Excited states by quantum Monte Carlo methods: Imaginary time evolution with projection operators

Blume, D.; Lewerenz, Marius; Niyaz, Parhat; Whaley, K. Birgitta

doi:10.1103/physreve.55.3664

Cited by 69 publications

(79 citation statements)

References 47 publications

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“…In this scheme, the excited state energies are extracted from the two-sided inverse Laplace transform of an imaginary time correlation functionκ(τ ) that is obtained by a multidimensional Monte Carlo integration combined with zero temperature diffusion Monte Carlo sidewalks. The basic formulation of the method has been previously discussed [23], and thus we present here only a brief summary of the main ideas, together with details specific to the present study.…”

Section: Projection Operator Imaginary Time Spectral Evolution (Poitsmentioning

confidence: 99%

“…[24]. Our use of this approach is identical to that employed in previous POITSE applications [23,[25][26][27]. Since the maximum entropy analysis requires independent samples ofκ(τ ), the initial configuration for each DMC sidewalk is taken from the VMC walk every 200 steps apart, to minimize correlations between successive sidewalks.…”

Section: Projection Operator Imaginary Time Spectral Evolution (Poitsmentioning

confidence: 99%

“…It has been shown in ref. [23] that this dependence on the arbitrary reference energy can be eliminated by introducing the normalization factor…”

Section: Projection Operator Imaginary Time Spectral Evolution (Poitsmentioning

confidence: 99%

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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: Projection Operator Imaginary Time Spectral Evolution (Poitsmentioning

confidence: 99%

Section: Projection Operator Imaginary Time Spectral Evolution (Poitsmentioning

confidence: 99%

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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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“…The scarcity of low-lying excited states typical of superfluid systems makes it possible in this case to extract information on the location and intensity of the spectral lines from an analysis of the time series generated by quantum Monte Carlo random walks. 6,7,8 The rotational spectrum of OCS@He N has been studied along these lines in Refs. [9,10].…”

Section: Introductionmentioning

confidence: 99%

Rotational dynamics of CO solvated in small He clusters: A quantum Monte Carlo study

Cazzato

Paolini

Moroni

et al. 2004

The Journal of Chemical Physics

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The rotational dynamics of CO single molecules solvated in small He clusters (CO@HeN ) has been studied using Reptation Quantum Monte Carlo for cluster sizes up to N = 30. Our results are in good agreement with the roto-vibrational features of the infrared spectrum recently determined for this system, and provide a deep insight into the relation between the structure of the cluster and its dynamics. Simulations for large N also provide a prediction of the effective moment of inertia of CO in the He nano-droplet regime, which has not been measured so far.

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“…Exact calculation of excitation energies is possible with the projection operation imaginary time spectral evolution approach (POITSE) which is generally very time-consuming. [30,31] Alternatively, excited states can be calculated with fixed node approximations, but these generally introduce an unknown amount of bias by the choice of nodal surfaces [31]. The methods described below share the element of approximation with fixed node in the sense that they are approximative, but possess the advantage that it is very straightfor-ward and can be implemented with the same efficiency as ground state energy calculations.…”

Section: Methods For Rotational Excitationsmentioning

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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Excited states by quantum Monte Carlo methods: Imaginary time evolution with projection operators

Cited by 69 publications

References 47 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

Rotational dynamics of CO solvated in small He clusters: A quantum Monte Carlo study

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

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