-node Diffusion Monte Carlo approach to provide insights into the nature and strength of the rotation-vibration coupling present in the rotationally excited states of highly fluxional asymmetric tops is discussed. Finally, the sensitivity of the methodology to the details of its implementation, such as the choice of embedding scheme, is explored.
David C. Clary - One of the best experts on this subject based on the ideXlab platform.
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Torsional Diffusion Monte Carlo: A method for quantum simulations of proteins
The Journal of Chemical Physics, 2001Co-Authors: David C. ClaryAbstract:The quantum Diffusion Monte Carlo (DMC) method is extended to the treatment of coupled torsional motions in proteins. A general algorithm and computer program has been developed by interfacing this torsional-DMC method with all-atom force-fields for proteins. The method gives the zero-point energy and atomic coordinates averaged over the coupled torsional motions in the quantum ground state of the protein. Application of the new algorithm is made to the proteins gelsolin (356 atoms and 142 torsions) and gp41-HIV (1101 atoms and 452 torsions). The results indicate that quantum-dynamical effects are important for the energies and geometries of typical proteins such as these.
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Diffusion Monte Carlo simulations of methanol–water clusters
Chemical Physics Letters, 1999Co-Authors: Jaime L Iosue, David M. Benoit, David C. ClaryAbstract:Abstract We report the ground-state properties of several partially deuterated CH 3 OH[H 2 O] n ( n =1, 2) clusters calculated with the rigid-body Diffusion Monte Carlo method. The dimer has two distinct isomers with methanol-donor or water-donor structures. The dimer complex was found to prefer a water-donor structure and the calculated rotational constants for this isomer compare favorably with results obtained in recent experiments. The ground-state structure of the CH 3 OH[H 2 O] 2 trimer was found to be cyclic.
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Diffusion Monte Carlo studies of isotope-substituted water trimers
Chemical Physics Letters, 1996Co-Authors: Jon M. Sorenson, Jonathon K. Gregory, David C. ClaryAbstract:Abstract We report the ground-state properties of several partially deuterated water trimers calculated with the rigid-body Diffusion Monte Carlo method. Rotational constants are compared with recent experiments and good agreement is found. Tunneling splittings for the hydrogen flip in (HDO) 3 are predicted. New results are predicted for the experimentally unexamined mixed trimers.
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Calculation of expectation values of molecular systems using Diffusion Monte Carlo in conjunction with the finite field method
The Journal of Chemical Physics, 1994Co-Authors: P. Sandler, V. Buch, David C. ClaryAbstract:A scheme employing the finite field method is proposed for extracting expectation values of molecular properties from Diffusion Monte Carlo calculations. The method enables calculation of expectation values of nonlocal operators, such as kinetic energies of molecular components of the system. The method is demonstrated for the H2O...N2 cluster.
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A comparison of conventional and rigid body Diffusion Monte Carlo techniques. Application to water dimer
Chemical Physics Letters, 1994Co-Authors: Jonathon K. Gregory, David C. ClaryAbstract:Abstract Diffusion Monte Carlo calculations on the ground state of the water dimer are reported. Two versions of the method have been used. The first is the conventional method with importance sampling which simulates all vibrations in the dimer and the second is one due to Buch which treats the water monomers as rigid bodies and therefore removes their vibrational motion. We find good agreement between the results of the two approaches in terms of their calculations of zero-point energies, geometries and rotational constants.
E. Curotto - One of the best experts on this subject based on the ideXlab platform.
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On Diffusion Monte Carlo in spaces with multi-valued maps, boundaries and gradient torsion
Chemical Physics Letters, 2021Co-Authors: Lena Jake, E. CurottoAbstract:Abstract Importance sampling in Diffusion Monte Carlo has a long history. However, only recently, simulations of ground state properties have been extended to spaces mapped with non-Cartesian coordinates. We demonstrate that in spaces with nonzero advection the Smoluchowski operator for any nontrivial trial wavefunction does not converge to the exact result. Rather, every drift term is equivalent to some advection in a manifold that contains the physical space of the system. Since these manifolds may be formulated with gradient torsion we demonstrate with several numerical experiments that Diffusion Monte Carlo is possible in these as well.
