Ab initio quantum dynamics with very weak van der Waals interactions: Structure and stability of small Li2(1Sigmag+)-(He)n clusters

Potential Energy Surface and Bound States of Ne-Li2+(X2Σg+) van der Waals Complex Based on Ab Initio Calculations

Theoretical studies of the potential energy surface and vibrational bound states calculations were performed for the ground state of the Ne-Li2+(X2Σg+) van der Waals (vdW) complex. The intermolecular interactions were investigated by using an accurate monoconfigurational RCCSD(T) method and large basis sets (aug-cc-pVnZ, n = T, Q, 5), extrapolated to the complete basis set (CBS) limit. In turn, the obtained raw data from RCCSD(T)/CBS(Q5) calculations were numerically interpolated using the Morse + vdW model and the Reproducing Kernel Hilbert Space (RKHS) polynomial method to generate analytic expressions for the 2D-PES. The RKHS interpolated PES was then used to assess the bound states of the Ne-Li2+(X2Σg+) system through nuclear quantum calculations. By studying the aspect of the potential energy surface, the analysis sheds light on the behavior of the Ne-Li2+(X2Σg+) complex and its interactions between repulsive and attractive forces with other particles. By examining the vibrational states and wave functions of the system, the researchers were able to gain a better understanding of the behavior of the Ne-Li2+(X2Σg+) complex. The calculated radial and angular distributions for all even and odd symmetries are discussed in detail. We observe that the radial distributions exhibit a more complicated nodal structure, representing stretching vibrational behavior in the neon atom along its radial coordinate. For the highest bound states, the situation is very different, and the energies surpass the angular barrier.

Structural, Spectroscopic, and Dynamic Properties of Li2+(X2∑g+) in Interaction with Krypton Atom

We report a computational study of the potential energy surface (PES) and vibrational bound states for the ground electronic state of Li2+Kr. The PES was calculated in Jacobi coordinates at the Restricted Coupled Cluster method RCCSD(T) level of calculation and using aug-cc-pVnZ (n = 4 and 5) basis sets. Afterward, this PES is extrapolated to the complete basis set (CBS) limit for correction. The obtained interaction energies were, then, interpolated numerically using the reproducing kernel Hilbert space polynomial (RKHS) approach to produce analytic expressions for the 2D-PES. The analytical PES is used to solve the nuclear Schrodinger equation to determine the bound states' eigenvalues of Li2+Kr for a J = 0 total angular momentum configuration and to understand the effects of orientational anisotropy of the forces and the interplay between the repulsive and attractive interaction within the potential surface. In addition, the radial and angular distributions of some selected bound state levels, which lie below, around, and above the T-shaped 90° barrier well, are calculated and discussed. We note that the radial distributions clearly acquire a more complicated nodal structure and correspond to bending and stretching vibrational motions "mode" of the Kr atom along the radial coordinate, and the situation becomes very different at the highest bound states levels with energies higher than the T-shaped 90° barrier well. The shape of the distributions becomes even more complicated, with extended angular distributions and prominent differences between even and odd states.

Structure, energetics, and spectroscopy of the K2+(X2Σ+g) interacting with the noble gas atoms Ar, Kr and Xe

The structure, energetic, and spectroscopy properties of the ionic system K₂⁺(X²Σ⁺_g) interacting with the noble gas atoms Argon, Krypton and Xenon are studied. The computations are done by an accurate ab initio approach based on the pseudo-potential technique, Gaussian basis sets, parameterized l-dependent polarization potentials and an analytic potential form for the K⁺Ar, K⁺Kr and K⁺Xe interactions. These interactions are added via the CCSD(T) potential taken from literature and fitted applying the analytical expression of Tang and Toennies. The application of the pseudo-potential approach reduces the number of active electrons of each complex to only one electron. The potential energy surfaces are analyzed on a large range of the Jacobi coordinates, R and θ. By the general interpolation outline based on the RKHS (Reproducing Kernel Hilbert Space) procedure, we have reproduced for each complex from our ab initio results the two-dimensional contour plots of an analytical potential. To evaluate the stability of each complex, we have determined from the potential energy surfaces the equilibrium distance (R_e), the well depth (D_e), the quantum energy (D₀), the zero-point-energy (ZPE) and the ZPE%. The results showed that the linear configurations, where the noble gas atom connected to the K₂⁺(X²Σ⁺_g) system, are the more stable.

