Path integral Monte Carlo investigations on doped helium clusters

32 págs.; 21 figs.; 5 tabs.One of the most commonly employed methods to study doped helium clusters is the path integral Monte Carlo (PIMC) approach. In this review we present results of recent investigations on a series of both atomic and diatomic dopants attached to droplets formed with up to 40 He atoms. Besides the comparison with similar studies existing in the literature, this work also gives the possibility to analyse different issues such as the role played by the He¿impurity interaction in the overall geometry of the clusters, the inclusion of internal molecular degrees of freedom and the exchange permutation symmetry in the PIMC calculations. The study of the structure and energies of and at thermal equilibrium presented in this work thus covers most of the usual aspects treated for these kinds of doped systems. © 2016 Taylor & Francis GroupThis work was supported by MICINN under FIS2011-29596-C02-01 and FIS2014-51933-P.Peer Reviewe

A configurational study of helium clusters doped with He∗− and He2 ∗−

9 pags.; 10 figs.; 4 tabs.Helium clusters doped with electronically excited atomic and molecular helium anions He∗− and He∗− 2 at T = 0.4 K are studied by means of path integral Monte Carlo calculations. Geometry and energetics of the systems with up to 32 solvating He atoms are characterised. The interactions between the anions and the neutral He atoms have been described by fitting previously reported ab initio points to analytical expressions. The HeN–He∗− clusters with N > 6 display a structure defined by a bipyramid which completely solvates the atomic anion, whereas the rest of surrounding He atoms form a dimple around that initial cage. On the contrary, the structures observed for the HeN–He∗−2 clusters clearly show the dopant located outside the helium droplet, thereby confirming the heliophobic character of He∗− 2. C 2015 AIP Publishing LLCThis work has been supported by MICINN Grant No. FIS2011-29596-C02-01. R.R.-C. acknowledges funding from Grant No. JAE-Pre-2010-01277. F.A.G. acknowledges several helpful discussions with A. Mauracher and thanks him for providing us with the original computed ab initio points for the He–dopant interactionsPeer Reviewe

Quantum features of anionic species He*− and He2*− in small HeN clusters

9 págs.; 11 figs.; 2 tabs.; Published as part of The Journal of Physical Chemistry A virtual special issue “Spectroscopy and Dynamics of Medium-Sized Molecules and Clusters: Theory, Experiment, and Applications”.We present variational calculations on systems containing a few boson helium atoms attached to electronically excited atomic and molecular helium anions He*– and He2*– and characterize their structures and energetics. Previously reported high-level ab initio results [Huber, S. E.; Mauracher, A. Mol. Phys. 2014, 112, 794] to describe the interactions between excited (metastable) anions and a neutral He atom have been employed. For the case of the atomic species He*–, the corresponding interaction with He suggests large anharmonicity effects due to the presence of a deep well of ∼17 500 cm–1 at short distances, together with a more external shallow secondary well of ∼4 cm–1, both supporting bound levels. Moreover, when a sum of pairwise interactions is assumed to describe the full PES corresponding to the presence of several neutral He atoms, geometrical constraints already predict the complete solvation of the anionic impurity by six helium atoms, giving rise to a bipyramidal structure. In turn, for the anisotropic weak interaction He–He2*–, where the anionic dimer is considered as a rigid rotor, the obtained structures show the tendency of the helium atoms to pack themselves together and largely far away from the dopant, thereby confirming the heliophobic character of He2*–.The authors thank the Centro de Calculo (IFF), CTI (CSIC), and Centro de Supercomputacion de Galicia (CESGA) for allocation of computer time. This work has been supported by MICINN, Spain, Grants FIS2011-29596-C02-01 and FIS2014- 51933-P. F.A.G. acknowledges several helpful discussions with A. Mauracher and thanks him for providing us with the original computed ab initio points for the He−dopant interactions. The aid of COST Action CM1405 (MOLIM) is also acknowledged.Peer Reviewe

Variational and Path Integral Monte Carlo calculations on Helium Clusters Doped with Metastable Anions He^*- and He_2^*-

1 pág.; 2 figs.; XXIX International Conference on Photonic, Electronic, and Atomic Collisions (ICPEAC2015); Open Access funded by Creative Commons Atribution Licence 3.0Variational calculations (T = 0 K) on small He_N...He^*- and He_N...He^2*- metastable clusters (N¿4), as well as Path Integral Monte Carlo (PIMC) simulations (T = 0.4 K) on larger species are presented and discussed.Peer Reviewe

