Post-Hartree-Fock studies of the He/Mg(0001) interaction: Anti-corrugation, screening, and pairwise additivity

12 págs.; 6 figs.; 5 tabs.The adsorption of noble gases on metallic surfaces represents a paradigmatic case of van-der-Waals (vdW) interaction due to the role of screening effects on the corrugation of the interaction potential [J. L. F. Da Silva et al., Phys. Rev. Lett. 90, 066104 (2003)]. The extremely small adsorption energy of He atoms on the Mg(0001) surface (below 3 meV) and the delocalized nature and mobility of the surface electrons make the He/Mg(0001) system particularly challenging, even for state-of-the-art vdW-corrected density functional-based (vdW-DFT) approaches [M. P. de Lara-Castells et al., J. Chem. Phys. 143, 194701 (2015)]. In this work, we meet this challenge by applying two different procedures. First, the dispersion-corrected second-order Möller-Plesset perturbation theory (MP2C) approach is adopted, using bare metal clusters of increasing size. Second, the method of increments [H. Stoll, J. Chem. Phys. 97, 8449 (1992)] is applied at coupled cluster singles and doubles and perturbative triples level, using embedded cluster models of the metal surface. Both approaches provide clear evidences of the anti-corrugation of the interaction potential: the He atom prefers on-top sites, instead of the expected hollow sites. This is interpreted as a signature of the screening of the He atom by the metal for the on-top configuration. The strong screening in the metal is clearly reflected in the relative contribution of successively deeper surface layers to the main dispersion contribution. Aimed to assist future dynamical simulations, a pairwise potential model for the He/surface interaction as a sum of effective He–Mg pair potentials is also presented, as an improvement of the approximation using isolated He–Mg pairs. Published by AIP Publishing.This work has been partly supported by the COST Action No. CM1405 “Molecules in Motion (MOLIM),” Grant Nos. FIS2011-29596-C02-01 and MAT2012-33633 from the Spanish Dirección General de Investigación Científica y Técnica, and the German Research Foundation (DFG) through Project No. VO 1711/2-1. The Cesga Super-Computer Center (Galicia), and the Centro Técnico de Informática (CTI, CSIC) are acknowledged for allocating computer time.Peer Reviewe

Carbon nanotubes immersed in superfluid helium: the Impact of quantum confinement on wetting and capillary action

7 págs.; 5 figs.; 1 tab. ; Associated content mp4 video: http://pubs.acs.org/doi/suppl/10.1021/acs.jpclett.6b02414A recent experimental study [Ohba, Sci. Rep. 2016, 6, 28992] of gas adsorption on single-walled carbon nanotubes at temperatures between 2 and 5 K reported a quenched propagation of helium through carbon nanotubes with diameters below 7 Å despite the small kinetic diameter of helium atoms. After assessing the performance of a potential model for the He−nanotube interaction via ab initio calculations with density functional theorybased symmetry adapted perturbation theory, we apply orbital-free helium density functional theory to show that the counterintuitive experimental result is a consequence of the exceptionally high zeropoint energy of helium and its tendency to form spatially separated layers of helium upon adsorption at the lowest temperatures. Helium filling factors are derived for a series of carbon nanotubes and compared to the available experimental data. © 2016 American Chemical SocietyThis work has been supported by the COST Action CM1405 “Molecules in Motion (MOLIM)”. M.P.d.L.-C. gratefully acknowledges support from MINECO (Spain) under Grant MAT2016-75354-P and thanks the CTI (CSIC) and CESGA supercomputer facilities (Spain) for the resources provided.Peer reviewe

An optimized full-configuration-interaction nuclear orbital approach to a “hard-core” interaction problem: Application to (3He)N–Cl2(B) clusters (N<4)