Solvation of lithium ion in helium clusters: Structural properties and relative stabilities

Structural study and relative stabilities of Li⁺-doped helium clusters Li⁺He_n (n = 1-18) has been reported in this work using two theoretical protocols. The first one is based on the basin-hopping optimization technique, where the total energy of each cluster is described by an additive model describing Li⁺-He and He-He interactions. The second one is the DFT calculations, in which the initial structures are generated by ABCluster algorithm and CALYPSO software. The CSA shape was found where the first solvation shell is completed at n = 10. The relative stabilities of Li⁺He_n (n = 1-18) clusters have been discussed based on the variation of the binding energy, second-order difference in energy, fragmentation energy and HOMO-LUMO energy gap as a function of the cluster size. The results showed that Li⁺He₁₀ is the most stable cluster. The dipole moment is calculated and showed the polar character of the Li⁺He_n clusters. Finally, the interatomic interactions have been examined topologically by the means of AIM and non-covalent reduced density gradient (NC-RDG) analyses.

Theoretical Study of Cationic Alkali Dimers Interacting with He: Li2+-He and Na2+-He van der Waals Complexes

We present a theoretical study on the potential energy surface and bound states of He-A₂⁺ complexes, where A is one of the alkali Li or Na atoms. The intermolecular interactions were systematically investigated by high-level ab initio electronic structure computations, and the corresponding raw data were then employed to reproduce accurate analytical expressions of the potential surfaces. In turn, we used these potentials to evaluate bound configurations of the trimers from nuclear quantum calculations and to extract information on the effect of orientational anisotropy of the forces and the interplay between repulsive and attractive interaction within the potential surfaces. The spatial features of the bound states are analyzed and discussed in detail. We found that both systems are going under large amplitude stretching and bending motions with high zero-point energies. Despite the large differences in the potential well-depths, the correct treatment of nuclear quantum effects provides insights on the effect of different strength of the ionic interaction on the spectral structure of such cationic alkali van der Waals complexes, related to the mobility of ions and the formation of cold-molecules in He-controlled environments.

ELECTRON DENSITY ANALYSIS OF WEAK VAN DER WAALS COMPLEXES

The nature of weak van der Waals interactions in different complexes of some atmospheric molecules such as CO 2, N 2 O , and N 2 was examined. Ab initio calculation was carried out at MP2 level of theory using Dunning's aug-cc-pVTZ basis set. Bader's theory of atoms in molecules (AIM) was employed to analyze electron density and to characterize the nature and properties of van der Waals interactions. A set of criteria, having been proposed in the context of AIM theory, was examined for these complexes. In spite of the parameter kinetics energy, per electron density is expected to be greater than unity for closed-shell interactions; we obtained values less than unity for many of these polyatomic systems. A set of limitations has also been outlined for the values of two AIM quantities: total energy density, H(r), and Laplacian of electron density, ∇2ρ, which correspond to different bond natures.

Spin-driven structural effects in alkali doped (4)He clusters from quantum calculations

In this paper, we carry out variational Monte Carlo and diffusion Monte Carlo (DMC) calculations for Li(2)((1)Sigma(g) (+))((4)He)(N) and Li(2)((3)Sigma(u) (+))((4)He)(N) with N up to 30 and discuss in detail the results of our computations. After a comparison between our DMC energies with the "exact" discrete variable representation values for the species with one (4)He, in order to test the quality of our computations at 0 K, we analyze the structural features of the whole range of doped clusters. We find that both species reside on the droplet surface, but that their orientation is spin driven, i.e., the singlet molecule is perpendicular and the triplet one is parallel to the droplet's surface. We have also computed quantum vibrational relaxation rates for both dimers in collision with a single (4)He and we find them to differ by orders of magnitude at the estimated surface temperature. Our results therefore confirm the findings from a great number of experimental data present in the current literature and provide one of the first attempts at giving an accurate, fully quantum picture for the nanoscopic properties of alkali dimers in (4)He clusters.

