Potential Energy Surface Of He Research Articles

Molecular beam scattering experiments on noble gas-propylene oxide: Total integral cross sections and potential energy surfaces of He- and Ne-C3H6O.

The interactions of He and Ne with propylene oxide have been investigated with the molecular beam technique by measuring the total (elastic + inelastic) integral cross section as a function of collision velocity. Starting from the analysis of these experimental data, potential energy surfaces, formulated as a function of the separation distance and orientation of propylene oxide with respect to the interacting partners, have been built: The average depth of potential wells (located at intermediate separation distances) has been characterized by analyzing the observed "glory" quantum effects, and the strength of long-range attractions has been obtained from the magnitude and the velocity dependence of the smooth component of measured cross sections. The surfaces, tested and improved against new ab initio calculations of minima interaction energies at the complete basis set level of theory, are defined in the full space of relative configurations. This represents a crucial condition to provide force fields useful to carry out, in general, important molecular property simulations and to evaluate, in the present case, the spectroscopic features and the dynamical selectivity of weakly bound complexes formed by propylene oxide, a prototype chiral species, during collisions in interstellar clouds and winds, in the space and planetary atmospheres. The adopted formulation of the interaction can be readily extended to similar systems, involving heavier noble gases or diatomic molecules (H2, O2, and N2) as well as to propylene oxide dimers.

Gas Transport across Carbon Nitride Nanopores: A Comparison of van der Waals Functionals against the Random-Phase Approximation

C2N is an ordered two-dimensional carbon nitride with a high density (1.7 × 1014 cm–2) of 3.1 Å-sized nanopores, making it promising for high-flux gas sieving for energy-efficient He and H2 purification. Herein, we discuss the accurate calculation of potential energy surfaces for He, H2, N2, and CO2 across C2N nanopores, to characterize the gas-sieving potential of C2N. We compare the potential energy surface derived from density-functional theory calculations using five commonly used van der Waals (vdW) approximations. While all five functionals point that the C2N nanopore yields He/N2 and H2/N2 selectivities over 1000, adsorption energies and energy barriers vary remarkably depending on the approximation chosen. To make progress, we compare the calculations against the results from the adiabatic connection fluctuation dissipation theory, with random-phase approximation, known to be accurate in capturing vdW interactions. The comparison indicates that the interaction energy is less accurate with vdW density functional theory. On the other hand, more empirical corrections work reasonably well, a finding that we also confirm for another carbon nitride lattice, poly(triazine imide). Overall, we recommend these for screening carbon nitride materials for gas separation, but also comparing functionals with higher-order approaches when dealing with different materials.

Zero- and high-pressure mechanisms in the complex forming reactions of OH with methanol and formaldehyde at low temperatures.

A recent Ring Polymer Molecular Dynamics study of the reactions of OH with methanol and formaldehyde, at zero pressure and below 100 K, has shown the formation of long lived complexes, with long lifetimes, longer than 100 ns for the lower temperatures studied, 20-100 K (del Mazo-Sevillano et al., 2019). These long lifetimes support the existence of multi collision events with the He buffer-gas atoms under experimental conditions, as suggested by several transition state theory studies of these reactions. In this work we study these secondary collisions, as a dynamical approach to study pressure effects on these reactions. For this purpose, the potential energy surfaces of He with H2CO, OH, H2O and HCO are calculated at highly accurate ab initio level. The stability of some of the complexes is studied using Path Integral Molecular dynamics techniques, determining that OH-H2CO complexes can be formed up to 100 K or higher temperatures, while the weaker He-H2CO complexes dissociate at approximately 50 K. The predicted IR intensity spectra shows new features which could help the identification of the OH-H2CO complex. Finally, the He-H2CO + OH and OH-H2CO + He collisions are studied using quassi-classical trajectories, finding that the cross section to produce HCO + H2O products increases with decreasing collision energy, and that it is ten times higher in the He-H2CO + OH case.

A new four-dimensional ab initio potential energy surface and predicted infrared spectra for the He–CS2 complex

