Exchange-coulomb potential energy curves for He-He, and related physical properties

Nonadiabatic Laser-Induced Alignment Dynamics of Molecules on a Surface

We demonstrate that a sodium dimer, Na_{2}(1^{3}Σ_{u}^{+}), residing on the surface of a helium nanodroplet, can be set into rotation by a nonresonant 1.0 ps infrared laser pulse. The time-dependent degree of alignment measured, exhibits a periodic, gradually decreasing structure that deviates qualitatively from that expected for gas-phase dimers. Comparison to alignment dynamics calculated from the time-dependent rotational Schrödinger equation shows that the deviation is due to the alignment dependent interaction between the dimer and the droplet surface. This interaction confines the dimer to the tangential plane of the droplet surface at the point where it resides and is the reason that the observed alignment dynamics is also well described by a 2D quantum rotor model.

The Solid Phase of 4He: A Monte Carlo Simulation Study

The thermodynamics of solid (hcp) 4He is studied theoretically by means of unbiased Monte Carlo simulations at finite temperature, in a wide range of density. This study complements and extends previous theoretical work, mainly by obtaining results at significantly lower temperatures (down to 60 mK) and for systems of greater size, by including in full the effect of quantum statistics, and by comparing estimates yielded by different pair potentials. All the main thermodynamic properties of the crystal, e.g., the kinetic energy per atom, are predicted to be essentially independent of temperature below ∼ 1 K. Quantum-mechanical exchanges are virtually non-existent in this system, even at the lowest temperature considered. However, effects of quantum statistics are detectable in the momentum distribution. Comparison with available measurements shows general agreement within the experimental uncertainties.

Nuclear quantum effects in thermal conductivity from centroid molecular dynamics

We show that the centroid molecular dynamics (CMD) method provides a realistic way to calculate the thermal diffusivity a = λ/ρc_V of a quantum mechanical liquid such as para-hydrogen. Once a has been calculated, the thermal conductivity can be obtained from λ = ρc_Va, where ρ is the density of the liquid and c_V is the constant-volume heat capacity. The use of this formula requires an accurate quantum mechanical heat capacity c_V, which can be obtained from a path integral molecular dynamics simulation. The thermal diffusivity can be calculated either from the decay of the equilibrium density fluctuations in the liquid or by using the Green-Kubo relation to calculate the CMD approximation to λ and then dividing this by the corresponding approximation to ρc_V. We show that both approaches give the same results for liquid para-hydrogen and that these results are in good agreement with the experimental measurements of the thermal conductivity over a wide temperature range. In particular, they correctly predict a decrease in the thermal conductivity at low temperatures-an effect that stems from the decrease in the quantum mechanical heat capacity and has eluded previous para-hydrogen simulations. We also show that the method gives equally good agreement with the experimental measurements for the thermal conductivity of normal liquid helium.

Nuclear quantum effects in the direct ionization process of pure helium clusters: path-integral and ring-polymer molecular dynamics simulations on the diatomics-in-molecule potential energy surfaces

The ionization dynamics of pure He_n clusters has been theoretically studied using path-integral and ring-polymer molecular dynamics simulations.

Photoexcited Ag ejection from a low-temperature He cluster: a simulation study by nonadiabatic Ehrenfest ring-polymer molecular dynamics

Nonadiabatic ring-polymer molecular dynamics simulations were performed to understand the photoexcitation dynamics of a low-temperature Ag·He₅₀₀ cluster.

Anisotropic superfluidity of (4)He on a C36 fullerene molecule

We have performed path-integral Monte Carlo calculations to study the adsorption of (4)He atoms on two different C36 isomers with the D6h and the D2d symmetries. The radial (4)He density distributions reveal layer-by-layer growth with the first layer being located at a distance of ∼5.5 Å from the C36 molecular center and the second layer at ∼8.3 Å. From the angular density profiles of (4)He, we find different quantum states as the number of (4)He adatoms N varies. For N = 20, we observe commensurate solid structures on both D6h and D2d isomers, where each of 8 hexagon and 12 pentagon centers of the fullerene surfaces is occupied by a single (4)He atom. The second-layer promotion starts beyond N = 38 on both isomers, where a compressible incommensurate structure is observed on the D6h isomer and another commensurate structure on D2d. Between N = 20 and N = 38, the (4)He monolayer on D6h shows several distinct rings of delocalized (4)He atoms along with strongly anisotropic superfluid responses at low temperatures, while isotropic but weak superfluid responses are observed in the (4)He layer on D2d.

Persistence of dual free internal rotation in NH4(+)(H2O)·Hen=0-3 ion-molecule complexes: expanding the case for quantum delocalization in He tagging

To explore the extent of the molecular cation perturbation induced by complexation with He atoms required for the application of cryogenic ion vibrational predissociation (CIVP) spectroscopy, we compare the spectra of a bare NH4(+)(H2O) ion (obtained using infrared multiple photon dissociation (IRMPD)) with the one-photon CIVP spectra of the NH4(+)(H2O)·He1-3 clusters. Not only are the vibrational band origins minimally perturbed, but the rotational fine structures on the NH and OH asymmetric stretching vibrations, which arise from the free internal rotation of the -OH2 and -NH3 groups, also remain intact in the adducts. To establish the location and the quantum mechanical delocalization of the He atoms, we carried out diffusion Monte Carlo (DMC) calculations of the vibrational zero point wave function, which indicate that the barriers between the three equivalent minima for the He attachment are so small that the He atom wave function is delocalized over the entire -NH3 rotor, effectively restoring C3 symmetry for the embedded -NH3 group.