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Smart darting Diffusion Monte Carlo: Applications to lithium ion-Stockmayer clusters
The Journal of chemical physics, 2016Co-Authors: Helen Christensen, L. C. Jake, E. CurottoAbstract:In a recent investigation [K. Roberts et al., J. Chem. Phys. 136, 074104 (2012)], we have shown that, for a sufficiently complex potential, the Diffusion Monte Carlo (DMC) random walk can become quasiergodic, and we have introduced smart darting-like moves to improve the sampling. In this article, we systematically characterize the bias that smart darting moves introduce in the estimate of the ground state energy of a bosonic system. We then test a simple approach to eliminate completely such bias from the results. The approach is applied for the determination of the ground state of lithium ion-n–dipoles clusters in the n = 8–20 range. For these, the smart darting Diffusion Monte Carlo simulations find the same ground state energy and mixed-distribution as the traditional approach for n < 14. In larger systems we find that while the ground state energies agree quantitatively with or without smart darting moves, the mixed-distributions can be significantly different. Some evidence is offered to conclude th...
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A rare event sampling method for Diffusion Monte Carlo using smart darting
The Journal of chemical physics, 2012Co-Authors: K. Roberts, R. Sebsebie, E. CurottoAbstract:We identify a set of multidimensional potential energy surfaces sufficiently complex to cause both the classical parallel tempering and the guided or unguided Diffusion Monte Carlo methods to converge too inefficiently for practical applications. The mathematical model is constructed as a linear combination of decoupled Double Wells [(DDW)n]. We show that the set (DDW)n provides a serious test for new methods aimed at addressing rare event sampling in stochastic simulations. Unlike the typical numerical tests used in these cases, the thermodynamics and the quantum dynamics for (DDW)n can be solved deterministically. We use the potential energy set (DDW)n to explore and identify methods that can enhance the Diffusion Monte Carlo algorithm. We demonstrate that the smart darting method succeeds at reducing quasiergodicity for n ≫ 100 using just 1 × 106 moves in classical simulations (DDW)n. Finally, we prove that smart darting, when incorporated into the regular or the guided Diffusion Monte Carlo algorithm,...
Michele Casula - One of the best experts on this subject based on the ideXlab platform.
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size consistent variational approaches to nonlocal pseudopotentials standard and lattice regularized Diffusion Monte Carlo methods revisited
Journal of Chemical Physics, 2010Co-Authors: Michele Casula, Sandro Sorella, S Moroni, Claudia FilippiAbstract:We propose improved versions of the standard Diffusion Monte Carlo (DMC) and the lattice regularized Diffusion Monte Carlo (LRDMC) algorithms. For the DMC method, we refine a scheme recently devised to treat nonlocal pseudopotential in a variational way. We show that such scheme—when applied to large enough systems—maintains its effectiveness only at correspondingly small enough time-steps, and we present two simple upgrades of the method which guarantee the variational property in a size-consistent manner. For the LRDMC method, which is size-consistent and variational by construction, we enhance the computational efficiency by introducing: (i) an improved definition of the effective lattice Hamiltonian which remains size-consistent and entails a small lattice-space error with a known leading term and (ii) a new randomization method for the positions of the lattice knots which requires a single lattice-space
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size consistent variational approaches to non local pseudopotentials standard and lattice regularized Diffusion Monte Carlo methods revisited
arXiv: Other Condensed Matter, 2010Co-Authors: Michele Casula, Saverio Moroni, Sandro Sorella, Claudia FilippiAbstract:We propose improved versions of the standard Diffusion Monte Carlo (DMC) and the lattice regularized Diffusion Monte Carlo (LRDMC) algorithms. For the DMC method, we refine a scheme recently devised to treat non-local pseudopotential in a variational way. We show that such scheme --when applied to large enough systems-- maintains its effectiveness only at correspondingly small enough time-steps, and we present two simple upgrades of the method which guarantee the variational property in a size-consistent manner. For the LRDMC method, which is size-consistent and variational by construction, we enhance the computational efficiency by introducing (i) an improved definition of the effective lattice Hamiltonian which remains size-consistent and entails a small lattice-space error with a known leading term, and (ii) a new randomization method for the positions of the lattice knots which requires a single lattice-space.