13 pages, 8 figures, 3 tables, 3 appendix.An efficient full-configuration-interaction nuclear orbital treatment has been recently developed as a benchmark quantum-chemistry-like method to calculate ground and excited “solvent” energies and wave functions in small doped AEest clusters (N<4) [M. P. de Lara-Castells, G. Delgado-Barrio, P. Villarreal, and A. O. Mitrushchenkov, J. Chem. Phys. 125, 221101 (2006)]. Additional methodological and computational details of the implementation, which uses an iterative Jacobi–Davidson diagonalization algorithm to properly address the inherent “hard-core” He–He interaction problem, are described here. The convergence of total energies, average pair He–He interaction energies, and relevant one- and two-body properties upon increasing the angular part of the one-particle basis set (expanded in spherical harmonics) has been analyzed, considering Cl2 as the dopant and a semiempirical model (T-shaped) He–Cl2(B) potential. Converged results are used to analyze global energetic and structural aspects as well as the configuration makeup of the wave functions, associated with the ground and low-lying “solvent” excited states. Our study reveals that besides the fermionic nature of 3He atoms, key roles in determining total binding energies and wave-function structures are played by the strong repulsive core of the He–He potential as well as its very weak attractive region, the most stable arrangement somehow departing from the one of N He atoms equally spaced on equatorial “ring” around the dopant. The present results for N=4 fermions indicates the structural “pairing” of two 3He atoms at opposite sides on a broad “belt” around the dopant, executing a sort of asymmetric umbrella motion. This pairing is a compromise between maximizing the 3He–3He and the He-dopant attractions, and suppressing at the same time the “hard-core” repulsion. Although the He–He attractive interaction is rather weak, its contribution to the total energy is found to scale as a power of three and it thus increasingly affects the pair density distributions as the cluster grows in size.This work has been partially supported by the CSICCAM, CICYT, and MICINN-CSIC Spanish Grants Nos. CCG08-CSIC/ESP-3680, FIS2007-62006, and 2007501004.Peer reviewe

Physisorption of helium on a TiO2(110) surface: periodic and finite clusters approaches

Workshop CMST Action CM1200 CODECS: Holistic Computational Spectroscopy, 16-18 November 2011, Pisa, Italy. http://codecs.sns.it/index.php/Holistic_Computational_SpectroscopyAs a proto-typical case of physisorption on an extended transition-metal oxide surface, the interaction of a helium atom with a TiO2(110) surface is studied here by using finite cluster and periodic approaches and both wavefunction-based (post Hartree-Fock) quantum chemistry methods, as well as density functional theory. The finite cluster approach is applied to provide reference results at coupled-cluster and perturbative second-order Möller-Plesset levels of theory. It is shown that once the basis set is specifically tailored to minimize the basis set superposition error, periodic calculations using the Perdew Burke-Ernzerhof functional yield short and medium-range interaction potentials in very reasonable agreement with those obtained using the correlated wave-function-based methods, while small long-range dispersion corrections are necessary to reproduce the correct asymptotic behavior. This study is aimed at the simulation of helium droplet-mediated deposition of metallic clusters on oxide surfaces.Peer Reviewe

Vibrational quenching of CO2(010) by collisions with O(3P) at thermal energies: A quantum-mechanical study

10 pages, 6 figures, 1 table, 1 appendix.-- PACS nrs.: 34.50.Ez; 33.20.Tp; 33.15.Mt; 31.15.Ar.The CO2(010)–O(3P) vibrational energy transfer (VET) efficiency is a key input to aeronomical models of the energy budget of the upper atmospheres of Earth, Venus, and Mars. This work addresses the physical mechanisms responsible for the high efficiency of the VET process at the thermal energies existing in the terrestrial upper atmosphere (150 K ≤ T ≤ 550 K). We present a quantum-mechanical study of the process within a reduced-dimensionality approach. In this model, all the particles remain along a plane and the O(3P) atom collides along the C(2v) symmetry axis of CO2, which can present bending oscillations around the linear arrangement, while the stretching C–O coordinates are kept fixed at their equilibrium values. Two kinds of scattering calculations are performed on high-quality ab initio potential energy surfaces (PESs). In the first approach, the calculations are carried out separately for each one of the three PESs correlating to O(3P). In the second approach, nonadiabatic effects induced by spin-orbit couplings (SOC) are also accounted for. The results presented here provide an explanation to some of the questions raised by the experiments and aeronomical observations. At thermal energies, nonadiabatic transitions induced by SOC play a key role in causing large VET efficiencies, the process being highly sensitive to the initial fine-structure level of oxygen. At higher energies, the two above-mentioned approaches tend to coincide towards an impulsive Landau-Teller mechanism of the vibrational to translational (V-T) energy transfer.This work has been partially supported by the European Project No. R113-2003-506079 and the DGICYT Spanish Grant Nos. FIS2004-02461 and CTQ-2004-02415/BQU. One of the authors (M.P.d.L.-C.) was supported by the "Ramón y Cajal" Programme and another author (M.L.-P.) by the Spanish projects Nos. REN2001-3249/CLI, ESP2004-01556, and by EC FEDER funds. The calculations presented here were performed at CINECA (the SuperComputer Center of the University of Bologna), the Instituto de Matemáticas y Física Fundamental (CSIC), and CESGA (the SuperComputer Center of Galicia).Peer reviewe

