Higher order diffusion Monte Carlo propagators for linear rotors as diffusion on a sphere: Development and application to O2@He<i>n</i>

Phys. Chem. Chem. Phys.

2013

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The structure and energetics of exciplexes M*((2)L)He(n) (M = Cu, Ag and Au; L = P and D) in their vibrational ground state are studied by employing diffusion Monte Carlo (DMC). Interaction potentials between the excited coinage metals and He atoms are built using the Diatomics-in-Molecule (DIM) approach and ab initio potential curves for the M((2)L)-He dimers. Extending our previous work [Cargnoni et al., J. Phys. Chem. A, 2011, 115, 7141], we computed the dimer potential for Au in the (2)P and (2)D states, as well for Cu and Ag in the (2)D state, employing basis set superposition error-corrected Configuration Interaction calculations. We found that the (2)Π potential correlating with the (2)P state of Au is substantially less binding than for Ag and Cu, a trend well supported by the M(+) ionic radiuses. Conversely, the interaction potentials between a (n - 1)d(9)ns(2 2)D metal and He present a very weak dependency on M itself or the projection of the angular momentum along the dimer axis. This is due to the screening exerted by the ns(2) electrons on the hole in the (n - 1)d shell. Including the spin-orbit coupling perturbatively in the DIM energy matrix has a major effect on the lowest potential energy surface of the (2)P manifold, the one for Cu allowing the formation of a "belt" of five He atoms while the one for Au being completely repulsive. Conversely, spin-orbit coupling has only a weak effect on the (2)D manifold due to the nearly degenerate nature of the diatomic potentials. Structural and energetic results from DMC have been used to support experimental indications for the formation of metastable exciplexes or the opening of non-radiative depopulation channels in bulk and cold gaseous He.

Section: P Statesmentioning

confidence: 99%

Coinage metal exciplexes with helium atoms: a theoretical study of M*(2L)Hen (M = Cu, Ag, Au; L = P,D)

Cargnoni

Ponti

Phys. Chem. Chem. Phys.

2013

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“…The diffusion of the H 2 center of mass is simulated sampling Gaussian distributed displacements with variance δt/m, with m = 3672.4 a.u. being the molecular mass of the hydrogen molecule; the rotational diffusion of H 2 needed in the simulations employing the full 5D ammonia-hydrogen molecules surface is simulated employing a recently proposed algorithm 80 capable of reducing the single step error to second order in the time step δt and third in the curvature R. This choice guarantees a robust second order error for the average potential estimator of E 0 , which is employed in this work due to the absence of the trial wave function.…”

Section: Quantum Simulationsmentioning

confidence: 99%

Quantum simulations of the hydrogen molecule on ammonia clusters

The Journal of Chemical Physics

Curotto

2013

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Mixed ammonia-hydrogen molecule clusters [H2-(NH3)n] have been studied with the aim of exploring the quantitative importance of the H2 quantum motion in defining their structure and energetics. Minimum energy structures have been obtained employing genetic algorithm-based optimization methods in conjunction with accurate pair potentials for NH3-NH3 and H2-NH3. These include both a full 5D potential and a spherically averaged reduced surface mimicking the presence of a para-H2. All the putative global minima for n ≥ 7 are characterized by H2 being adsorbed onto a rhomboidal ammonia tetramer motif formed by two double donor and two double acceptor ammonia molecules. In a few cases, the choice of specific rhombus seems to be directed by the vicinity of an ammonia ad-molecule. Diffusion Monte Carlo simulations on a subset of the species obtained highlighted important quantum effects in defining the H2 surface distribution, often resulting in populating rhomboidal sites different from the global minimum one, and showing a compelling correlation between local geometrical features and the relative stability of surface H2. Clathrate-like species have also been studied and suggested to be metastable over a broad range of conditions if formed.

“…In the case of the diffusion equation on the two‐sphere,

{double-struckS}^{2}

, a particular choice of coordinate involving the geodesic distance allows one to implement a perturbation treatment leading to a short time approximation of the diffusion Green's function that is a third‐order in

Δ τ

and fourth in the ratio between the diffusion displacement along a great circle and the sphere radius. Such approximation has been tested, and confirmed to have a cumulative second‐order error in

Δ τ

, when computing dynamical observables such as the average diffused angle along a great circle after a chosen elapsed time

n_{step} Δ τ

(

n_{step} \sim 10 - 100

) . The need for testing the diffusive dynamics directly, rather than the steady state estimate of a few observables as commonly done while testing DMC, is rooted into the fact that the

τ \to \infty

distribution for the diffusion in a curved space is a constant.…”

Section: Methodsmentioning

confidence: 99%

Quantum monte carlo methods for constrained systems

Wolf

Curotto

2014

Int. J. Quantum Chem.

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The torsional ground state for ethane, the torsional, rotational, and mixed torsional and rotational ground state of propane are computed with a version of diffusion Monte Carlo adapted to handle the geometric complexity of curved spaces such as the Ramachandra space. The quantum NVT ensemble average for the mixed torsional and rotational degrees of freedom of propane is computed, using a version of Monte Carlo path integral, also adapted to handle curved spaces. These three problems are selected to demonstrate the generality and the applicability of the approaches described. The spaces of coordinates can be best constructed from the parameters of continuous Lie groups, and alternative methods based on vector spaces, where extended Lagrangian terms would be too cumbersome to implement. We note that the geometric coupling between the torsions and the rotations of propane produces a substantial effect on the ground state energy of propane, and that the quantum effects on the energy of propane are quite large even well above room temperature.