We present a new four-dimensional ab initio potential energy surface for He–CS2 that is constructed at the coupled cluster and doubles with noniterative inclusion of connected triple [CCSD(T)] level with augmented correlation-consistent quadruplet-zeta (aug-cc-pVQZ) basis set plus midpoint bond functions. The $$Q_{1}$$ and $$Q_{3}$$ normal modes for the $$\nu_{1}$$ symmetric stretching vibration and $$\nu_{3}$$ antisymmetric stretching vibration of CS2 are involved in the construction of the He–CS2 potential. Two vibrationally averaged potentials with CS2 at the vibrational ground and the $$\nu_{1} \text{ + }\nu_{3}$$ excited states are generated from the integration of the four-dimensional potential over the $$Q_{1}$$ and $$Q_{3}$$ coordinates. Each potential is found to have a T-shaped global minimum. The radial discrete variable representation/angular finite basis representation method is employed to calculate the rovibrational states without separating the inter- and intramolecular vibrations. The calculated shift of band origin (0.2270 cm−1) agrees well with the experimental value (0.2278 cm−1). The frequencies and line intensities of the rovibrational transitions in the $$\nu_{1} \text{ + }\nu_{3}$$ region of CS2 for the vdW vibrational ground state are also in good agreement with the observed infrared spectra.

Ion collision cross sections and transport coefficients extended to intermediate energies and reduced electric fields forHe2+ions colliding with He

This work is devoted to the calculation of transport coefficients for He(2)(+) ions in gaseous He at intermediate reduced electric fields. These swarm data are of great interest for a better understanding of the mechanisms of formation and propagation of the fast plasma bullets or ionization waves observed in dielectric barrier plasma jet devices. For transport data, the collision cross sections required are determined from several theoretical methods based on quantum, semiclassical, and hybrid approaches and a diatomics-in-molecules model for the potential energy surfaces of He(2)(+). The corresponding collision cross sections are then used in an optimized Monte Carlo code to calculate the ion transport coefficients over a wide range of reduced electric fields extending over the experimental range. Calculated transport coefficients are compared with available experimental data at low electric fields. Moreover, an extrapolation method is used in order to determine the reduced mobility for stronger fields. A critical discussion has been performed on the pertinence and the reliability of these different methods of determination of collision cross sections needed for the calculation of ion transport data. Such ion data will be used in electrohydrodynamic and chemical kinetic models of the low-temperature plasma jet to quantify and to tune the active species production for a better use in biomedical applications.

Ab initio intermolecular potential energy surfaces of He–CS2, Ne–CS2 and Ar–CS2 complexes

The potential energy surfaces of the He–CS2, Ne–CS2 and Ar–CS2 van der Waals complexes were calculated for the first time at the CCSD(T) level of theory using the aug-cc-pVDZ basis set augmented with a set of midbond functions (3s3p2d1f1g). It was found that the calculated interaction potential, using the applied basis set, readily converges to the complete basis set limit. For a broad range of intermolecular separations and configurations, the interaction energies were obtained by the supermolecular approach with the full counterpoise correction for the basis set superposition error (BSSE). In addition, symmetry-adapted perturbation theory (SAPT) calculations were performed with the same basis set in order to determine the character of the interaction energy of the most stable configuration of each complex at different intermolecular separations in order to make a comparison with the CCSD(T) results. The CCSD(T) calculated potential energy surface of each complex was fitted to an analytic expression to obtain the values of the isotropic dipole–dipole () and dipole–quadruple () dispersion coefficients of each complex. Finally, the interaction second virial coefficients (B12) were obtained using the calculated potential energy surface and used together with the experimental second virial coefficients of pure gases (CS2, Ar, Ne and He) to obtain the second virial coefficient of mixtures of CS2 with rare gas at different temperatures and mole fractions.

( H Cl ) 2 and (HF)2 in small helium clusters: Quantum solvation of hydrogen-bonded dimers

We present a rigorous theoretical study of the solvation of (HCl)(2) and (HF)(2) by small ((4)He)(n) clusters, with n=1-14 and 30. Pairwise-additive potential-energy surfaces of He(n)(HX)(2) (X=Cl and F) clusters are constructed from highly accurate four-dimensional (rigid monomer) HX-HX and two-dimensional (rigid monomer) He-HX potentials and a one-dimensional He-He potential. The minimum-energy geometries of these clusters, for n=1-6 in the case of (HCl)(2) and n=1-5 for (HF)(2), correspond to the He atoms in a ring perpendicular to and bisecting the HX-HX axis. The quantum-mechanical ground-state energies and vibrationally averaged structures of He(n)(HCl)(2) (n=1-14 and 30) and He(n)(HF)(2) (n=1-10) clusters are calculated exactly using the diffusion Monte Carlo (DMC) method. In addition, the interchange-tunneling splittings of He(n)(HCl)(2) clusters with n=1-14 are determined using the fixed-node DMC approach, which was employed by us previously to calculate the tunneling splittings for He(n)(HF)(2) clusters, n=1-10 [A. Sarsa et al., Phys. Rev. Lett. 88, 123401 (2002)]. The vibrationally averaged structures of He(n)(HX)(2) clusters with n=1-6 for (HCl)(2) and n=1-5 for (HF)(2) have the helium density localized in an effectively one-dimensional ring, or doughnut, perpendicular to and at the midpoint of the HX-HX axis. The rigidity of the solvent ring varies with n and reaches its maximum for the cluster size at which the ring is filled, n=6 and n=5 for (HCl)(2) and (HF)(2), respectively. Once the equatorial ring is full, the helium density spreads along the HX-HX axis, eventually solvating the entire HX dimer. The interchange-tunneling splitting of He(n)(HCl)(2) clusters hardly varies at all over the cluster size range considered, n=1-14, and is virtually identical to that of the free HCl dimer. This absence of the solvent effect is in sharp contrast with our earlier results for He(n)(HF)(2) clusters, which show a approximately 30% reduction of the tunneling splitting for n=4. A tentative explanation for this difference is proposed. The implications of our results for the interchange-tunneling dynamics of (HCl)(2) in helium nanodroplets are discussed.