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beyond the locality approximation in the standard Diffusion Monte Carlo method
Physical Review B, 2006Co-Authors: Michele CasulaAbstract:We present a way to include nonlocal potentials in the standard Diffusion Monte Carlo method without using the locality approximation. We define a stochastic projection based on a fixed node effective Hamiltonian, whose lowest energy is an upper bound of the true ground-state energy, even in the presence of nonlocal operators in the Hamiltonian. The variational property of the resulting algorithm provides a stable Diffusion process, even in the case of divergent nonlocal potentials, like the hard-core pseudopotentials. It turns out that the modification required to improve the standard Diffusion Monte Carlo algorithm is simple.
Joaquim Casulleras - One of the best experts on this subject based on the ideXlab platform.
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Quadratic Diffusion Monte Carlo and pure estimators for atoms
The Journal of Chemical Physics, 2002Co-Authors: Antonio Sarsa, Jordi Boronat, Joaquim CasullerasAbstract:The implementation and reliability of a quadratic Diffusion Monte Carlo method for the study of ground-state properties of atoms are discussed. We show in the simple yet non-trivial calculation of the binding energy of the Li atom that the method presented is effectively second-order in the time step. The fulfilment of the expected quadratic behavior relies on some basic requirements of the trial wave function used for importance sampling, in the context of the fixed-node approximation. Expectation values of radial operators are calculated by means of a pure estimation based on the forward walking methodology. It is shown that accurate results without extrapolation errors can be obtained with a pure algorithm that can be easily implemented in any previous Diffusion Monte Carlo program.
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Ground state of a homogeneous Bose gas: A Diffusion Monte Carlo calculation
Physical Review A, 1999Co-Authors: Simona Giorgini, Jordi Boronat, Joaquim CasullerasAbstract:We use a Diffusion Monte Carlo method to calculate the lowest energy state of a uniform gas of bosons interacting through different model potentials, both strictly repulsive and with an attractive well. We explicitly verify that at low density the energy per particle follows a universal behavior fixed by the gas parameter na^3. In the regime of densities typical for experiments in trapped Bose-condensed gases, the corrections to the mean-field energies greatly exceed the differences due to the details of the potential.
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VORTEX EXCITATION IN SUPERFLUID 4HE : A Diffusion Monte Carlo STUDY
Physical Review Letters, 1996Co-Authors: Simona Giorgini, Jordi Boronat, Joaquim CasullerasAbstract:We present a Diffusion Monte Carlo study of a single vortex in two-dimensional superfluid liquid $^4$He within the fixed node approximation. We use both the Feynman phase and an improved phase which includes backflow correlations to model the nodal surface of the vortex wavefunction. Results for the particle density, core radius and excitation energies are presented.
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Vortex Excitation in Superfluid4He: A Diffusion Monte Carlo Study
Physical review letters, 1996Co-Authors: Simona Giorgini, Jordi Boronat, Joaquim CasullerasAbstract:We present a Diffusion Monte Carlo study of a single vortex in two-dimensional superfluid liquid $^4$He within the fixed node approximation. We use both the Feynman phase and an improved phase which includes backflow correlations to model the nodal surface of the vortex wavefunction. Results for the particle density, core radius and excitation energies are presented.
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Diffusion Monte Carlo study of two-dimensional liquid 4He.
Physical review. B Condensed matter, 1996Co-Authors: Simona Giorgini, Jordi Boronat, Joaquim CasullerasAbstract:The ground-state properties of two-dimensional liquid $^{4}\mathrm{He}$ at zero temperature are studied by means of a quadratic Diffusion Monte Carlo method. As interatomic potential we use a revised version of the HFDHE2 Aziz potential which is expected to give a better description of the interaction between helium atoms. The equation of state is determined with great accuracy over a wide range of densities in the liquid phase from the spinodal point up to the freezing density. The spinodal decomposition density is estimated and other properties of the liquid, such as the radial distribution function, static form factor, and density dependence of the condensate fraction, are all presented.