4He adsorption on a H(2)-plated C20 molecular surface: the formation of helium buckyballs

We perform path-integral Monte Carlo calculations to study the adsorption of 4He atoms on a H2-plated C20 molecular surface. It is found that 32 H2 molecules form a complete solid layer on C20, where each H2 molecule is located either above one of the 12 pentagon centers or above one of the 20 carbon atoms. The angular density profiles of the first 4He layer on the (H2)32-C20 surface reveal different quantum states as the number of 4He atoms N varies. Especially, the helium layer exhibits an icosidodecahedron structure for N=30, where each 4He atom is located at one of the vertices of 20 corner-sharing triangles. While the 4He density peaks for N=60 constitute a truncated icosahedron with 12 pentagonal and 20 hexagonal faces, the additional atoms beyond N=60 are found to be placed at the hexagon centers of the truncated icosahedron to form a hexakis truncated icosahedron for N=80. The superfluid response of the 4He layer at a temperature of T=0.6 K is found to be completely quenched for N=30 and to be significantly suppressed for N=60 and 80, reflecting the formation of compact buckyball structures.

DEVELOPMENT OF A THREE-DIMENSIONAL AB INITIO POTENTIAL ENERGY SURFACE FOR THE He–Cl2(X) SYSTEM AND ITS APPLICATION TO SOLVATION STRUCTURES IN THE HenCl2 CLUSTERS

High-quality ab initio electronic structure calculations for the van der Waals interaction of He with Cl 2 in the electronic ground state have been carried out to develop a new three-dimensional potential energy surface for this system. The calculations were performed at the single and double excitation coupled-cluster level of theory with non-iterative perturbational treatment of triple excitations [CCSD(T)] with a very large basis set including an additional set of bond functions. The analytical potential surface developed were then used in the path-integral molecular dynamics calculations for the He n Cl 2 cluster, where quantum solvation structures of helium atoms in clusters were investigated. It has been found that the helium solvation structures are quite different between the electronic ground state and the electronically excited B 3Π state.

An exchange-Coulomb model potential energy surface for the Ne-CO interaction. II. Molecular beam scattering and bulk gas phenomena in Ne-CO mixtures

Four potential energy surfaces are of current interest for the Ne-CO interaction. Two are high-level fully ab initio surfaces obtained a decade ago using symmetry-adapted perturbation theory and supermolecule coupled-cluster methods. The other two are very recent exchange-Coulomb (XC) model potential energy surfaces constructed by using ab initio Heitler-London interaction energies and literature long range dispersion and induction energies, followed by the determination of a small number of adjustable parameters to reproduce a selected subset of pure rotational transition frequencies for the (20)Ne-(12)C(16)O van der Waals cluster. Testing of the four potential energy surfaces against a wide range of available experimental microwave, millimeter-wave, and mid-infrared Ne-CO transition frequencies indicated that the XC potential energy surfaces gave results that were generally far superior to the earlier fully ab initio surfaces. In this paper, two XC model surfaces and the two fully ab initio surfaces are tested for their abilities to reproduce experiment for a wide range of nonspectroscopic Ne-CO gas mixture properties. The properties considered here are relative integral cross sections and the angle dependence of rotational state-to-state differential cross sections, rotational relaxation rate constants for CO(v=2) in Ne-CO mixtures at T=296 K, pressure broadening of two pure rotational lines and of the rovibrational lines in the CO fundamental and first overtone transitions at 300 K, and the temperature and, where appropriate, mole fraction dependencies of the interaction second virial coefficient, the binary diffusion coefficient, the interaction viscosity, the mixture shear viscosity and thermal conductivity coefficients, and the thermal diffusion factor. The XC model potential energy surfaces give results that lie within or very nearly within the experimental uncertainties for all properties considered, while the coupled-cluster ab initio surface gives results that agree similarly well for all but one of the properties considered. When the present comparisons are combined with the ability to give accurate spectroscopic transition frequencies for the Ne-CO van der Waals complex, only the XC potential energy surfaces give results that agree well with all extant experimental data for the Ne-CO interaction.