Structural changes in water and Ar-water clusters under high pressure

1 pág.; 1 fig.; XXIX International Conference on Photonic, Electronic, and Atomic Collisions (ICPEAC2015); Open Access funded by Creative Commons Atribution Licence 3.0Specific size gas-water clusters are currently receiving considerable attention, as models for inclusion compounds of different type of clathrate hydrates. As model microsolutions they retain many characteristics of the bulk, are theoretically tractable, and can be used to probe the relevant guest/host interactions, as well as to derive and to test intermolecular potentials that can be also used under different thermodynamic conditions.Peer Reviewe

Coronene molecules in helium clusters: Quantum and classical studies of energies and configurations

Under the terms of the Creative Commons Attribution (CC BY) license to their work.Coronene-doped helium clusters have been studied by means of classical and quantum mechanical (QM) methods using a recently developed He–C24H12 global potential based on the use of optimized atom-bond improved Lennard-Jones functions. Equilibrium energies and geometries at global and local minima for systems with up to 69 He atoms were calculated by means of an evolutive algorithm and a basin-hopping approach and compared with results from path integral Monte Carlo (PIMC) calculations at 2 K. A detailed analysis performed for the smallest sizes shows that the precise localization of the He atoms forming the first solvation layer over the molecular substrate is affected by differences between relative potential minima. The comparison of the PIMC results with the predictions from the classical approaches and with diffusion Monte Carlo results allows to examine the importance of both the QM and thermal effects.This work has been supported by MICINN Grant Nos. FIS2011-29596-C02-01, FIS2013-48275-C2-1-P, and FIS2014-51993-P. R.R.-C. acknowledges funding from the Grant No. JAE-Pre-2010-01277. J.H.-R. and J.B. acknowledge the financial support from Ministerio de Economía y Competitividad under Grant No. FIS2013-41532-P.Peer reviewe

Examination of the Feynman-Hibbs Approach in the Study of NeN_N-Coronene Clusters at Low Temperatures

Feynman-Hibbs (FH) effective potentials constitute an appealing approach for investigations of many-body systems at thermal equilibrium since they allow us to easily include quantum corrections within standard classical simulations. In this work we apply the FH formulation to the study of Ne$_N$-coronene clusters ($N=$ 1-4, 14) in the 2-14 K temperature range. Quadratic (FH2) and quartic (FH4) contributions to the effective potentials are built upon Ne-Ne and Ne-coronene analytical potentials. In particular, a new corrected expression for the FH4 effective potential is reported. FH2 and FH4 cluster energies and structures -obtained from energy optimization through a basin-hoping algorithm as well as classical Monte Carlo simulations- are reported and compared with reference path integral Monte Carlo calculations. For temperatures $T> 4$ K, both FH2 and FH4 potentials are able to correct the purely classical calculations in a consistent way. However, the FH approach fails at lower temperatures, especially the quartic correction. It is thus crucial to assess the range of applicability of this formulation and, in particular, to apply the FH4 potentials with great caution. A simple model of $N$ isotropic harmonic oscillators allows us to propose a means of estimating the cut-off temperature for the validity of the method, which is found to increase with the number of atoms adsorbed on the coronene molecule

Estudio de agregados de helio dopados con impurezas atómicas y moleculares mediante el método Path Integral Monte Carlo

Tesis inédita de la Universidad Complutense de Madrid, Facultad de Ciencias Físicas, Departamento de Óptica, leída el 19/12/2014Depto. de ÓpticaFac. de Ciencias FísicasTRUEunpu

Theoretical investigations on doped rare gas clusters with neutral or ionic atomic and diatomic impurities

MOLEC 2016, Toledo (Spain), 11 - 16 September 2016Helium clusters formed with up to N ~ 70 atoms doped with an impurity X have been studied by means of Monte Carlo (MC) simulations and classical optimization algorithms [1]. The dopants under consideration are neutral atoms (Ca) [2,3] and diatoms (Rb2) [4-7], and anionic species He*- and He2 *-[8]. Energetics and structure of the corresponding HeN-X systems has been analyzed for different temperature ranges with a path integral MC (PIMC) approach. A variety of interaction potentials have been employed and the effect of their strength was studied in detail. Special emphasis is made on general issues such as the position of the impurity with respect to the helium cluster and the role of its internal degrees of freedom such as rotation.This work has the support of MICINN under FIS2014-51933-P.Peer Reviewe