Estudio teórico de agregados atómicos mediante el método Path Integral Monte Carlo

El objetivo principal de esta tesis ha sido desarrollar un código Path Integral Monte Carlo(PIMC), para posteriormente aplicarlo a sistemas compuestos por átomos de muy distinta naturaleza en orden creciente de complejidad. En cada caso, se investigan las propiedades de estos sistemas en equilibrio a una temperatura finita, siempre dentro de la aproximación de Born-Oppenheimer y considerando el estado electrónico fundamental. En primer lugar, se han estudiado las propiedades termodinámicas de agregados pequeños de argón (dímero y trímero) con el propósito de entender procesos macroscópicos como la evaporación en términos de variables microscópicas. Se hará especial hincapié en discutir la validez de la técnica comunmente utilizada de restringir el espacio configuracional accesible al sistema con el fin de evitar evaporaciones espúreas

Exact and quantum chemistry-like calculations in helium doped clusters: The He2Br2(X) example

7 pages, 2 figures, 2 tables.-- Issue title: "Proceedings from the Eleventh European Workshop on Quantum Systems in Chemistry and Physics", edited by Oleg Vasyutinskii, Jean Maruani, Piotr Piecuch, Gerardo Delgado-Barrio, Stephen Wilson.A quantum chemistry-like approach has been recently developed in our group to deal with HeN-BC doped helium clusters, where the BC dopant is a conventional diatomic molecule. The central idea is to consider the He atoms as electrons while the B and C atoms play the role of the nuclei in standard electronic structure calculations. The procedure provides energies and wavefunctions allowing to perform spectral simulations and, hence, making feasible to do proper comparisons with current experiments. However, because of the large difference of masses of He and electrons, and also to the replacement of Coulomb potentials by molecular interactions, it is worthy to assess to what extent the approximations involved in this model (decoupling of orbital angular momenta of the He atoms from the BC rotation and adiabatic separation of the BC stretch versus the He motions) lead to accurate results. In this work we address these issues on the 4He2-Br2(X) system, containing a couple of bosonic He atoms for which variational calculations can be performed.Funded by DGICYT Spanish grants (FIS2004-02461, CTQ2004-02415/BQU) and Spanish "Ramón y Cajal" Programme, Ministerio de Educación y Ciencia; Grant Number: PDRyC-2003-001015, PDRyC-2006-001017.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 teórico de agregados atómicos mediante el método Path Integral Monte Carlo

El objetivo principal de esta tesis ha sido desarrollar un código Path Integral Monte Carlo(PIMC), para posteriormente aplicarlo a sistemas compuestos por átomos de muy distinta naturaleza en orden creciente de complejidad. En cada caso, se investigan las propiedades de estos sistemas en equilibrio a una temperatura finita, siempre dentro de la aproximación de Born-Oppenheimer y considerando el estado electrónico fundamental. En primer lugar, se han estudiado las propiedades termodinámicas de agregados pequeños de argón (dímero y trímero) con el propósito de entender procesos macroscópicos como la evaporación en términos de variables microscópicas. Se hará especial hincapié en discutir la validez de la técnica comunmente utilizada de restringir el espacio configuracional accesible al sistema con el fin de evitar evaporaciones espúreas.Peer Reviewe

Detection of parity violation in chiral molecules by external tuning of electroweak optical activity

A proposal is made to measure the parity-violating energy difference between enantiomers of chiral molecules by modifying the dynamics of the two-state system using an external chiral field, in particular, circularly polarized light. The intrinsic molecular parity-violating energy could be compensated by this external chiral field, with the subsequent change in the optical activity. From the observation of changes in the time-averaged optical activity of a sample with initial chiral purity and minimized environment effects, the value of the intrinsic parity-violating energy could be extracted. A discussion is made on the feasibility of this measurement

Full-dimensional quantum calculations of the dissociation energy, zero-point, and 10 K properties of H 7 + / D 7 + clusters using an ab initio potential energy surface

8 pags. ; 9 figsDiffusion Monte Carlo (DMC) and path-integral Monte Carlo computations of the vibrational ground state and 10 K equilibrium state properties of the H+7 / D+7 cations are presented, using an ab initio full-dimensional potential energy surface. The DMC zero-point energies of dissociated fragments H+5 ( D+5 )+H2(D2) are also calculated and from these results and the electronic dissociation energy, dissociation energies, D0, of 752 ± 15 and 980 ± 14 cm−1 are reported for H+7 and D+7 , respectively. Due to the known error in the electronic dissociation energy of the potential surface, these quantities are underestimated by roughly 65 cm−1. These values are rigorously determined for first time, and compared with previous theoretical estimates from electronic structure calculations using standard harmonic analysis, and available experimental measurements. Probability density distributions are also computed for the ground vibrational and 10 K state of H+7 and D+7 . These are qualitatively described as a central H+3 / D+3 core surrounded by “solvent” H2/D2 molecules that nearly freely rotate.This work has been supported by MICINN, Spain, Grant No. FIS2011-29596-C02-01, Consolider-Ingenio 2010 Programme CSD2009-00038 (MICINN), and COST Action CM1002 (CODECS).Peer reviewe

Vibrational ground state properties of the H+7 /D+7 clusters.

1 pag. ; 1 fig. PACS: 36.40.Wa Charged clusters; 33.15.Mt Rotation, vibration, and vibration-rotation constants; 34.20.-b Interatomic and intermolecular potentials and forces, potential energy surfaces for collisions. XXVIII International Conference on Photonic, Electronic and Atomic Collisions (ICPEAC 2013)We present the first attempt for an analytical representation of the 15-dimensional potential energy surface (PES) of the H+7 cluster. Such surfaces are of particular interest in studying the unusual dynamics and spectroscopy of these cations. Diffusion Monte Carlo (DMC) and path-integral Monte Carlo (PIMC) simulations based on this surface are employed to investigate the quantum effect and structural fluctuations of the vibrational ground state of H+7 and its deuterated isotopomer. Published under licence by IOP Publishing LtdPeer Reviewe