Crystallographic point and space group symmetries in the one-and two-body density of crystals and generalized Patterson functions: Fourier path integral Monte Carlo case studies of novel high-temperature/high-pressure 4He quantum crystals

We present a complete formal analysis of crystallographic point and space group symmetries in the local one- and two-body density ρ(x) and ρ 2(x 1,x 2) of crystals and from there develop the mathematical theory of generalized Patterson functions. The two-body density ρ 2(x 1,x 2) and ρ(x 1) ρ(x 2) may be written as symmetrized lattice Fourier series over all reciprocal lattice vectors K with Fourier transforms that are functions of the relative vector r = x 1 – x 2. In the new formalism derived here the usual Patterson function turns out to be the lowest-lying K = 0 lattice Fourier transform in the Fourier series representation of the uncorrelated product ρ(x 1)ρ(x 2) of one-body densities. All other K ≠ 0 Fourier transforms in the Fourier series of ρ(x 1)ρ(x 2) may be regarded as generalized Patterson functions. In complete analogy to these generalized Patterson functions the Fourier transform functions in the Fourier series of the fully correlated two-body density ρ 2(x 1,x 2) may be regarded as K = 0 and K ≠ 0 fully correlated generalized Patterson functions. It is shown that the former generalized Patterson functions represent the uncorrelated long-range limits for large interparticle distances of the latter, fully correlated, generalized Patterson functions. Both types of generalized Patterson functions may be cast in the form of symmetry-adapted trigonometric series with expansion functions that depend on the relative distance r = |r| and on the polar angle of r. The symmetrized trigonometric series representations of the generalized Patterson functions are given here explicitly in specific applications of the new formalism to crystallographic space group P63/mmc. In exact path integral Monte Carlo case studies of novel high-temperature/high-pressure 4He quantum crystals in the hexagonal close-packed structure we present and discuss exact numerical results for the one-body density ρ(x) and for expansion functions, for both types of generalized Patterson functions, in the symmetrized trigonometric series representations of the generalized Patterson functions. The numerical results for the two types of generalized Patterson functions are compared to each other.

An exchange-Coulomb model potential energy surface for the Ne-CO interaction. I. Calculation of Ne-CO van der Waals spectra

Exchange-Coulomb model potential energy surfaces have been developed for the Ne-CO interaction. The initial model is a three-dimensional potential energy surface based upon computed Heitler-London interaction energies and literature results for the long-range induction and dispersion energies, all as functions of interspecies distance, the orientation of CO relative to the interspecies axis, and the bond length of the CO molecule. Both a rigid-rotor model potential energy surface, obtained by setting the CO bond length equal to its experimental spectroscopic equilibrium value, and a vibrationally averaged model potential energy surface, obtained by averaging the stretching dependence over the ground vibrational motion of the CO molecule, have been constructed from the full data set. Adjustable parameters in each model potential energy surface have been determined through fitting a selected subset of pure rotational transition frequencies calculated for the (20)Ne-(12)C(12)O isotopolog to precisely known experimental values. Both potential energy surfaces provide calculated results for a wide range of available experimental microwave, millimeter-wave, and midinfrared Ne-CO transition frequencies that are generally far superior to those obtained using the best current literature potential energy surfaces. The vibrationally averaged CO ground state potential energy surface, employed together with a potential energy surface obtained from it by replacing the ground vibrational state average of the CO stretching dependence of the potential energy surface by an average over the first excited CO vibrational state, has been found to be particularly useful for computing and/or interpreting mid-IR transition frequencies in the Ne-CO dimer.

Accuracy of recent potential energy surfaces for the He–N2 interaction. I. Virial and bulk transport coefficients

A new exchange-Coulomb semiempirical model potential energy surface for the He-N2 interaction has been developed. Together with two recent high-level ab initio potential energy surfaces, it has been tested for the reliability of its predictions of second-virial coefficients and bulk transport phenomena in binary mixtures of He and N2. The agreement with the relevant available measurements is generally within experimental uncertainty for the exchange-Coulomb surface and the ab initio surface of Patel et al. [J. Chem. Phys. 119, 909 (2003)], but with slightly poorer agreement for the earlier ab initio surface of Hu and Thakkar [J. Chem. Phys. 104, 2541 (1996)].

Transport properties of normal liquid helium: Comparison of various methodologies

We revisit the problem of self-diffusion in normal liquid helium above the lambda transition. Several different methods are applied to compute the velocity autocorrelation function. Since it is still impossible to determine the exact result for the velocity autocorrelation function from simulation, we appeal to the computation of short-time moments to determine the accuracy of the different approaches at short times. The main conclusion reached from our study is that both the quantum mode-coupling theory and the numerical analytic continuation approach must be regarded as a viable and competitive methods for the computation of dynamical properties of quantum systems.

A reliable new three-dimensional potential energy surface for H2–Kr

An improved three-dimensional potential energy surface for the H(2)-Kr system is determined from a direct fit of new infrared spectroscopic data for H(2)-Kr and D(2)-Kr to a potential energy function form based on the exchange-Coulomb model for the intermolecular interaction energy. These fits require repetitive, highly accurate simulations of the observed spectra, and both the strength of the potential energy anisotropy and the accuracy of the new data make the "secular equation perturbation theory" method used in previous analyses of H(2)-(rare gas) spectra inadequate for the present work. To address this problem, an extended version of the "iterative secular equation" method was developed which implements direct Hellmann-Feynman theorem calculation of the partial derivatives of eigenvalues with respect to parameters of the Hamiltonian which are required for the fits.