Importance sampling in rigid body diffusion Monte Carlo

Viel, Alexandra; Patel, Mehul V.; Niyaz, Parhat; Whaley, K. Birgitta

doi:10.1016/s0010-4655(02)00145-5

Cited by 46 publications

(61 citation statements)

References 61 publications

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“…We employ here the rigid-body diffusion Monte Carlo (RB-DMC) scheme that is described in ref. [20]. In the following we provide only a summary of the main concepts and give details specific to the OCS(p-H 2 ) N system.…”

Section: Diffusion Monte Carlo (Dmc) Methodsmentioning

confidence: 99%

OCS in small para-hydrogen clusters: Energetics and structure with N=1–8 complexed hydrogen molecules

Paesani

Zillich

Whaley

2003

The Journal of Chemical Physics

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We determine the structure and energetics of complexes of the linear OCS molecule with small numbers of para-hydrogen molecules, N =1-8, using zero temperature quantum Monte Carlo methods. Ground state calculations are carried out with importance-sampled rigid body diffusion Monte Carlo (IS-RBDMC) and excited state calculations with the projection operator imaginary time spectral evolution (POITSE) methodology. The ground states are found to be highly structured, with a gradual build up of two axial rings as N increases to 8. Analysis of the azimuthal density correlations around the OCS molecule shows that these rings are quite delocalized for small N values, but become strongly localized for N ≥ 5 . Excited state calculations are made for a range of total cluster angular momentum values and the rotational energy levels fitted to obtain effective rotational and distortion constants of the complexed OCS molecule as a function of cluster size N . Detailed analysis of these spectroscopic constants indicates that the complexes of OCS with para-hydrogen have an unusually rich variation in dynamical behavior, with sizes N =1-2 showing near rigid behavior, sizes N =3-4 showing extremely floppy behavior, and the larger sizes N =5-8 showing more rigid behavior again. The large values of the distortion constant D obtained for N =3-4 are rationalized in terms of the coupling between the OCS rotations and the "breathing" mode of the first, partially filled ring of para-hydrogen molecules.

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Section: Diffusion Monte Carlo (Dmc) Methodsmentioning

confidence: 99%

OCS in small para-hydrogen clusters: Energetics and structure with N=1–8 complexed hydrogen molecules

Paesani

Zillich

Whaley

2003

The Journal of Chemical Physics

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“…[31] for the specific implementation). The second step is that of using the optimized wavefunction in a Diffusion Monte Carlo (DMC) algorithm [32,33,34] in order to obtain the energy and the geometric distributions of the cluster. We have discussed our DMC procedure more specifically in earlier work [35,36,37] and will therefore not be repeating it here.…”

Section: The Monte Carlo Calculationsmentioning

confidence: 99%

Bosonic helium droplets with cationic impurities: Onset of electrostriction and snowball effects from quantum calculations

Coccia

Bodo

Marinetti

et al. 2007

The Journal of Chemical Physics

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Variational MonteCarlo and Diffusion MonteCarlo calculations have been carried out for cations like Li + , Na + and K + as dopants of small helium clusters over a range of cluster sizes up to about 12 solvent atoms.The interaction has been modelled through a sum-of-potential picture that disregards higher order effects beyond atom-atom and atom-ion contributions. The latter were obtained from highly correlated ab-initio calculations over a broad range of interatomic distances.This study focuses on two of the most striking features of the microsolvation in a quantum solvent of a cationic dopant: electrostriction and snowball effects. They are here discussed in detail and in relation with the nanoscopic properties of the interaction forces at play within a fully quantum picture of the clusters features.

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“…∂ϕ x and ∂ϕ y are the infinitesimal angles of rotation of the principal axis frame of OCS, r ≡ |r X − r 0 | is the distance between atom and center of mass of OCS, and θ the angle between the OCS axis and the location of the atom. We employ the rigid body diffusion Monte Carlo algorithm [26,27] in which the rotational degrees of freedom ϕ x and ϕ y are sampled by random walks in the angular variable taken in the principal axis frame of the molecule. Although it is not essential for a system of only 8 coordinates (r X , r 0 , ϕ x , ϕ y ), of which only r and u ≡ cos θ enter in the potential calculation, we employ biased DMC here, sampling the state Φ 0 Φ T instead of Φ 0 , with the trial wave function…”

Section: Ground Statementioning

confidence: 99%

“…This trial function has both angular and radial dependence, and thus it requires the full rotational importance sampled algorithm developed in Ref. [27]. Importance-sampled DMC yields exact values for the ground state energy and also for expectation values of operators commuting with the Hamiltonian.…”

Section: Ground Statementioning

confidence: 99%

Comparison of rotational energies and rigidity of OCS–para-H2 and OCS–4He complexes

Zillich¹,

Whaley²

2003

Chemical Physics

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We analyze the nature of the rotational energy level structure of the OCS-He and OCS-H 2 complexes with a comparison of exact calculations to several different dynamical approximations. We compare with the clamped coordinate quasiadiabatic approximation that introduces an effective potential for each asymmetric rotor level, with an effective rotation Hamiltonian constructed from ground state averages of the inverse of the inertial matrix, and investigate the usefulness of the Eckart condition to decouple rotations and vibrations of these weakly bound complexes between linear OCS and 4 He or H 2 . Comparison with exact results allows an assessment of the accuracies of the different approximate methods and indicates which approaches are suitable for larger clusters of OCS with 4 He and with H 2 . We find the OCS-H 2 complex is considerably more rigid than the OCS-He complex, suggesting that semi-rigid models are useful for analysis of larger clusters of H 2 with OCS.

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Importance sampling in rigid body diffusion Monte Carlo

Cited by 46 publications

References 61 publications

OCS in small para-hydrogen clusters: Energetics and structure with N=1–8 complexed hydrogen molecules

OCS in small para-hydrogen clusters: Energetics and structure with N=1–8 complexed hydrogen molecules

Bosonic helium droplets with cationic impurities: Onset of electrostriction and snowball effects from quantum calculations

Comparison of rotational energies and rigidity of OCS–para-H2 and OCS–4He complexes

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