Ab initio calculations, potential representation and vibrational dynamics of He2Br2 van der Waals complex

An intermolecular potential energy surface for He(2)Br(2) complex in the ground state is calculated at the levels of fourth-order (MP4) Moller-Plesset and coupled-cluster [CCSD(T)] approximations, using large-core pseudopotential for Br atoms and the aug-cc-pV5Z basis set for He. The surface is characterized by three minima and the minimum energy pathways through them. The global minimum corresponds to a linear He-Br(2)-He configuration, while the two other ones to "police-nightstick" and tetrahedral structures. The corresponding well depths are -90.39/-89.18, -81.23/-80.78 and -74.40/-74.02 cm(-1), respectively, at MP4/CCSD(T) levels of theory. It is found that results obtained by summing three-body parametrized HeBr(2) interactions and the He-He interaction are in very good accord with the corresponding MP4/CSSD(T) configuration energies of the He(2)Br(2). Variational calculations using a sum of three-body interactions are presented to study the bound states of the vdW He(2)Br(2) complex. The binding energy D(0) and the corresponding vibrationally averaged structure are determined for different isomers of the cluster and their comparison with the available experimental data is discussed.

An empirical potential energy surface for He–Cl2(B 3Πu) based on a multiproperty fit

An empirical interaction surface for the He–Cl2(B 3Πu) complex based on additive pairwise potentials is reported. A novelty of the present surface with respect to previous empirical potentials is that a dependence on the Cl–Cl separation is introduced in some of the potential parameters, which makes more flexible the analytic form used. The parameters of the surface are adjusted in order to reproduce the available data for several properties such like spectral blueshifts, predissociation lifetimes, and Cl2 product state distributions. The fitted surface yields very good agreement with experiment for most of the properties measured. As compared with previous surfaces, the present potential is found to improve significantly on the description of the vibrational dependence of the lifetime, in all the range of vibrational excitations probed by the experiment. In previous works this property has revealed difficult to describe accurately for He–Cl2(B). For the remaining properties, the current surface provides a level of accuracy as good (or better in some cases) as the best one achieved by earlier interaction potentials. The features of the proposed potential surface and its range of validity are discussed. It is found that validity of the potential is essentially limited to the range of vibrational levels probed experimentally.

A new potential energy surface for He–H 2CO

The intermolecular potential energy surface (PES) for He–H 2CO has been calculated using the CCSD(T) method. A basis set extrapolation procedure was employed to estimate the complete basis set limit. For comparison, analogous calculations were also carried out on He–CO. Three minima were identified for He–H 2CO, all arising from planar structures of the complex. The global minimum corresponds to an approximately T-shaped arrangement of the helium atom with respect to the CO bond. A well depth of 59.5 cm −1 was obtained for the global minimum, more than double that reported in an earlier ab initio study of He–H 2CO. The implications of this new PES for spectroscopic studies of He–H 2CO and for understanding He + H 2CO scattering processes is discussed.

Morphing the He–OCS intermolecular potential

A potential energy surface for He–OCS that agrees with experimental rotational spectra to within 1 MHz is presented. The potential was first calculated at a grid defined in prolate spheroidal coordinates, which give stabler interpolations than Jacobi coordinates. Coupled cluster calculations at the CCSD(T) level were carried out with an aug-cc-pVTZ basis set. The potential was then morphed, a procedure that scales the energy and the intermolecular distance in a coordinate-dependent way. The parameters of the function used for morphing were determined by a least-squares fit to the experimental data. The global minimum of the recommended potential, at −50.2 cm−1, is 4.8 cm−1 deeper than the unscaled potential of Higgins and Klemperer [J. Chem. Phys. 110, 1383 (1999)]. The morphing procedure increases the well depth by more at the sulfur end than at the oxygen end.