Helium mediated deposition: Modeling the He−TiO2(110)-(1×1) interaction potential and application to the collision of a helium droplet from density functional calculations

This paper is the first of a two-part series dealing with quantum-mechanical (density-functional-based) studies of helium-mediated deposition of catalytic species on the rutile TiO2(110)-(1×1) surface. The interaction of helium with the TiO2(110)-(1×1) surface is first evaluated using the Perdew-Burke-Ernzerhof functional at a numerical grid dense enough to build an analytical three-dimensional potential energy surface. Three (two prototype) potential models for the He-surface interaction in helium scattering calculations are analyzed to build the analytical potential energy surface: (1) the hard-corrugated-wall potential model; (2) the corrugated-Morse potential model; and (3) the three-dimensional Morse potential model. Different model potentials are then used to study the dynamics upon collision of a 4He300 cluster with the TiO2(110) surface at zero temperature within the framework of a time-dependent density-functional approach for the quantum fluid [D. Mateo, D. Jin, M. Barranco, and M. Pi, J. Chem. Phys. 134, 044507 (2011)] and classical dynamics calculations. The laterally averaged density functional theory-based potential with an added long-range dispersion interaction term is further applied. At variance with classical dynamics calculations, showing helium droplet splashing out of the surface at impact, the time evolution of the macroscopic helium wave-function predicts that the helium droplet spreads on the rutile surface and leads to the formation of a thin film above the substrate. This work thus provides a basis for simulating helium mediated deposition of metallic clusters embedded within helium nanodroplets

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

A Combined Periodic Density Functional and Incremental Wave-Function-Based Approach for the Dipersion-Accounting Time-Resolved Dynamics of 4He Nanodroplets on Surfaces: 4He/Graphene

Helium-mediated Synthesis, Soft-landing and Spectroscopy of Metal Nanoparticles on Surfaces,CSIC, Madrid, Spain, October 10-11, 2014A general strategy to calculate accurate He-surface interaction potentials is proposed [1]. It extends the dispersionless density functional (dlDF) approach by Pernal et al. [2] to adsorbatesurface interactions by including periodic boundary conditions [1b]. A scheme to parametrize the dispersion interaction is introducced by calculating two- and three-body dispersion terms at CCSD(T) level via the method of increments [3]. The performance of the composite approach is tested on the low-lying selective adsorption states of 4He/graphene [5]. Second, its capability to describe dispersionless correlation effects realistically is used to extract dispersion effects in time-dependent density functional simulations on the collision of 4He droplets with graphene [1b]. Dispersion effects play a key role in the fast spreading of the 4He nanodroplet [1b,6], the evaporation-like process of helium atoms, and the formation of solid-like helium structures. These characteristics are expected to be quite general and highly relevant to explain experimental measurements with the newly developed helium droplet mediated deposition technique [7].Peer Reviewe

Exact, Born–Oppenheimer, and quantum-chemistry-like calculations in helium clusters doped with light molecules: The He2N2(X) system

9 pages, 2 figures, 4 tables.-- PACS nrs.: 34.20.-b; 31.50.-x; 31.15.A-; 33.15.Mt; 33.20.Vq; 36.40.-c.Helium clusters doped with diatomic molecules, He(N)–BC, have been recently studied by means of a quantum-chemistry-like approach. The model treats He atoms as “electrons” and dopants as “nuclei” in standard electronic structure calculations. Due to the large mass difference between He atoms and electrons, and to the replacement of Coulomb interactions by intermolecular potentials, it is worth assessing up to what extent are the approximations involved in this model, i.e., decoupling of the BC rotation from the He-atom orbital angular momenta and Born–Oppenheimer separation of the BC stretch versus the He motions, accurate enough. These issues have been previously tackled elsewhere for the 4He2–Br2(X) system, which contains a heavy dopant [Roncero et al., Int. J. Quantum Chem. 107, 2756 (2007)]. Here, we consider a similar cluster but with a much lighter dopant such as N2(X). Although the model does not provide the correct energy levels for the cluster, positions and intensities of the main detectable lines of the vibrotational Raman spectrum at low temperature are accurately reproduced.This work has been partially supported by the DGICYT Spanish Grant Nos. FIS2007-62006 and CTQ2004-02415/BQU. M.P.de L.-C. acknowledges the support of a MEC-CSIC Spanish Grant No. 2007501004. The calculations presented here were performed at Centro de Cálculo of IMAFF (CSIC).Peer reviewe