Bosonic helium droplets with cationic impurities: onset of electrostriction and snowball effects from quantum calculations

Variational Monte Carlo and diffusion Monte Carlo calculations have been carried out for cations such as Li(+), Na(+), and K(+) as dopants of small helium clusters over a range of cluster sizes up to about 12 solvent atoms. The interaction has been modeled through a sum-of-potential picture that disregards higher order effects beyond atom-atom and atom-ion contributions. The latter were obtained from highly correlated ab initio calculations over a broad range of interatomic distances. This study focuses on two of the most striking features of the microsolvation in a quantum solvent of a cationic dopant: electrostriction and snowball effects. They are discussed here in detail and in relation with the nanoscopic properties of the interaction forces at play within a fully quantum picture of the cluster features.

Microsolvation of Cationic Dimers in 4He Droplets: Geometries of (He)N (A = Li, Na, K) from Optimized Energies

Ab initio computed interaction forces are employed to describe the microsolvation of the A+2(2Sigma) (A=Li, Na, K) molecular ion in 4He clusters of small variable size. The minimum energy structures are obtained by performing energy minimization based on a genetic algorithm approach. The symmetry features of the collocation of solvent adatoms around the dimeric cation are analyzed in detail, showing that the selective growth of small clusters around the two sides of the ion during the solvation process is a feature common to all three dopants.

Microsolvation of an Ionic Dopant in Small4He Clusters: OH+(3Σ)(4He)N via Genetic Algorithm Optimizations

The optimized spatial structures of the small clusters (with N up to 33) formed by an increasing number of (4)He atoms, which act as a microsolvent surrounding the OH(+) ionic molecular dopant, are obtained using a sum-of-potentials scheme corrected by three-body (3B) effects. The most stable structures are generated using the type of genetic algorithm described herein, and the sequential formation of regular shell structures is analyzed in detail. Possible quantum corrections for both the solvent distributions and the stable energetics are analyzed and discussed.

Ionic dimers in He droplets: Interaction potentials for Li2+–He,Na2+–He, and K2+–He and stability of the smaller clusters

We present post Hartree-Fock calculations of the potential energy surfaces (PESs) for the ground electronic states of the three alkali dimer ions Li(2) (+),Na(2) (+), and K(2) (+) interacting with neutral helium. The calculations were carried out for the frozen molecular equilibrium geometries and for an extensive range of the remaining two Jacobi coordinates, R and theta, for which a total of about 1000 points is generated for each surface. The corresponding raw data were then fitted numerically to produce analytic expressions for the three PESs, which were in turn employed to evaluate the bound states of the three trimers for their J=0 configurations: The final spatial features of such bound states are also discussed in detail. The possible behavior of additional systems with more helium atoms surrounding the ionic dopants is gleaned from further calculations on the structural stability of aggregates with up to six He atoms. The validity of a sum-of-potential approximation to yield realistic total energies of the smaller cluster is briefly discussed vis-a-vis the results from many-body calculations.

Microsolvation of LiH+ in helium clusters: many-body effects and additivity models for the interaction forces

The ab initio calculation of the interaction forces between the LiH+ molecular ion, at its equilibrium geometry, and several He atoms is carried out in order to isolate and assess the importance of many-body contributions in the search for realistic energy and geometry data. The full potential energy surface (PES) with a single helium partner is obtained first by using an aug-cc-pVQZ basis set for He and higher quality ones for Li and H. The calculations were performed at the CAS-SCF plus MRCI level for the lowest potential energy surface over a total of 480 grid points of the two intermolecular Jacobi coordinates, whereas the excited state surface has also been examined in order to exclude the presence of any significant nonadiabatic interaction between the two PESs. A numerical fit of the lower surface is presented and the general physical changes of the ionic interaction when going from the lower to the upper of the two potentials are described and discussed. The fairly limited importance of many-body effects for such systems is seen from further ab initio calculations including several He atoms: our results suggest that, at least in the present case, no strong charge migration occurs after He attachment, and therefore, one could realistically model larger clusters by implementing a sum-of-potentials approach via the presently computed PES.