Three-dimensional ab initio potential energy surface for He–O2

An interaction potential with vibrational coordinate dependence is presented for the He–O2(X 3Σg−) van der Waals complex. The interaction is calculated with the partially spin-restricted open-shell single and double excitation coupled cluster method with perturbative triples [RCCSD(T)] in the supermolecule approach, with correction for the basis set superposition error. The augmented correlation consistent polarized triple zeta basis set is used with an additional set of bond orbitals. The single reference configuration method breaks down at an O–O separation of r≈3.5 a0. No points with r>3.1 a0 are used in the construction of the potential surface. A high quality 120 parameter fit to 754 ab initio points is presented. Special attention is given to the extraction of the long range coefficients.

Ab initio study of the intermolecular potential surface of He–NH3

The intermolecular potential surface for the van der Waals complex He–NH3 is studied by ab initio calculation using fourth-order Møller–Plesset perturbation theory (MP4) with a bond function basis set. The calculation gives a global minimum at R=3.26 Å, θ=89°, φ=60° (in a Jacobi coordinate system) with a well depth of De=32.96 cm−1, along with barriers of 23.09 and 20.86 cm−1 for in-plane rotations at θ=0° and 180°, respectively, and a barrier of 10.45 cm−1 for out-of-plane rotation at θ=89°, φ=0°. The potential energy surface of He–NH3 is compared to those of Ar–NH3 and He–H2O, respectively.

Ab initio potential energy surfaces for He–Cl2, Ne–Cl2, and Ar–Cl2

The three-dimensional ground state potential energy surfaces for He–Cl2, Ne–Cl2, and Ar–Cl2 have been calculated using the single and double excitation coupled-cluster approach with noniterative perturbational treatment of triple excitations [CCSD(T)]. Calculations have been performed with the augmented correlation consistent triple zeta basis sets supplemented with an additional set of bond functions. Single point calculations for approximate minima have also been performed with several other basis sets including the quadruple zeta basis set (aug-cc-pVQZ) with bond functions. For He–Cl2 and Ar–Cl2 the CCSD(T) results show that the linear configuration is lower in energy than the T-shaped one. For Ne–Cl2 the CCSD(T) approach predicts the T-shaped configuration to be lower in energy. The linear configuration has been found to be more sensitive than the T-shaped one to the changes of the Cl–Cl bond length with the interaction becoming weaker when the Cl–Cl bond length is shortened from its equilibrium value and stronger when it is lengthened. More detailed analysis shows that sensitivity of component energies such as exchange, dispersion, and induction is much greater than that of supermolecule results. The interaction in the T-shaped configuration becomes slightly stronger for shorter Cl–Cl bonds. For He–Cl2 and Ar–Cl2 the larger zero-point vibrational energy of the linear configuration is responsible for making the T-shaped configuration the ground vibrational state. Vibrational effects further increase the difference in energy between the ground state T-shaped configuration of Ne–Cl2 and its linear counterpart.

An ab initio study of He–F2, Ne–F2, and Ar–F2 van der Waals complexes

Single and double excitation coupled-cluster approach with noniterative perturbational treatment of triple excitations [CCSD(T)] has been used to calculate the ground state potential energy surfaces for He–F2, Ne–F2, and Ar–F2 van der Waals complexes. Calculations have been performed with the augmented correlation consistent triple zeta basis sets supplemented with an additional set of bond functions (aug-cc-pVTZ+bf). Single point calculations for approximate minima have also been performed with a larger quadruple zeta basis set (aug-cc-pVQZ+bf). For He–F2 and Ar–F2 the CCSD(T) results show that the linear configuration is lower in energy than the T-shaped one. For Ne–F2 the CCSD(T) interaction energies of the two configurations are virtually the same. The linear configuration of each complex has been found to be much more sensitive than the T-shaped one to the changes of the F–F bond length with the interaction becoming weaker when the F–F bond length is shortened from its equilibrium value and stronger when it is lengthened. More detailed analysis shows that sensitivity of component energies such as exchange, dispersion, and induction is much greater than that of supermolecule results. High-order correlation corrections have been found to play an important role in determining the relative stability of the linear and T-shaped configurations. The harmonic approximation zero-point vibrational energy for He–F2 exceeds the depth of both wells. For Ne–F2 the zero-point vibrational energy is greater for the linear configuration and, because of that, the complex has a T-shaped ground vibrational state. When the zero-point vibrational energy is taken into account for the Ar–F2 complex the linear and the T-shaped configurations are found to have nearly identical energies.

The potential energy surfaces of He 2+: implications for femtosecond spectroscopy

The potential energy surfaces of the ground state and excited states of the helium dimer ion (He 2 +) are calculated using MRSDCI wavefunctions. Wavepacket dynamics simulations are performed on the excited B-state ( 2Π u) of He 2 + to illustrate the interesting aspects of this prototypical 3-electron system. Implications for femtosecond and sub-femtosecond pump-probe laser experiments are discussed.

The potential energy surface of He–HCN determined by fitting to high-resolution spectroscopic data

Two potential energy surfaces for He–HCN are determined by least-squares fitting of parameterised functional forms to data from high-resolution microwave and millimeter-wave spectroscopy [Drucker et al., J. Phys. Chem. 99, 2646 (1995)]. The two potentials both have significantly deeper wells than suggested by the ab initio supermolecule calculations of Drucker et al. Both potentials have linear or near-linear equilibrium geometries, He–H–C–N, but the shapes of the well depth functions away from the linear geometry are significantly different. The existing experimental data are thus not sufficient to probe this potential feature in detail. Predictions of spectroscopic properties that would allow the new potentials to be tested and refined are given.

An accurate ab initio potential energy surface of HeH 2O

We present an accurate calculation of the intermolecular potential surface for the van der Waals complex HeH 2O using complete fourth-order Møller-Plesset perturbation theory (MP4) with an efficient basis set containing bond functions. The calculation gives a global minimum at R=3.15 A ̊ , θ=105°, φ=0° (in a Jacobi coordinate system) with a minimum energy D e=31.8 cm −1, along with barriers of 13.4 and 12.6 cm −1 for in-plane rotation at θ=0° and 180°, respectively, and a barrier of 20.0 cm −1 for out-of-plane rotation at θ=105°, φ=90°. The potential energy surface is compared with previously published surfaces for HeH 2O, and with the potential energy surface for ArH 2O.

Infrared spectrum and potential energy surface of He–CO

For 3He–CO and 4He–CO van der Waals bimers, fully resolved infrared spectra in the 4.7 μm region near the fundamental band origin of the CO monomer have been measured for the first time. Only a small fraction of the observed lines could be assigned using conventional empirical spectroscopic techniques, and little additional insight was gained from synthetic spectra generated from a published ab initio potential for this system. However, a complete set of unique assignments was made on the basis of comparisons with synthetic spectra generated from a variety of trial potential energy surfaces, and least-squares fits to the observed transition frequencies were used to determine a new anisotropic potential energy surface for this system. This new surface is much deeper and has a much stronger well depth anisotropy than the best previous one, and its predictions of very low temperature microwave line broadening cross sections raise serious questions regarding the methodology for calculating that property.

An accurate ab initio potential energy surface of the He–H2 interaction

An accurate potential energy surface of the He–H2 interaction is calculated with a large basis set at the complete fourth-order Mo/ller–Plesset approximation. The basis set—a combination of a nucleus-centered set 6s4p2d and a bond function set 3s3p2d centered at the midpoint between He and the H2 center of mass—is designed to give the optimal description of both the intra- and intersystem correlation effects. The validity of the basis set is confirmed by extensive preliminary calculations on the linear (orientation angle θ=0°), bent (45°), and T-shaped (90°) structures at a fixed separation (R=6.5a0) with a series of large basis sets containing different polarization functions and/or bond functions. Bond functions are found more effective than polarization functions in recovering the intersystem correlation energy and they are particularly useful in removing the geometric bias of a basis to give an accurate description for the potential anisotropy and the relative energies of different structures. The effect of bond functions is insensitive to the displacement of bond functions and the geometric midpoint of the van der Waals bond is a satisfactory choice for the center of bond functions. The potential energy surface of He–H2 is calculated at 15 values of R from 2.0 to 15.0a0 along each of the three main configurations (θ=0°, 45°, and 90°) with the vibrationally averaged H2 bond length r=1.449a0. Additional calculations are given for r=1.28 and 1.618a0 to show the effect of H2 zero-point vibration. While our potential at the self-consistent field (SCF) level is essentially the same as the previous calculations, our potential at the correlated level is globally deeper in the attractive region and less repulsive in the shorter range. Our calculated well depth (47.19 μhartrees) corresponding to the global minimum at θ=0° and R=6.5a0, is very close to the estimated experimental value of 48 μhartrees. In the Legendre expansion, our potential compares very well with the empirical potential of Rodwell and Scoles, but differs considerably from the empirical potential of Tang and Toennies and the previous ab initio potentials.